Cascode broadband power amplifier. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / RF power amplifiers
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When creating transistorized KB transmitters, radio amateurs face difficulties in designing band power amplifiers. If you use conventional resonant amplifiers, to obtain an output power of 1-1,5 W (with an input of 0,3-0,6 mW), you will need several stages that need to be switched. The use of cascode broadband amplifiers with a choke load on multi-emitter transistors makes it possible to significantly simplify the amplifier circuit and makes it possible to obtain a high gain.
On fig. 1 shows a diagram of such an amplifier. It has a power gain of up to 16 dB in the frequency range from 3 to 35 MHz with a non-uniformity of no more than 2,5 dB. The output power was determined by an M3-3A power meter with an input impedance of 75 ohms.
The amplifier operates in class A mode. The initial current (80-90 mA) is set by selecting resistors R2. R3. At the connection point of the collector of transistor V1 with the emitter of transistor V2, the voltage should be from half to two thirds of the supply voltage Uк. Thermal stabilization is carried out using the chain R4, C5. If the amplifier will be operated at a constant temperature (indoor operation), then its use is not necessary. In the absence of a chain, the output power increases slightly.
The amplifier can also work in class AB, for which the initial current is reduced to 2-3 mA by selecting resistors R20, R30, but the power gain decreases.
It should also be noted that this amplifier can operate at frequencies up to 300 MHz, but already at 250 MHz its gain drops to 10 dB.
The amplifier used a factory inductor L1 type D-0,12 with an inductance of 43 μH, but it can be replaced with a home-made one. In the case of use at high frequencies (above 100 MHz), the choke can be wound on MLT-0,5 resistors with a resistance of more than 1 kOhm with a PEV-2 wire 0,2 turn to turn until filled.
The best results can be obtained when using transformers with a short-circuited volume turn as a load. The diagram of switching on transformers is shown in fig. 2, drawing of transformers - in fig. 3. The connection between their magnetic fluxes is carried out by a short-circuited volumetric coil. made in the form of a rod soldered to the covers of a brass cylinder.
Each of the windings is wound on a K10x6x3 core made of 1000NM-9 ferrite. The winding of the transformer T1 contains 12 turns of wire PEV-2 0,31, the transformer T2 - 5-7 turns of wire PEV-2 0,64. The transformer housing is connected to a common wire. The dependence of the output power on the input for the second amplifier is shown in fig. 4, and the dependence of the maximum output power on the magnitude of the collector voltage at a constant input power - in Fig.5. Good results have been obtained by amplifying a single-sideband signal. It should be noted that when using more powerful transistors, for example, KT904, the amplifier at output power (Pout> 2 W) is prone to self-excitation, however, when the gain is reduced, it works quite stably.
Authors: A. Wenger, V. Yashchenko (UB5LBZ), Kharkov; Publication: N. Bolshakov, rf.atnn.ru
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