CIRCUIT DESCRIPTION
4-3
December 1997
Part No. 001-7500-001
Figure 4-2 APC Circuit
The audio signal changes the reactance of diode
D6 (VHF) or D404 (UHF) to modulate the VCO sig-
nal at Q7 and Q8. The VCO signal is amplified by
buffer-amplifiers Q4 and Q6 and then applied to T/R
switching circuit D3 and D4.
4.2.3 DRIVE/POWER AMPLIFIER CIRCUITS
The signal from the VCO circuit passes through
T/R switch D3 and is amplified by buffer Q3 (VHF)
or buffers Q3 and Q403 (UHF), driver Q2, and power
amplifier Q1. Typical output power is 5 watts VHF or
4 watts UHF with a supply voltage of 9.6 volts DC.
The amplified signal passes through antenna switch
D1 and a low-pass filter and is applied to the antenna
jack. The bias current of driver Q2 and power ampli-
fier Q1 is controlled by the APC circuit (see Section
4.2.6).
4.2.4 CURRENT DETECTOR CIRCUIT
The current detector circuit consisting of Q9 and
Q28 detects the total current of the driver and power
amplifier stages using current sensor R161. Differen-
tial amplifier Q9 detects the voltage difference of the
current sensor input and output voltages then outputs
control voltage to the APC circuit (VHF only) and the
CPU (IC8, pin 97).
4.2.5 POWER DETECTOR (UHF ONLY)
Power detector D2 detects the transmit power
output level and converts it to a DC voltage. The
detected signal is applied to the APC circuit (see fol-
lowing).
4.2.6 APC CIRCUIT
The APC (Automatic Power Control) circuit con-
sisting of IC3A and Q37 protects the driver and power
amplifier stages from excessive drive current and
selects high and low power output. A diagram of this
circuit is shown in Figure 4-2.
The signal output from the current detector cir-
cuit (VHF) or the power detector circuit (UHF) is
applied to differential amplifier IC3A on pin 2. The
“T4” signal from expander IC10 on pin 14 is con-
trolled by CPU IC8 and applied to the other input as a
reference.
When driving current increases, input voltage on
pin 2 of the differential amplifier also increases. The
differential amplifier output voltage on pin 1 then
decreases to reduce driving current.