7MHz Class C SEPP power amplifier using 2SC2120/2SA950 complementary pair



This is an experimental results of 7MHz Class-C SEPP power amplifier using audio complementary pair 2SC2120/2SA950, held in 1991.

Both devices are for audio output stage of 1[W] output power, which have maximum rate of 600[mW] PC, 0.8[A] IC and 120[MHz] fT. The results has been reflected to my homebrew rigs such as 7T9521 “Komukai40” and obsoleted 7MHz DC transceiver.

The circuit under test was designed by the following procedure;

  1. Provide supply voltage VCC and output power PO.
  2. Calculate load resistor as
  3. Input impedance of the amplifier can be then calculate as
For example,

are obtained under VCC=12 [V], PO=1 [W], VCEsat=1 [V] and hfe=17(=120[MHz]/7[MHz]) condition. Then, the amplifier can be inserted to 50[Ω] line by connecting two 1:4 transformers at both input and output nodes.
The base bias is set to half of the supply voltage so that the bias resistors are given by 15 times of input impedance Rin.

Schematic of the circuit under test (CUT), input vs output response and harmonic response are shown in Fig.1 through 3.

Fig.1  Circuit under test.

Output characteristics
Fig.2 Input vs output response at 7[MHz]

Harmonics characteristics
Fig.3 Harmonic output with respect to input level.

Maximum power gain and collector efficiency at 1[W] output are 12[dB] and 60[%], respectively, and these values are suitable for 7MHz QRP transmitters.

Even order harmonics are well suppressed as same as most push-pull amplifier does.

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