High frequency power amplifier um 83.1 circuit. Simple high frequency (UHF) amplifiers for receivers. Arrangement of elements on the circuit board

Current consumption - 46 mA. The bias voltage V bjas determines the output power level (gain) of the amplifier

Fig. 33.11. Internal structure and pinout of TSH690, TSH691 microcircuits

Rice. 33.12. Typical inclusion of TSH690, TSH691 microcircuits as an amplifier in the frequency band 300-7000 MHz

and can be adjusted within 0-5.5 (6.0) V. The transmission coefficient of the TSH690 (TSH691) microcircuit at a bias voltage V bias = 2.7 V and a load resistance of 50 Ohms in a frequency band up to 450 MHz is 23 (43) dB, up to 900(950) MHz - 17(23) dB.

Practical inclusion of TSH690, TSH691 microcircuits is shown in Fig. 33.12. Recommended element values: C1=C5=100-1000 pF; C2=C4=1000 pF; C3=0.01 µF; L1 150 nH; L2 56 nH for frequencies not exceeding 450 MHz and 10 nH for frequencies up to 900 MHz. Resistor R1 can be used to regulate the output power level (can be used for an automatic output power control system).

The broadband INA50311 (Fig. 33.13), manufactured by Hewlett Packard, is intended for use in mobile communications equipment, as well as in household electronic equipment, for example, as antenna amplifier or radio frequency amplifier. The operating range of the amplifier is 50-2500 MHz. Supply voltage - 5 V with current consumption up to 17 mA. Average gain

Rice. 33.13. internal structure microcircuits ΙΝΑ50311

10 dB. The maximum signal power supplied to the input at a frequency of 900 MHz is no more than 10 mW. Noise figure 3.4 dB.

A typical connection of the ΙΝΑ50311 microcircuit when powered by a 78LO05 voltage stabilizer is shown in Fig. 33.14.

Rice. 33.14. broadband amplifier on the INA50311 chip

Shustov M. A., Circuitry. 500 devices on analog chips. - St. Petersburg: Science and Technology, 2013. -352 p.

A simple amplifier with just one transistor can be made to amplify a weak RF signal for a radio, TV or radio station.

The article below presents two circuits of simple amplifiers. H I buy it in a store, it’s cheaper to assemble an amplifier yourself, with characteristics sometimes no worse than store-bought ones.

Only a few parts are needed to assemble it. Even a novice radio amateur can handle assembling the amplifier. There are no inductors in it, the amplifiers are broadband and cover the entire range of the amplified signal, including UHF. In any case, the result was more than I expected. Most VHF local television and radio broadcasts began to be received with better quality, the picture became clearer.

Amplifier circuit diagram

The main part of this circuit is a high frequency reverse conduction (npn) transistor Q1 (2SC2570), specially designed for VHF gain signal circuit without an inductor.

If you plan to use the amplifier constantly, then you can exclude S2, which is needed to bypass the amplifier.

The amplifier is assembled on a circuit board.

Circuit board

Arrangement of elements on the circuit board

The second version of the circuit with an additional amplifier for the HF range

Schematic diagram of a dual-band HF/VHF amplifier

In this circuit, an HF amplifier is added to field effect transistor(Q1 MFE201 N-channel two gate and Q2 (and 2SC2570 n-p-n RF silicon transistor), which provide two independent amplifiers, switched by switch S1. The result is a simple active antenna designed to amplify signals from 3 to 3000 MHz (three ranges: 3-30 MHz high-frequency (HF) signals; 3-300 MHz very-high-frequency (VHF) signals; 300-3000 MHz ultra-high-frequency (UHF) signals.

Most audio lovers are quite categorical and are not ready to compromise when choosing equipment, rightly believing that the perceived sound must be clear, strong and impressive. How to achieve this?

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Perhaps the main role in resolving this issue will be played by the choice of amplifier.
Function
The amplifier is responsible for the quality and power of sound reproduction. At the same time, when purchasing, you should pay attention to the following symbols, which indicate the implementation high technology in the production of audio equipment:

• Hi-fi. Provides maximum purity and accuracy of sound, freeing it from extraneous noise and distortion.
• Hi-end. The choice of a perfectionist who is willing to pay a lot for the pleasure of discerning the smallest nuances of his favorite musical compositions. Hand-assembled equipment is often included in this category.

Specifications you should pay attention to:

• Input and output power. The rated output power is of decisive importance, because edge values ​​are often unreliable.
• Frequency range. Varies from 20 to 20000 Hz.
• Nonlinear distortion factor. Everything is simple here - the less the better. The ideal value, according to experts, is 0.1%.
• Signal to noise ratio. Modern technology assumes a value of this indicator over 100 dB, which minimizes extraneous noise when listening.
• Dumping factor. Reflects the output impedance of the amplifier in its relation to the nominal load impedance. In other words, a sufficient damping factor (more than 100) reduces the occurrence of unnecessary vibrations of equipment, etc.

It should be remembered: the manufacture of high-quality amplifiers is a labor-intensive and high-tech process; accordingly, too low a price with decent characteristics should alert you.

Classification

To understand the variety of market offers, it is necessary to distinguish the product according to various criteria. Amplifiers can be classified:

• By power. Preliminary is a kind of intermediate link between the sound source and the final power amplifier. The power amplifier, in turn, is responsible for the strength and volume of the output signal. Together they form a complete amplifier.

Important: the primary conversion and signal processing takes place in the preamplifiers.

• Based on the element base, there are tube, transistor and integrated minds. The latter arose with the goal of combining the advantages and minimizing the disadvantages of the first two, for example, the sound quality of tube amplifiers and the compactness of transistor amplifiers.
• Based on their operating mode, amplifiers are divided into classes. The main classes are A, B, AB. If Class A amplifiers use a lot of power, but produce high-quality sound, Class B amplifiers are exactly the opposite, Class AB seems to be the optimal choice, representing a compromise between signal quality and fairly high efficiency. There are also classes C, D, H and G, which arose with the use of digital technologies. There are also single-cycle and push-pull operating modes of the output stage.
• Depending on the number of channels, amplifiers can be single-, double- and multi-channel. The latter are actively used in home theaters to create volumetric and realistic sound. Most often there are two-channel ones for right and left audio systems, respectively.

Attention: studying the technical components of the purchase is, of course, necessary, but often the decisive factor is simply listening to the equipment according to the principle of whether it sounds or not.

Application

The choice of amplifier is largely justified by the purposes for which it is purchased. We list the main areas of use of audio amplifiers:

1. As part of a home audio system. It's obvious that best choice is a tube two-channel single-ended class A, also optimal choice can form a three-channel class AB, where one channel is designated for a subwoofer, with a Hi-fi function.
2. For car audio system. The most popular are four-channel AB or D class amplifiers, depending on the financial capabilities of the buyer. Cars also require a crossover function for smooth frequency control, allowing frequencies in the high or low range to be cut as needed.
3. In concert equipment. The quality and capabilities of professional equipment are reasonably subject to higher demands due to the large propagation space of sound signals, as well as the high need for intensity and duration of use. Thus, it is recommended to purchase an amplifier of at least class D, capable of operating almost at the limit of its power (70-80% of the declared one), preferably in a housing made of high-tech materials that protects against negative influences. weather conditions and mechanical influences.
4. In studio equipment. All of the above is also true for studio equipment. We can add about the largest frequency reproduction range - from 10 Hz to 100 kHz in comparison with that from 20 Hz to 20 kHz in a household amplifier. Also noteworthy is the ability to separately adjust the volume on different channels.

In the way that for a long time To enjoy clear and high-quality sound, it is advisable to study the variety of offers in advance and choose the audio equipment option that best suits your needs.

High-frequency power amplifiers are built according to a circuit containing amplification stages, a filter and automation circuits. Amplifiers are characterized by nominal output and minimum input powers, operating frequency range, efficiency, sensitivity to load changes, level of unwanted fluctuations, stability and reliability of operation, weight, dimensions, and cost.

The currently obtained maximum output power values ​​at frequencies up to 100 MHz are several tens of kilowatts. With significantly less power supplied by individual transistors (no more than 200 W), these values ​​are achieved by special signal combining devices, among which the most common are power dividers and adders. There are many varieties of these devices. Based on the magnitude of the phase shift, they are divided into in-phase (with a phase shift of the summed signals φ = 0), antiphase (φ = π), quadrature (φ = n/2), etc.; by type of execution - with distributed and concentrated elements; according to the method of connection to the load - serial and parallel, etc.

One of the main requirements for signal summing devices is to ensure the least mutual influence of individual modules, the powers of which are summed up (the so-called module decoupling). Let's see how this requirement is met in a simple common-mode adder using transformers. The circuit of such an adder on transformers T4- T6 together with a divider (on transformers T1- TK) and summable cascades (on transistors VT1 And VT2) without bias and power circuits is shown in Fig. 5.4. Transformers T4- T6 have transformation ratios of 1.1 and 1/V2, respectively (here r n is the load resistance, R B is the ballast resistor, the resistance of which is 2g n). Under normal operating conditions, when the voltages on the collectors are in phase and their amplitudes are equal, there is no current in the ballast resistor. Transformer T6 leads to two series-connected transformer windings T4 And T5 resistance is 2r n, so at the collector of each transistor the load resistance is r n. Let us now imagine that the collector of the transistor VT2 turned out to be short-circuited with its emitter. In this case, the secondary winding of the transformer T5 represents an extremely low resistance for an RF signal, so that the resistance is 2r n, reduced to the primary winding of the transformer T6, completely driven to the secondary winding of the transformer T4, a therefore, to the collector of the transistor VT1. But in parallel VT1 in this case, a ballast resistor of the same resistance turns out to be connected, i.e., despite the change in operating mode, in the second stage the operating conditions of the first stage have not changed - it still operates on load resistance r n. But, since half of its power now goes to the ballast resistor, only half the power of one stage remains in the load, which is 4 times less than the power delivered by the amplifier to the load before normal operating conditions change. How larger number cascades are used to obtain output power, the less the effect a change in operating conditions in one or another cascade has on the total power in the load. For example, in an amplifier with an output power of 4.5 kW, obtained by summing the powers of 32 transistor stages, if one stage failed, the output power was reduced to only 4.3 kW. Thus, the very small mutual influence of the cascades in the power summing device allows, by making maximum use of the amplifying properties of each transistor, to ensure high reliability of its operation, and therefore, trouble-free operation of the power amplifier as a whole.

Rice. 5.4. Amplifier circuit with power addition on transformers

The adding device is selected based on the nature and operating conditions of the amplifier, since when solving the main problem - adding signals - it is possible, using certain features of a particular type of adder, to improve other characteristics of the amplifier, for example, to weaken certain types of unwanted oscillations or reduce sensitivity to load mismatch .

Satisfactory isolation of the modules, as well as a low level of unwanted third-order oscillations, low sensitivity to load changes and weak influence of the summed stages on the preamplifier are obtained by using quadrature power adders. Antiphase adders with satisfactory isolation suppress unwanted second-order oscillations. Alternating quadrature and antiphase addition devices, for example, when two modules are added in antiphase, and the pairs of modules combined in this way are added in quadrature, largely combines the advantages of both types of adding devices. For these reasons, quadrature and anti-phase adders and power dividers, made, for example, on long coaxial or strip lines, transformers, are widely used in amplifiers with output powers of 10 W and above.

The next parameter of the amplifier - the minimum input power - is determined by the permissible noise level and stability of operation and, in this regard, depends on the circuit, operating mode and design of the amplifier. The effect of noise on the sensitivity of the amplifier is explained as follows. It is known that the noise power brought to the input of the amplifier is determined by the formula P w = = 4kTF w Df, where k - Boltzmann constant; T- absolute temperature; F m - noise factor;

Af is the frequency bandwidth in which it is determined

R sh. But for a given signal-to-noise ratio TO w at the amplifier output power of the input signal R With should not be less than R Sh TO Sh . It follows that the minimum permissible value of the input signal, thus characterizing the sensitivity of the amplifier, is defined as R C tsh = 4kTF y K w Df. For given TO w and Af all quantities included in this expression are known, with the exception of F JI. Using well-known relationships, it is easy to show that in a nonlinear amplifier, which in the general case is a power amplifier, with a sufficiently large power gain of the first stage

where F sh1 is the noise figure of the first stage; at t+1 is the ratio of the noise power gain to the signal power gain in the (m+1)th stage of the amplifier containing P cascades. Depending on the operating mode of the cascade, this ratio is determined by the formula

The coefficients included in this formula are found in the tables. For example, for a four-stage amplifier with a power of 50 W at F m 1 = 6, Y 2 =1.6, Yз=1.7, Y 4 =1.9 we have F w =31, which at Kw = 120 dB, Df = 20 kHz and 4kT = 1.62*10-20 W/Hz gives P sh = 1*10 -14 W and P cmin = 10 MW, i.e., under the specified conditions, the minimum the permissible value of the input signal is characterized by a voltage of about 1 V at a resistance of 75 Ohms. Note that the specified definition of sensitivity is valid if there is a signal at the input of the amplifier in which the noise power is at least an order of magnitude lower than the self-noise power of the amplifier Psh reduced to the input, since otherwise an acceptable signal-to-noise ratio will not be obtained Ksh. If this difference in the noise levels at the input is not observed, then to ensure the required value of Ksh, a selective circuit must be installed between the signal sources and the amplifier, leading to the necessary noise suppression at a given detuning from the operating frequency.

Rice. 5.7. Scheme amplifier with 15 W output power for the frequency range 2 - 30 MHz

Table 5.1

Rice. 5.8. Amplifier circuit with an output power of 80 W for the frequency range 2 - 30 MHz

Table 5.2

End of table. 5.2

The frequency bandwidth at high power levels is largely determined by interstage matching circuits, which use specially designed broadband transformers, as well as amplitude-frequency response correction circuits and feedback circuits. So, in Fig. Figures 5.7 and 5.8 show amplifier circuits with output powers of 15 and 80 W for radio transmitters with powers of 10 and 50 W operating in the range 2 - 30 MHz. Their main characteristics are given in table. 5.1, and the data of the transformers and chokes used are in table. 5.2. The features of these amplifiers are a relatively low level of unwanted vibrations and a relatively small unevenness of the amplitude-frequency response. These parameters, for example, in an 80 W amplifier are achieved by using frequency-dependent negative feedback in the output stage (from the secondary winding of the transformer TK through resistors R11 And R12 to transistor bases VT3 And VT4) and in the pre-final stage (using resistors R4 - R7), and also with correction chains C2 R2, C3 R3 And R1 L1 C1.

You can also reduce gain unevenness in the frequency band by using correction circuits at the input of the final stage (capacitor C7 and conductor inductance AB And VG, which are strips of foil 30 mm long and 4 mm wide) and at the amplifier output (transformer inductance T4 and capacitor C 13). The wideband transformers used in these amplifiers are capable of providing satisfactory matching not only in the range of 2 - 30 MHz, but also at higher frequencies. However, at frequencies above 30 MHz, better performance is obtained with stripline transformers without ferrite materials. Such transformers, for example, were used in an amplifier with an output power of 80 W in the range of 30 - 80 MHz (Table 5.3), the circuit of which is shown in Fig. 5.9. A special feature of this amplifier is the use of bipolar and field-effect transistors simultaneously. This combination made it possible to improve the noise characteristics in relation to the use of only bipolar transistors, and, in comparison with the use of only field devices, to improve the energy characteristics of the amplifier.

Table 5.3

Rice. 5.9 Amplifier circuit with an output power of 80 W for the frequency range 30---80 MHz

An important parameter of an RF amplifier is its efficiency. This parameter depends on the purpose of the amplifier, its operating conditions and, as a consequence, on the construction circuit and the semiconductor devices used. It is 40 - 90% for signal amplifiers with constant or switched amplitude (for example, with frequency and phase modulation, frequency and amplitude telegraphy) and 30 - 60% for linear signal amplifiers with amplitude modulation. The lower of the indicated values ​​is explained by the use of energetically unfavorable, but providing linear amplification of understressed modes in all stages, as well as mode A in the preliminary, and often in the pre-final stage of the amplifier. Higher values ​​are typical for the key mode of amplification of signals with constant or switched amplitude (80 - 90%) or for amplitude-modulated signals (50 - 60%) when using the method of separate amplification of signal components. For example, an efficiency of at least 80% was obtained in a 4.5 kW wideband amplifier with an output stage of 32 transistors, built taking into account general recommendations for the switching mode and taking measures to eliminate through currents. However, despite the obvious energy advantages of the key operating mode, it is still relatively rarely used in RF amplifiers. This is explained by a number of features, which, for example, include criticality to load changes, a high level of unwanted oscillations, a high probability of exceeding the maximum permissible transistor voltages and the difficulty of adjustment in obtaining the necessary phase-frequency characteristics, the stability of which must be ensured under conditions of changing load, supply voltage and temperature environment. In addition, to implement the switching mode at high frequencies, transistors with extremely short duration of transient processes when turned on and off are required.

A promising direction for increasing the energy characteristics of amplitude-modulated signal amplifiers is quantization of the signal by level with separate amplification of discrete components and their subsequent summation, taking into account phase shifts.

In increasing the efficiency of amplifiers, an important role is played by the quality of matching with the load, taking into account the possibility of its change. Currently, this issue is simply and at the same time most effectively solved by using ferrite valves and circulators. However, this is the case at relatively high frequencies, at least above 80 MHz. As the frequency decreases, the efficiency of using ferrite decoupling devices drops sharply. In this regard, it is of interest to study and subsequent industrial development of semiconductor non-reciprocal devices with circulator properties, which in principle allow operation at low frequencies. If the use of valves or circulators is not possible, satisfactory results are obtained by combining conventional matching devices with automatic control of the operating mode of the amplifier. Thus, by increasing the supply voltage with increasing load resistance (with a constant or slightly reduced excitation) and decreasing it with a decrease in load resistance with increasing excitation, it is possible to obtain not only a constant output power, but also to maintain, under changing load conditions, the high efficiency value that was received in nominal mode. The capabilities of this method of stabilizing the output power, however, are limited by the maximum permissible currents and voltages of the transistor used, as well as the technical capabilities of matching low resistances. For these reasons, the currently implemented range of load resistances, in which it is still possible to achieve relatively stable output power in this way, is limited, as tests of an amplifier with an output power of 4.5 kW have shown, to a VSWR value not exceeding 3.

The effect of low sensitivity to load mismatch can also be obtained by constructing an amplifier using a power addition circuit using quadrature adders and power dividers. With an appropriate excitation voltage, such an amplifier can be achieved, despite changing the operating mode of each of the summed stages, a slight change in the total current consumption and total output power. When testing such amplifiers, it was noted that the change in output power during load mismatch is the same as in linear circuits, i.e., it is described by an expression close to P/P n = 4p/(1+p) 2, where P n And R- power in the rated and mismatched load, ap - VSWR, characterizing the degree of mismatch. Such a change on average, as comparative tests have shown, is approximately half that of an amplifier built, for example, using a push-pull circuit.

There are other ways to reduce the sensitivity of an amplifier to load mismatch, but all of them are, to one degree or another, inferior to those considered.

Recently, the main parameters of an amplifier have included the level of unwanted oscillations that arise during the process of amplifying the useful signal. Such oscillations appear in the power amplifier due to nonlinear processes under the influence of the useful signal f and interference coming from the signal generation path (f f), the power source (f p) and the radio transmitter antenna (f a). Extraneous oscillations (interference) from the signal generation path lead to unwanted emissions from the radio transmitting device not only at the frequencies of these oscillations ff, but also at the frequencies formed under the influence of combination oscillations mf± nf f . The level of such radiation is determined by the relative level of unwanted oscillations at the output of the shaping path, its change (conversion) in the power amplifier, as well as the filtering and radiating properties of the radio transmitting device nodes following the amplifier. The change in the interference/signal ratio in the amplifier (K ​​y) is determined by the transistor switching circuit, the mode of operation of the cascades, the value and frequency of the useful signal and interference.

The greatest change in the noise/signal ratio is observed in an amplifier with an OE, as well as at a low output resistance of the signal source r G in an amplifier with OB and at low load resistance r n in an amplifier with OK. With increasing r g in an amplifier with OB and r n in an amplifier with OK K y -> 1. When the amplifier operates in modes A and B with any transistor turned on, the relative noise level does not change; a shift in the operating mode towards mode C leads to an increase, and towards mode AB, on the contrary, to decrease in the relative level of interference; in this case, the increase is more noticeable than the decrease. Increasing the intensity of the mode reduces the relative level of interference. The greater the value of the useful signal, the more the interference/signal ratio changes under the same operating mode. With increasing frequency of the signal and interference, the change The interference/signal ratio decreases.

Raman oscillations arising under the influence of interference are especially dangerous when the amplifier is operating in mode C, where their level at the amplifier output is commensurate with the level of interference. With a change in operating mode from C to A, the level of combination oscillations of the second order (f±fф) monotonically decreases, and the third (2f±fф) passes through 0 in mode B and upon reaching a minimum in the region of negative values, indicating a change in the phase of oscillations to the opposite , when approaching mode A tends to 0.

All other things being equal, the amplifier with OK is characterized by the greatest suppression of combination oscillations, followed by amplifiers with OB and OE. In a multi-stage amplifier, unlike a single-stage amplifier, the interference for each subsequent stage, starting from the second, is not only amplified unwanted oscillations of the formation path, but also combinational and harmonic oscillations of the previous stages. The influence of the second harmonic is especially great; it increases the levels of second- and third-order Raman oscillations and reduces the noise/signal ratios. This is mainly manifested in mode C and is actually absent in A. Under its influence, the linear mode of operation (K y = 1) is shifted from mode B to C. These changes are directly opposite if the phase of the second harmonic is somehow artificially changed to l.

A low level of combination oscillations, a slight deterioration in the noise/signal ratio and at the same time acceptable energy characteristics are characteristic of an amplifier whose preliminary stages operate in modes A - B, and the output stage in B - C. When the transistors are switched on according to the OK circuit, modes B - C can be used and in the preliminary stages, but in the output stage, switching on according to the OK circuit is unacceptable due to the high susceptibility of the amplifier to signals from extraneous radio transmitters. The best option for the output stage is to switch on the device according to the OB or OE circuit. In this case, the deterioration of the interference/signal ratio in the amplifier at a low level of combination oscillations can amount to a maximum of 3 dB. But if the amplifier is poorly designed, this value can increase to 20 dB, and the highest level of unwanted oscillations will be not only at the interference frequency, but also at the frequencies caused by this interference of combination oscillations.

When the frequency is detuned between the useful signal and the interference, interference is most effectively suppressed in amplifiers with filters. Suppression is realized both with electronically commutated filters and by constructing an amplifier based on a powerful self-oscillator controlled using a phase-locked loop system. In the latter case, it is possible to obtain attenuation of unwanted components - up to 70 - 80 dB, starting from a 5% detuning of their frequency from the frequency of the useful signal.

Currently existing transistors in the undervoltage operating mode of the cascade make it possible to obtain a level of third-order intermodulation oscillations - (15 - 30) dB in relation to the interference that caused them when switched on according to the OE circuit, approximately 15 dB less when switched on according to the OB circuit and, vice versa, 15 dB more when switched on according to the OK scheme. Additional suppression of about 15 - 20 dB can be obtained using quadrature summation of the module signals in the output stage and at least another 15 dB by using a ferrite valve or circulator at the amplifier output.

The highest level of unwanted oscillations is observed at the harmonics of the useful signal. In a single-stage amplifier, without taking any measures to suppress them, this level for the second and third harmonics is usually - (15 - 20) dB. By switching on cascades according to a power addition circuit using quadrature and antiphase adders and dividers, it can be reduced to - (30 - 40) dB. If a filter bank is installed behind the amplifier, then this level is further reduced by the amount of attenuation of the corresponding filter in the stopband.

Using filters, you can achieve a high level of suppression of harmonic components. However, it should be emphasized that attenuate harmonics; to a level below - 120 dB is only possible with very careful shielding of the RF stages and the elimination of various contact connections in the path after the power amplifier, including RF connectors, in which harmonic oscillations with the same level can form.

As can be seen, existing technical solutions provide high suppression of unwanted vibrations. However, in a number of cases it still turns out to be insufficient for normal operation of the equipment. Thus, when transceivers located on mobile vehicles are brought closer together or when working as part of radio complexes, where a wide variety of equipment is concentrated and must function in extremely limited space, radio receivers often cannot work with their correspondents as soon as a nearby radio transmitter of another communication line is turned on. This situation occurs due to the receivers being exposed to some unwanted emissions from the radio transmitter. These primarily include noise. Despite their low level, they are the ones who fly

the greatest danger in these conditions, since, having a continuous spectrum and slightly varying spectral density with detuning, they can, if the necessary measures are not taken, almost completely paralyze the work of nearby receivers.

The greatest danger in the situation under consideration is represented by interference from the signal generation path of the transmitter and the combination oscillations they generate in the power amplifier, which, like noise, occupy a wide frequency range and cannot be significantly minimized when constructing an amplifier according to the previously discussed principle of direct cascade power amplification.

High frequency amplifiers (UHF) are used to increase the sensitivity of radio receiving equipment - radios, televisions, radio transmitters. Placed between the receiving antenna and the input of the radio or television receiver, such UHF circuits increase the signal coming from the antenna (antenna amplifiers).

The use of such amplifiers allows you to increase the radius of reliable radio reception; in the case of radio stations (receive-transmit devices - transceivers), either increase the operating range, or, while maintaining the same range, reduce the radiation power of the radio transmitter.

Figure 1 shows examples of UHF circuits often used to increase radio sensitivity. The values ​​of the elements used depend on specific conditions: on the frequencies (lower and upper) of the radio range, on the antenna, on the parameters of the subsequent stage, on the supply voltage, etc.

Figure 1 (a) shows broadband UHF circuit according to the common emitter circuit(OE). Depending on the transistor used, this circuit can be successfully applied up to frequencies of hundreds of megahertz.

It is necessary to recall that the reference data for transistors provides maximum frequency parameters. It is known that when assessing the frequency capabilities of a transistor for a generator, it is enough to focus on the limiting value of the operating frequency, which should be at least two to three times lower than the limiting frequency specified in the passport. However, for an RF amplifier connected according to the OE circuit, the maximum nameplate frequency must be reduced by at least an order of magnitude or more.

Fig.1. Examples of circuits of simple high-frequency (UHF) amplifiers using transistors.

Radio elements for the circuit in Fig. 1 (a):

• R1=51k (for silicon transistors), R2=470, R3=100, R4=30-100;
• C1=10-20, C2= 10-50, C3= 10-20, C4=500-Zn;

Capacitor values ​​are given for VHF frequencies. Capacitors such as KLS, KM, KD, etc.

Transistor stages, as is known, connected in a common emitter (CE) circuit, provide relatively high gain, but their frequency properties are relatively low.

Transistor stages connected according to a common base (CB) circuit have less gain than transistor circuits with OE, but their frequency properties are better. This allows the same transistors to be used as in OE circuits, but at higher frequencies.

Figure 1 (b) shows wideband high frequency amplifier circuit (UHF) on one transistor turned on according to a common base scheme. The LC circuit is included in the collector circuit (load). Depending on the transistor used, this circuit can be successfully applied up to frequencies of hundreds of megahertz.

Radio elements for the circuit in Fig. 1 (b):

• R1=1k, R2=10k. R3=15k, R4=51 (for supply voltage ZV-5V). R4=500-3 k (for supply voltage 6V-15V);
• C1=10-20, C2=10-20, C3=1n, C4=1n-3n;
• T1 - silicon or germanium RF transistors, for example. KT315. KT3102, KT368, KT325, GT311, etc.

Capacitor and circuit values ​​are given for VHF frequencies. Capacitors such as KLS, KM, KD, etc.

Coil L1 contains 6-8 turns of PEV 0.51 wire, brass cores 8 mm long with M3 thread, 1/3 of the turns are drained.

Figure 1 (c) shows another broadband circuit UHF on one transistor, included according to a common base scheme. An RF choke is included in the collector circuit. Depending on the transistor used, this circuit can be successfully applied up to frequencies of hundreds of megahertz.

Radioelements:

• R1=1k, R2=33k, R3=20k, R4=2k (for supply voltage 6V);
• C1=1n, C2=1n, C3=10n, C4=10n-33n;
• T1 - silicon or germanium RF transistors, for example, KT315, KT3102, KT368, KT325, GT311, etc.

The values ​​of capacitors and circuit are given for frequencies of the MF and HF ranges. For higher frequencies, for example, for the VHF range, the capacitance values ​​should be reduced. In this case, D01 chokes can be used.

Capacitors such as KLS, KM, KD, etc.

L1 coils are chokes; for the CB range these can be coils on rings 600NN-8-K7x4x2, 300 turns of PEL 0.1 wire.

Higher gain value can be obtained by using multi-transistor circuits. These can be various circuits, for example, made on the basis of an OK-OB cascode amplifier using transistors of different structures with serial power supply. One of the variants of such a UHF scheme is shown in Fig. 1 (d).

This UHF circuit has significant gain (tens or even hundreds of times), but cascode amplifiers cannot provide significant gain at high frequencies. Such schemes are usually used at frequencies in the LW and SV ranges. However, with the use of ultra-high frequency transistors and careful design, such circuits can be successfully applied up to frequencies of tens of megahertz.

Radioelements:

• R1=33k, R2=33k, R3=39k, R4=1k, R5=91, R6=2.2k;
• C1=10n, C2=100, C3=10n, C4=10n-33n. C5=10n;
• T1 -GT311, KT315, KT3102, KT368, KT325, etc.
• T2 -GT313, KT361, KT3107, etc.

The capacitor and circuit values ​​are given for frequencies in the CB range. For higher frequencies, such as the HF band, capacitance values ​​and loop inductance (number of turns) must be reduced accordingly.

Capacitors such as KLS, KM, KD, etc. Coil L1 - for the CB range contains 150 turns of PELSHO 0.1 wire on 7 mm frames, trimmers M600NN-3-SS2.8x12.

When setting up the circuit in Fig. 1 (d), it is necessary to select resistors R1, R3 so that the voltages between the emitters and collectors of the transistors become the same and amount to 3V at a circuit supply voltage of 9 V.

The use of transistor UHF makes it possible to amplify radio signals. coming from antennas, in television bands - meter and decimeter waves. In this case, antenna amplifier circuits built on the basis of circuit 1(a) are most often used.

Antenna amplifier circuit example for frequency range 150-210 MHz is shown in Fig. 2 (a).

Fig.2.2. MV antenna amplifier circuit.

Radioelements:

• R1=47k, R2=470, R3= 110, R4=47k, R5=470, R6= 110. R7=47k, R8=470, R9=110, R10=75;
• C1=15, C2=1n, C3=15, C4=22, C5=15, C6=22, C7=15, C8=22;
• T1, T2, TZ - 1T311(D,L), GT311D, GT341 or similar.

Capacitors such as KM, KD, etc. The frequency band of this antenna amplifier can be expanded in the low frequency region by a corresponding increase in the capacitances included in the circuit.

Radio elements for the antenna amplifier option for the range 50-210 MHz:

• R1=47k, R2=470, R3= 110, R4=47k, R5=470, R6= 110. R7=47k, R8=470. R9=110, R10=75;
• C 1=47, C2= 1n, C3=47, C4=68, C5=47, C6=68, C7=47, C8=68;
• T1, T2, TZ - GT311A, GT341 or similar.

Capacitors such as KM, KD, etc. When repeating this device, all requirements must be met. requirements for installation of HF structures: minimum lengths of connecting conductors, shielding, etc.

An antenna amplifier designed for use in the television signal range (and higher frequencies) can be overloaded with signals from powerful CB, HF, and VHF radio stations. Therefore, a wide frequency band may not be optimal because this may interfere with the amplifier's normal operation. This is especially true in the lower region of the amplifier's operating range.

For the circuit of the given antenna amplifier, this can be significant, because The slope of the gain decay in the lower part of the range is relatively low.

You can increase the steepness of the amplitude-frequency response (AFC) of this antenna amplifier by using 3rd order high pass filter. To do this, an additional LC circuit can be used at the input of the specified amplifier.

The connection diagram for an additional LC high-pass filter to the antenna amplifier is shown in Fig. 2(b).

Additional filter parameters (indicative):

• C=5-10;
• L - 3-5 turns PEV-2 0.6. winding diameter 4 mm.

It is advisable to adjust the frequency band and frequency response shape using appropriate measuring instruments (sweeping frequency generator, etc.). The shape of the frequency response can be adjusted by changing the values ​​of capacitors C, C1, changing the pitch between turns L1 and the number of turns.

Using the described circuit solutions and modern high-frequency transistors (ultra-high-frequency transistors - microwave transistors), you can build an antenna amplifier for the UHF range. This amplifier can be used either with a UHF radio receiver, for example, part of a VHF radio station, or in conjunction with a TV.

Figure 3 shows UHF antenna amplifier circuit.

Fig.3. UHF antenna amplifier circuit and connection diagram.

Main parameters of the UHF range amplifier:

• Frequency band 470-790 MHz,
• Gain - 30 dB,
• Noise figure -3 dB,
• Input and output impedance - 75 Ohm,
• Current consumption - 12 mA.

One of the features of this circuit is the supply of supply voltage to the antenna amplifier circuit through the output cable, through which the output signal is supplied from the antenna amplifier to the radio signal receiver - a VHF radio receiver, for example, a VHF radio receiver or TV.

The antenna amplifier consists of two transistor stages connected in a circuit with a common emitter. A 3rd order high-pass filter is provided at the input of the antenna amplifier, limiting the range of operating frequencies from below. This increases the noise immunity of the antenna amplifier.

Radioelements:

• R1 = 150k, R2=1k, R3=75k, R4=680;
• C1=3.3, C10=10, C3=100, C4=6800, C5=100;
• T1, T2 - KT3101A-2, KT3115A-2, KT3132A-2.
• Capacitors C1, C2 are type KD-1, the rest are KM-5 or K10-17v.
• L1 - PEV-2 0.8 mm, 2.5 turns, winding diameter 4 mm.
• L2 - RF choke, 25 µH.

Figure 3 (b) shows a diagram of connecting the antenna amplifier to the antenna socket of the TV receiver (to the UHF selector) and to a remote 12 V power source. In this case, as can be seen from the diagram, power is supplied to the circuit through the coaxial cable used and for transmitting an amplified UHF radio signal from an antenna amplifier to a receiver - a VHF radio or to a TV.

Radio connection elements, Fig. 3 (b):

• C5=100;
• L3 - RF choke, 100 µH.

The installation is carried out on double-sided fiberglass SF-2 in a hinged manner, the length of the conductors and the area of ​​the contact pads are minimal, it is necessary to provide careful shielding of the device.

Setting up the amplifier comes down to setting the collector currents of the transistors and are regulated using R1 and RЗ, T1 - 3.5 mA, T2 - 8 mA; the shape of the frequency response can be adjusted by selecting C2 within 3-10 pF and changing the pitch between turns of L1.

Literature: Rudomedov E.A., Rudometov V.E - Electronics and spy passions-3.