Kenwood TK-250 Service Manual page 19

TK-250
CIRCUIT DESCRIPTION
4.3 Driver and final power amplifier
The transmit signal is directly generated by the VCO. The
transmit signal obtained from the VCO buffer amplifier 012,
is amplified by 015, to approximately 2dBm. This amplified
signal is passed to the power module(IC5). The power mod-
ule consist of a 3-stage amplifier and is capable of prod~c-
ing up to 4.8W .
4.4 ANT switching
The power module output signal is passed through a 3-
stage low-pass filter network and a transmit/receive switch-
ing circuit before it is passed to the anten na terminal. The
transmit/receive switching circuit is comprised of 08 and 09.
08 and 09 are turned on (conductive) in transmit mode and
turned off (isolated) in receive mode.
4.5 Automatic power control and transmitter output
level switch
The automatic power control (APC) circuit stabilizes the
transmitter output power at a pre-determined level by sens-
ing the collector current of the final amplifier transistor in the
power module. The voltage comparator (IC4) compares the
voltage obtained by the above collector current with a refer-
en ce voltage, set using the microprocessor and 018. An
APC voltage proportional to the difference between the
sensed voltage and the reference voltage appears at the
output of IC4. This output voltage controls 016, which in
turn controls the voltage at pin 2 of the power module, which
keeps the transmitter output power constant. The transmit-
ter output power can be varied to 1W or 2W by the micro-
processor, which in turn changes the reference voltage of
IC4, and hence the output power.
CI07 Cl!!
+
o o
c::;
I A62
I
CI06
I
C112
ANT
C123
IRF
swi
08
I
LPFI
CI04 C!08 C113
LIl
I
L13
I
L!4
A57
B
5T
IC5
I
RF POWER AMP
I
<D
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(J
AMP.
I
Q16
R53
A5!
loc swi
Q19
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L17
(Drive and final power amplifier, ANT switching, APC and transmitter
output level switch)
Fig. 5 Transmit power system
19