5 - 3
5-2-3 TRANSMIT AMPLIFIERS (PA UNIT)
The transmit signal from the VCO is amplifi ed to the transmit
output level by the transmit amplifi ers.
The transmit signal from the TX/RX switches (MAIN UNIT; D16
is ON, D17 is OFF) is amplified by two pre-drive amplifiers
(Q704, Q703), drive amplifier (Q702) and power amplifier
(Q701) in sequence to obtain 0.5 W (approx.) of transmit
output power.
The power-amplified transmit signal is passed through the
antenna switch (D701), then applied to the antenna via the
LPF (ANT UNIT; L801, L802, C802–C806).
5-2-4 APC CIRCUIT (PA UNIT)
The APC (Automatic Power Control) circuit prevents the
transition of the transmit output power level which is caused by
load mismatching or heat effect, etc.
A portion of transmit signal is detected by the transmit power
detectors (D702, D703) to produce DC voltage corresponding
to the transmit output power level. The detected voltage is
applied to the APC amplifi er (IC701, pin 3). The transmit power
setting voltage “T1” from the D/A converter (MAIN UNIT;
IC23, pin 1) is applied to another input terminal (pin 1) as the
reference voltage.
The APC amplifier compares the detected voltage and
reference voltage, and the difference of the voltage is output
from pin 4. The voltage controls the bias of the drive (Q702)
and power (Q701) amplifiers to reduce/increase the gain of
these amplifi ers for stable transmit output power.
The transmit power muting is carried out by the TX mute switch
(MAIN UNIT; Q46), using the “TMUT” signal from the CPU
(MAIN UNIT; IC22, pin 35).
5-2-5 OVER CURRENT DETECTION CIRCUIT
(PA UNIT)
The driving current of the drive (Q702) and power (Q701)
amplifiers is detected at the current detector (Q705, Q706)
by detecting the difference of voltage between both terminals
of R729. The detected voltage “ISENS” is applied to the CPU
(MAIN UNIT; IC22, pin 47).
In case of the over current is detected, the CPU outputs “TMUT”
signal from pin 35 to TX mute switch (MAIN UNIT; Q46) to
stop the transmitting to protect the transmit amplifi ers (Q701–
Q704).
Power
amp.
APC
amp.
Drive
amp.
+
1
3
4
–
VCC
MAIN UNIT PA UNIT
to the
antenna
T1
TMUT
Transmit signal
from the TX/RX
switches
(D16, D17)
T5V
Q702
Q703 Q704
APC
P_DET
IC701
Q701
to the receiver circuits
LPF
LPF
ANT
SW
D703
D702
D701
TX power
detectors
Q46
Pre-drive
amp.
Pre-drive
amp.
ANT UNIT
Current
detector
ISENS
Q705, Q706
• APC CIRCUIT
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