Icom IC-F50V Service Manual page 11

TRANSMIT AMPLIFIERS
The transmit amplifiers consist several RF amplifiers (pre-
driver, driver, power), and amplify the TX VCO output to the
transmit output level.
The TX VCO output is applied to the pre-drive amplifi er via
the TX/RX switch. The amplified TX signal is amplified by
drive amplifier, then power-amplified by the power amplifier
to obtain 5 W (max.) of TX output power.
The power-amplified TX signal is passed through the LPFs
(as a harmonic filter), ANT SW (TX), and another LPF, then
applied to the antenna.
• TRANSMIT AMPLIFIERS AND APC CIRCUIT
Pre-
TX/RX
DRIVE
SW
D14
00 C93
TX
D15
R
LPF
L14
C49
L2 0
C86
DRIVE
AMP
Q7
Q8
C92
Q9
R9
APC CIRCUIT
APC CIRCUIT
The APC (Automatic Power Control) circuit stabilizes
transmit output power to prevent transmit output power level
change which is caused by load mismatching or heat effect,
etc. The APC circuit also selects transmit output power from
high, middle and low power.
A portion of the TX signal is rectifi ed by the power detector
to be converted into DC voltage which is in proportion to the
transmit output power level. The detected voltage is applied
to the comparator. The transmit power setting voltage is ap-
plied to another input terminal as the reference voltage.
The comparator compares the detected voltage and refer-
ence voltage, and the difference of voltage is output. The
output voltage controls the bias of the TX amplifiers to re-
duce/increase the gain of these amplifi ers for stable transmit
output power.
The change of transmit power is carried out by changing
reference voltage.
PWR
LPF LPF
AMP
L1 6 L4 L3
C10
C302
TX POWER
DETECTOR
R2
C79
R8 D25
COMPARATOR
IC2
5 R5
3
4
1
2
4 - 5
To ANT
LPF
D2
C6 L1
C7
L2
C2
C4
ANT SW
(TX)