5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS
5.1 POWER AMPLIFIER (MODULE 1) PART NO. 625641
The power amplifier has a gain of 50 dB ±3 dB.
The power input is approx. 0 dBm and the output is approx. 250W.
The power amplifier has three stages each working as a class B push-pull amplifier.
The pre-amplifier consists of two 2N3553 (Q03, Q04) transistors working in push-pull. The bias supply
for the pre-driver consists of Q01, D01 and R01, R02. R01, R02 divide the 15V down to 9.6V and Q01
is a current amplifier.
Q02 is used to switch off the bias and D01 produces the basis-emitter diode of Q01 from reverse break-
down when Q02 is in saturation.
The signal from the pre-driver is fed through the impedance matching transformer TR02 to the driver. The
two driver transistors MRF426 (Q06, Q07) are working as push-pull class B. There is a current feed-back
from TR03 through R26 to the basis of Q06, Q07. C10, R21, and C11, R22 form an input impedance
matching network.
The bias supply is built around U01/1 OP.Amp.
D02 is a temperature compensating diode and is thermal connected to Q06. When Q06 gets hotter the
voltage drop over D02 gets lower and thus the output of U01/1 and Q08 is lowered (Q08 is a current
amplifier for U01/1). The result is that the current through Q06, Q07 is stabilized or lowered.
The bias current through Q06, Q07 is adjusted by means of the potentiometer R14.
Q05 is used to switch off the bias of Q06, Q07.
The output from Q06, Q07 is fed through the impedance matching transformer TR04 to the output stage
which consists of two TH416 (Q09, Q10) transistors working in class B.
Current feed-back is taken from TR05 through R38, R39 to the basis of Q09, Q10.
C17, R31, R32, R33, and C18, R34, R35, R56 form an input impedance matching network for Q09, Q10.
The biasing network is built around U01/2 and is in principle equal to the circuit for the driver. D03 is
thermal connected to Q10 and the bias is adjusted by means of the potentiometer R30.
Q11 is used to switch off the bias to Q09, Q10.
TR06 is transforming the low collector impedance of Q09, Q10 up to 50 ohm.
The power output is approx. 250W PEP.
A temperature sensor R45 sensors the temperature of one of the output transistors. The sensor is
connected to the TX processor which takes care of reducing the power when the transistors get too hot.
PAGE 5-1
T2130
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