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Hybrid linear power amplifier. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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In shortwave transceivers, the transmission path usually contains a powerful final amplifier based on an electric vacuum tube and a preamplifier based on transistors. At the same time, resonant circuits are used to match the pre-amplifier with the final amplifier. Similar circuits are also included between the pre-amplifier and the last mixer of the transmitting path.

Such a construction of the transmitting path of the transceiver cannot be considered optimal. The use of two switchable resonant circuits at the input and output of the preamplifier complicates the device. In addition, the inclusion of the collector of a powerful transistor in the circuit of the resonant circuit can lead to the appearance of nonlinear distortions due to the large nonlinearity of the capacitance of the collector junction of the transistor.

The figure shows a diagram of a hybrid power amplifier, in the output stage of which a cascode connection of a VT4 bipolar transistor connected according to a common emitter circuit and a VL1 lamp connected according to a common grid circuit is used.

Hybrid linear power amplifier. Scheme
(click to enlarge)

Such a construction not only allowed the low output impedance of a powerful transistor to be well matched to the lamp input, but also ensured the exceptional linearity of the amplitude-frequency characteristic of the cascade.

Another important advantage is that three electrodes turned out to be "grounded" in the lamp - the first and second grids and beam-forming plates. The through capacitance of the lamp became negligible, as a result of which there was no need to neutralize it.

To increase the input resistance of the terminal stage, an emitter follower on a VT3 transistor is included at its input. Since the emitter of this transistor is directly connected to the base of the VT4 transistor, the quiescent current of the output stage can be controlled by a trimmer resistor R20 included in the VT3 base circuit.

To increase the linearity and temperature stability of the amplifier, the cascode stage is covered by serial negative feedback through two resistors R23 and R25 connected in parallel. With a quiescent current of 25 mA, an anode voltage of 600 V and a signal power at the input of the emitter follower of 8 ... 10 mW, the amplifier outputs at least 130 W of power on all HF bands. In this case, the constant component of the anode current is 330 mA. Intermodulation distortion of the third and fifth order at an output power of 140 W does not exceed -37 dB.

The amplifier provides protection for the VT4 transistor from breakdown in case of lamp malfunctions, as well as during transients when it is heated. To do this, the collector of the transistor VT4 through the diodes VD2, VD3 is connected to the zener diode VD4 with a stabilization voltage of 50 V. During normal operation of the amplifier, the diodes VD2, VD3 are closed, since the voltage on the VT4 collector does not exceed 35 V. If for any reason the instantaneous voltage on the collector exceeds 50 V, the diodes VD2, VD3 will open and it will be shunted by the low differential resistance of the zener diode VD4.

The input impedance of the cascode stage (from the input of the emitter follower) is practically active, depends little on the frequency and is close to 400 ohms. To get an output power of 130 W, it is enough to have a 1,8 V RF signal at the input of the emitter follower. Such a level may well be provided by a transistor mixer. (If in the transceiver the last mixer of the transmitting path is made on diodes, then the power of the RF signal at the output of the mixer does not, as a rule, exceed 0,05 ... 0,1 mW).

To increase the gain at the input of the emitter follower, a two-stage broadband amplifier based on transistors VT1 and VT2 is included. The input impedance of the amplifier is about 200 ohms, which is in good agreement with the output impedance of conventional diode mixers. The gain in the frequency range 1...30 MHz is almost constant and equal to 26 dB. To obtain an output power of 130 W, it is enough to apply a signal with a power of 0,05 mW to the input of the pre-amplifier, i.e., the amplifier can be turned on directly at the output of the diode mixer of the HF transceiver's transmitting path.

When there is no RF signal at the input, the amplifier draws a current of about 40 mA from a +15 V source and 25 mA from a +600 V source. Therefore, it is beneficial to "close" the amplifier in receive mode. For this purpose, the outputs of the inverters D1 -DD3 are connected to the power circuits of the bases of the three transistors VT1.1-VT1.3. In the receive mode, logic 1 is applied to their inputs. In this case, the potential at the outputs of the inverters is lower than the opening voltage of the silicon transistors, as a result of which all stages of the amplifier are closed. In transmit mode, the inputs of the inverters are logic low. The potential at the outputs of the elements DD1.1-DD1.3 becomes high, and the amplifier opens.

The equivalent resistance of the output stage of the amplifier is about 900 ohms. The calculated values ​​of the reactive elements of the P-loop for matching the amplifier with the antenna are given in the table.

The value of the elements of the P-loop
Operating frequency, MHz Capacitance of the first capacitor, pF Inductance, uH Capacitance of the second capacitor, pF
Rн=50 Ohm Rн=75 Ohm Rн=50 Ohm Rн=75 Ohm Rн=50 Ohm Rн=75 Ohm
1,853,6
7,05
14,15
21,2
28,5
40332073
1058
527
352
262
34331764
901
449
300
223
2,81,4
0,73
0,36
0,24
0,18
2,81,4
0,73
0,36
0,24
0,18
137657074
3612
1800
1201
894
91774716
2408
1200
801
596

The passport value of the allowable power dissipation at the anode of the 6P45S lamp is 35 watts. In this amplifier, at an anode current of 330 mA, about 70 watts of power is dissipated at the anode of the lamp. However, this does not significantly reduce the reliability of the lamp, since the power dissipation reaches 70 W only at the peaks of the SSB signal envelope or during telegraph bursts. The average power dissipation usually does not exceed the allowable value.

Structurally, the 6P45S lamp and the elements of the matching P-circuit are placed in a shielded compartment, the conclusions from which are made by means of KTP feed-through capacitors. To improve lamp cooling, the top and bottom covers must be perforated. It should be noted that the lamp cools better when it is in a horizontal position. Transistors VT1 and VT3 are placed in close proximity to the lamp panel and mounted on the chassis so that good heat dissipation is ensured. The remaining elements of the amplifier can be placed on the printed circuit boards of the transceiver.

The inductor L6 is made on a cylindrical dielectric frame with a diameter of 14 mm and contains 270 turns of PEV 0,33 wire, wound round to round.

Inductor L7 contains 3 turns of PEV 0,11 wire placed on resistor R21. With proper installation, the amplifier does not require tuning, the only necessary adjustment is to set the quiescent current of the output stage with a tuning resistor R20.

Publication: cxem.net

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