Transmitter Circuits - Icom IC-A200 Service Manual

3-2 TRANSMITTER CIRCUITS

3-2-1 MIC AMPLIFIER CIRCUIT (MAIN UNIT)
The mic amplifier circuit amplifies the mic audio with the
limiter circuit to a level needed for the AM modulator.
Mic audio signals pass through the mic switch (Q4) and are
then applied to the limiter amplifier (IC13, pin 2). The mic
switch cuts out the mic audio signal while receiving.
IC13 has an ALC limiter circuit that controls the amplifier
gain to prevent signal distortion. A portion of the output
signals from IC13 (pin 3) is detected by D18 and its
detected voltages feed back to IC13 (pin 4) to control the
gain.
The output signals from IC13 (pin 3) pass through the active
low-pass filter (Q47) to cut out components higher than 2.5 kHz
and are then amplified at the mic amplifier (Q40). The signals
are then applied to the AM modulator (Q36, Q31).
3-2-2 MODULATOR CIRCUIT (MAIN UNIT)
The modulator circuit controls the collector voltage of the
transmitter linear amplifier to obtain amplitude modulation.
The modulator circuit consists of a current amplifier and a
linear amplifier. Amplified audio signals are applied to the
current amplifier (Q36) which controls the collector voltage
of the linear amplifier (Q31). The LO signal is amplified at
Q31 with variable voltage related to the mic input level so
that the LO signal level is varied as an AM modulation.
3-2-3 LOCAL SIGNAL AMPLIFIER CIRCUIT
(MAIN UNIT)
The LO signal (118.00–136.975 MHz) is obtained by mixing
the PLL output frequency (150.00–168.975 MHz) and
reference oscillator signal (32.00 MHz).
MIC AMP AND MODULATOR CIRCUITS
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The PLL output and the reference oscillator signals are
amplified at Q29 and Q20 respectively and are then applied
to the transmitter mixer circuit (IC15). The mixed signal
is passed through the transmitter bandpass filter to re
move the out-of-band signals. Q30 is a buffer amplifier to
compensate for bandpass filter attenuation.
The passed signal is modulated at Q31 and amplified at
Q44. The attenuator-type ALC control circuit (D33, D34,
R356) reduces the signal level when a mismatched load
occurs.
3-2-4 DRIVE AND POWER AMPLIFIERS (PA UNIT)
The signal from the MAIN unit is amplified at 3 amplifiers,
the pre-driver (Q5001), the driver (Q5003) and the final
amplifier (Q5005) in the PA unit to obtain the stable 7 W
output power.
The amplified signal passes thorough the matching circuit
(C34, C35, C59–C61), the ALC detector circuit (L13, D3–
D6), the antenna switching circuit (D7), and the low-pass
filter (L14–L16, C54–C57, C66). The signal is then applied
to the antenna connector.
3-2-5 ALC CIRCUIT (MAIN AND PA UNITS)
The RF ALC circuit protects the final amplifier (Q5005) from
a mismatched output load.
A portion of the output power from the final amplifier
(Q5005) is detected at the ALC detector (D5003–D5005)
on the PA unit. This detected voltage is set at a minimum
value when the output load is correct. However, when
mismatching occurs the detected voltage is increased.
The detected voltage is applied to the MAIN unit as an ALC
signal. The comparator (IC18) in the MAIN unit compares
the difference between the detected voltage (pin 5) and
reference voltage (pin 6). The comparator outputs "HIGH"
when mismatching occurs. The output current flows through
the attenuator-type ALC circuit (D33, D34, R356) to change
the attenuation ratio and to decrease the RF signal level.
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