X-2 Technical Service Manual Technical Description 4-17
Publication No: 15-02047
When PTT is selected, the microcontroller sends serial data to IC1 to select
pin 12 to ground. This energises the transmit/receive changeover relay K7
and forward biases DC switch V1
[D3]
providing DC to the input and predriver
stages. The control circuit to the output bias circuit is enabled and the input
DC is supplied to the +5 volt regulator IC2.
4.6.1 Gain Control Stage
104-02908
The RF input from the Exciter is terminated by R10
[G2]
and drives the
common base long tail pair V8
[F6]
and V9 through R29 and R34 in parallel
for signal currents. R30 sets the DC condition of the long tail pair and is RF
bypassed by C77
[G6]
.
The gain of V9 is controlled by the ratio of emitter DC currents in V8 and
V9. The RF input is split between the emitters of V8 and V9 inversely
proportional to their input impedance. The gain of V9 reduces when the
ALC increases the current in V8. The collector load of V9, consists of L19
and R33 which are frequency compensating components to reduce low
frequency gain.
4.6.2 Predriver Stages
The collector output of V9 is coupled, via C76, to emitter follower V10
where the output drives the transformer feedback predriver stage V11. High
frequency peaking in the emitter circuit is provided by L20 and C79.
4.6.3 Driver Stage
The push-pull class B driver stage, V13 and V14, is voltage driven from the
secondary of T2
[E8]
. The transformer output of T3
[E10]
provides the current
drive to the output stage and R49, L24 and C85 provide the low impedance
source required below 3 MHz.
Bias for the driver stage is provided by the total current of the emitter
follower and predriver stages passing through transistor V12 connected as a
diode. The bias is set by SOT resistor R39 and changes the voltage across
the collector-emitter of V12
[G8]
.
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