Johnson DM3474, Service Manual

Page: 35 / 64

CIRCUIT DESCRIPTION
4-11
Part No. 001-3474-002
4.4.2 PRE-DRIVER (Q501), DRIVER (Q521)
Pre-driver Q501 is biased class A by R501 and R502 and R506. L501 and C502 match Q501 to Q851. C520
and C508 bypass RF from the DC line, and R503 provides supply voltage isolation. R507 ties the +7.5V supply to
the circuit for high power applications and R508 ties the circuit to +5V for low power applications. Impedance
matching between Q501 and Q521 is provided by L502, L503 and C511. R504 and C504 provide negative
feedback to prevent
oscillation.
Driver Q521 is biased nearly Class C by R521 and R522. Impedance matching with Q541 is provided by
L521, C525, C527, L522 and C526.
4.4.3 -5V POWER CONTROL SUPPLY
The 17.5 MHz from the TCXO is coupled through C902 to Q901. Bias for Q901 is provided by R903, R904,
R901, R902 and R905. C901 and C903 provide RF decoupling. The amplified signal rectified by CR901/CR902 to
produce a -5V DC source. C909 stabilizes the voltage level and C910 and C911 provide RF decoupling. This -5V
source is used in the transmit power control circuit U542.
4.4.4 FINAL (Q541), POWER CONTROL (U542)
Q541 is biased for Class C operation. The output is matched to the low-pass filter by L541, C552, and several
capacitors. The supply voltage is isolated from RF by ferrite bead EP541.
Power control is provided by U542. The 5.5V transmit supply is passed by U542 to power adjust R542. The
other end of R542 is the rectified -5V from Q901. This negative voltage is required when low power is used to
pinch off Q541 to the required output.
The low-pass filter consists of L561, C561, L562, C562, L563, C563 and L564. The filter attenuates spurious
frequencies occurring above the transmit frequency band. The transmit signal is then fed through the antenna
switch to antenna jack J501.
4.4.5 ANTENNA SWITCH (CR561, CR562)
The antenna switching circuit switches the antenna to the receiver in the receive mode and the transmitter in
the transmit mode. In the transmit mode, +7.5V is applied to L565 and current flows through diode CR561, L566,
diode CR562, and R562/R563. When a diode is forward biased, it presents a low impedance to the RF signal;
conversely, when it is reverse biased (or not conducting), it presents a high impedance (small capacitance).
Therefore, when CR561 is forward biased, the transmit signal has a low-impedance path to the antenna through
coupling capacitor C568.