Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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A broadband transistor power amplifier makes it possible to significantly simplify the design of a modern transceiver and ensure (unlike tube devices) untuned operation of the final stage. As the author of the article reported, this silo was repeated by several shortwaves, and it works flawlessly for everyone.

Having suffered with the manufacture and adjustment of several variants of silos, I analyzed the circuits of the output stages of foreign factory-made transceivers intended for amateur radio communications, as well as domestic military circuitry of equipment of a similar class. As a result, a certain approach has appeared to the design of broadband transistor power amplifiers for shortwave transceivers. By adhering to it in the manufacture of silos, the radio amateur is more likely to avoid trouble both during their setup and during subsequent operation. Here are the main provisions of this approach.

1. In the silo it is necessary to use transistors specially designed for linear amplification in the frequency band of 1,5 ... 30 MHz (series KT921, KT927, KT944, KT950, KT951, KT955, KT956, KT957, KT980).

2. The output power of the device must not exceed the maximum value of the power of one transistor of a push-pull silo (in military technology, this figure does not exceed 25% of the maximum power of the transistor).

3. Prestages must work in class A.

4. Transistors for push-pull stages must be selected in pairs.

5. You should not strive to get the maximum gain (Kus) from each stage. This will lead to their unstable work. It is more expedient to introduce an additional cascade, and reduce the Kus of the remaining cascades by negative feedbacks.

6. Mounting must be rigid and element leads must be kept to a minimum length. The easiest way is to use PCB mounting with support pads.

7. Savings on blocking capacitors and decoupling chains adversely affect the stability of the amplifier as a whole.

8. Savings on the size of the radiator is not justified. Here attempts to "microminiaturize" the equipment usually end in nervous stress followed by material costs.

The rated output power of the proposed amplifier at a supply voltage of +24 V and an excitation voltage of 0,5 V (rms) is about 100 watts. The output impedance of the amplifier is 50 ohms, and the input impedance is 8 10 ohms. Without additional filtering, the level of the second harmonic at the amplifier output does not exceed -34 dB, and the third - -18 dB. The level of combination components of the third order at the peak of the two-tone signal envelope does not exceed -36 dB. These measurements were carried out with an SK4-59A spectrum analyzer. Current consumption - up to 9 A (at maximum output power). The operating frequency band is from 1,8 to 30 MHz. The amplifier was successfully operated in long tests (without the use of forced airflow).

Three stages of the power amplifier (Fig. 1) are placed on a common board with dimensions of 165x85 mm, fixed directly on the rear wall - the radiator of the transceiver. In the first stage, a KT913A transistor was used. It can be replaced by KT904A, KT911A. The quiescent current of the transistor (within the feedback of C2, R3 and C4, R4, R5 form the frequency response of the cascade. The frequency response of the cascade can be raised by the capacitor C4 in the 24 ... 28 MHz band. The values ​​​​of C2 and R3 affect the overall frequency response. powered from a source with a voltage of +12 V, then it can be performed on a KT939A transistor, which is specially designed for class A linear amplifiers.Transformer T1 is made on an annular magnetic core made of ferrite grade 1000NM-3, size K10x6x5 mm. The windings contain 8 turns of wire PEV 0,2, XNUMX mm.

(click to enlarge)

The second stage is assembled on a KT921A transistor. This transistor is designed for linear amplifiers KB and VHF bands. The quiescent current of this stage - 300 ... 350 mA is set by selecting the resistor R7. The characteristic of the cascade is formed by the elements R8, R9, C7, R6 and C8. The so-called "binoculars" were used as transformer T2 (see, for example, an article in "Radio", 1984, No. 12, p. 18). Two columns of the transformer are assembled from ring magnetic cores made of ferrite grade 1000NM-3 or 2000NM-3 with an outer diameter of 10 mm. The length of the typed column is about 12 mm (3-4 rings). Primary winding - 2-3 turns of MGTF wire 0,25 mm, secondary - 1 turn MGTF 0,8 mm.

The output stage of the amplifier is push-pull. Here you can use transistors of types KT956A, KT944A, KT957A. The best in terms of margin of safety - KT956A. Transistors KT944A give a "blockage" of the frequency response in the HF ranges, and KT957 are less reliable. A matched pair of transistors ensures high amplifier efficiency and good harmonic suppression. The quiescent current of transistors VT3, VT4 is set by selecting the resistor R14. It should be 150 ... 200 mA (for each transistor). The frequency response of the cascade is formed by the elements R10-R13, C10, C11. Capacitors C10, C11 affect Kus on low-frequency ranges, and resistors R10-R13 - on high-frequency. The capacitance of the capacitor C15 determines the rise in the frequency response in the frequency band 28 ... 30 MHz. Sometimes it is useful to include a capacitor with a capacity of 750 ... 1500 pF in parallel with the secondary winding of the transformer. This will also help raise the frequency response at frequencies above 24 MHz. In this case, the Kuss of the cascade should be controlled at 10 ... 14 MHz, so that there is no “blockage” of the characteristic here. It is necessary to check the correct selection of these elements at operating power, since at low powers the "impedances" are not the same as in the "cruising" mode.

The design of the T3 transformer fundamentally affects the quality of the amplifier. The magnetic circuit is an annular one made of ferrite grade 100NN-4, size K16x8x6 mm. The winding with a tap has 6 turns of 16 PEV-2 0,31 mm wires twisted together, divided into two groups of 8 wires. The withdrawal is made from the connection point of the end of the first group with the beginning of the second. The other winding is 1 turn of wire MGSHV-0,35 mm, 10 cm long. The output transformer T4 is a "binocular" of 2 columns of 7 ring magnetic cores of their ferrite grade 400NN-4, size K16x8x6 mm each. Primary winding - 1 turn of braid from a coaxial cable, secondary - 2 turns of 10 MPO-0,2 wires connected in parallel. The secondary winding is located inside the primary. Experiments with various design options for this transformer showed its performance with ferrites with a permeability of 400-1000 with ring diameters from 12 to 18 mm. The secondary winding can also be wound in one wire, for example, MGTF - 0,8 ... 1 mm. Do not just forget that the transformer noticeably heats up during operation and, accordingly, the insulation of the wires must be heat-resistant.

The ohmic resistance of the inductors L4, L5 must be minimal so that they do not have auto-bias. Here you can use, for example, DM-1,2 with an inductance of 8 ... 15 μH. Transistor VT5 (bias voltage stabilizer for output transistors) is fixed through a mica gasket on a common heat sink with them. Diodes VD3 and VD4 must be in thermal contact with one of the output transistors. Relay K1 type RES34 (passport RS4. 524. 372), although RES10, reliably serve for several years. The relay housing should be connected to a common wire.

To the output of the transformer T4 is connected "fool protection" - two-watt resistors R23, R24 with a total resistance of 470 ... 510 ohms. From the point of their connection, the RF voltage is removed for the output power indicator (detector on VD5) and the ALC system. In the event of a failure of the K1 relay, the low-pass filter board relay, or an open antenna, all the power will be dissipated by these resistors, and the SWR will be 10. This is not so bad, since the ALC system will work out and reduce the output power. If the ALC also fails, then the "fool protection" will work: the "spirit of burnt paint" will come from these resistors. Transistors can easily withstand such an execution. For power up to 100 W, the manufacturer guarantees "the degree of load mismatch (at Pout = 70 W) for 1 s 30:1". In our case, it will be 10:1, so we can work on the transfer for three seconds and think: "What does it smell like?".

A two-section low-pass filter (L7L8C21C23C25) with a cutoff frequency of 32 MHz is soldered directly on the amplifier board.

Power (+24 V) to the amplifier is constantly supplied from the moment the transceiver is turned on, and when switching to the transmit mode, the control voltage of +12 V is applied to the +TX bus.

The adjustment of the amplifier is carried out in the following sequence. After setting the quiescent currents of transistors VT1 - VT4, we unsolder the output of the capacitor C5 from the VT2 base circuits and connect it through a 10 ... 20 Ohm (1 W) resistor to a common wire. Having applied a signal from the GSS to the input of the silo with a frequency of 29 MHz, we select the capacitor C4, equalizing the frequency response at this frequency. Having restored the connection C5, VT2, we load the transformer T4 with a non-inductive resistor 50 ... 60 Ohm (25 W) with leads of the minimum length. By setting the input signal level to 0,2..0,3 V (rms), we measure the current consumption of transistors VT3, VT4 and the RF voltage at the load. By swapping the conclusions of the primary winding of the transformer T3, we determine their optimal connection - by the maximum voltage at the load. By increasing the input signal level to 0,5 V (rms), we measure Ipot and Pout. By selecting the capacitor C15, we achieve the highest power at the output of the amplifier at a frequency of 29 MHz (470 ... 2200 pF, depending on the permeability of the magnetic circuit of the transformer T3).

Without changing the signal level at the input, we measure Pout and Iout at frequencies of 14, 7 and 1,8 MHz. The measurement results are recorded. According to the maximum output power at the minimum current consumption, we sequentially select the number of turns of the primary winding, first of the T2 transformer (no more than 5 turns), and then of the T3 transformer (2-3 turns). At the same time, we compare the data on the output power at frequencies of 29, 14 and 1,8 MHz.

Since the output of bandpass filters rarely produces the same signal levels for all ranges, then it is necessary to form the final frequency response by selecting resistors R6, R10-R13 and capacitors C10, C11 with a real exciter (in the transceiver), and not with GSS. 57.

The preamplifier (Fig. 2) is assembled on a separate board along with band pass filters (BPF) and a receiver attenuator (ATT). Transistor VT1 (it is possible to replace it with transistors of types KT325, KT355 with any letter index) operates in linear mode. The gain of the cascade is about 10. The load is a broadband transformer T1, made on an annular magnetic circuit made of ferrite grade 600HH, size K10x6x5 mm. The windings contain 8 turns of 0,2 mm PEV wire. The quiescent current of the transistor (20 mA) is set by selecting the resistor R4. The amplitude-frequency characteristic of the cascade is formed by the elements R7, C4.

The key on the transistor VT2 controls the relay K3, which connects the input of the PA line to the DFT in the transmission mode. Bandpass filters - two-circuit. For inductors, frames with a diameter of 8 mm from TVs were used. This, of course, is not the best option, but the DFT copes well with the task of selection by mirror and side channels.

The transceiver has three stages of protection for the output stage of the power amplifier in case of overloads. On fig. Figure 3 shows ALC (Automatic Signal Level Control) and high SWR protection.

These protection circuits operate through a DSB amplifier based on a double-gate field-effect transistor. The voltage at the second gate of this transistor determines the Kus cascade and, accordingly, the output power of the entire line of the output cascade. The signal from the VD5 detector (see Fig. 1 in the first part of the article) and the signal from the SWR meter (Fig. 3) through the isolation diodes VD2, VD3 are fed to the transistor switch (VT1, VT2). The output of the emitter of the transistor VT2 through a variable resistor (output power regulator) with a resistance of 4,7 ... 10 kOhm is connected to a common wire. The moving contact of this resistor is connected to the second gate of the DSB amplifier. If the load is not connected to the output stage (for example, the relay of the low-pass filter unit is out of order), the RF voltage at the output T4 increases. It is rectified by the VD5 diode and closes the transistor switch VT1, VT2. The voltage at the second gate of the DSB amplifier and, accordingly, the buildup of the output stage are reduced. The same thing happens when the SWR exceeds the permissible level, with the only difference being that the diode VD1 of the SWR meter serves as a rectifier.

Having loaded the output stage on the equivalent of the antenna, trimming resistors R2 and R3 set the levels of operation of the protection system. With an output power of 100 W, the KT956A pair can withstand SWR up to 5 or more. You can limit yourself to SWR = 3 ... 4, at which the protection system is already starting to work. To do this, instead of an equivalent, you should connect a load with approximate values ​​​​of 20 or 150 Ohms and set the protection operation level with resistors R2 and R3. The overall gain of the PA line can be limited by the selection of resistor R5. When using transistors of the KPZ50 or KP306 type in the DSB amplifier, the voltage on the second gate should be set to no more than +5 ... 7 V. Capacitors C7 and C9 ensure smooth operation of the ALC system. If their capacitances are too small, the signal is distorted, a sharp limitation occurs, which is unpleasant to the ear, if the capacitances are large, the system reacts with a delay to changes in the load of the output stage, and the whole meaning of this protection is lost. By controlling the signal quality with an additional receiver, you can achieve a good signal by adjusting the ALC depth and its response time by selecting R3, R2, C7, C9. The transformer of the SWR meter T1 is wound on an annular ferrite magnetic circuit of the M50VCh-2 brand, size K12x6x4 mm. The secondary winding has 28 turns of PELSHO wire 0,2 mm. The primary winding is a coaxial cable threaded through the transformer ring and connecting the low-pass filter to the antenna connector of the transceiver.

The third stage of amplifier protection is the limitation of the current consumed from the +24 V power source. With an output power of the amplifier up to 100 W, the stabilizer protection operation current is set at 8,5 ... 9 A.

A few words about ferrite magnetic circuits sold on radio markets. When buying, never say what kind of permeability you need. It is better to ask which one is, since the seller always has a "duty box" at hand, where there is exactly the permeability that you name. With a high degree of risk, but still it is possible to distinguish ferrite in appearance, which has a high permeability. It usually has a darker color ("baked coal"), a larger grain, and it "rings" with a tester (HM brand). Ferrites of small permeability are gray in color, sometimes with a coating of "rust", very fine grain and are not "ringed" by the tester. In the amateur radio environment, there are various rumors about the use of ferrites of the NN and NM brands. I have not been able to find any difference in the performance of these ferrites, at least not in the amp design being replicated. But in military equipment, especially in transistor transmitters, ferrites of the NM brand can be found more often. This information is non-binding. Perhaps someone will want to conduct a detailed study in this direction and in the future share the findings with the amateur radio fraternity.

Author: Alexander Tarasov (UT2FW), Reni, Ukraine

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