Icom IC-22S PLL Synthesized 2-Meter Transceiver
Instruction Manual and Service Notes
10
Interstage Amplifier
BPF output is amplified up to about 2 mW in the Q22 stage that is a MOSEFT with good linearity.
Low Level Amplifier
Interstage amplifier output is amplified to about 100 mW by stage Q19, which also functions as an
ALC circuit.
Driver stage
Low level output is amplified to approximately 1.6 W by Q18.
ALC Circuit
Spurious signals, which might occur when voltage is reduced or when the degree of excitation is
low, are prevented by a small amount of forward bias applied by bias circuit D14.
Part of this output is brought up to excitation level in the threshold type voltage-doubling rectifying
circuit constituted by D19, D29. When AI..C is not applied, the self-bias of Q5 causes Q16 to
conduct. When ALC is applied, both 015 and Q16 are close to cut-off, collector voltage of Q19 falls,
and the excitation level is lowered.
ALC is effective for both high power and low modes. For high power, the threshold level is
controlled by R73, and for low power, the threshold is set by R149.
Power Amplification
Output is amplified by Q7 to give an output of 10 watts, including low pass filter losses. Since
power handled in this stage is particularly high, use is make of an aluminum die-cast radiator
which is in direct contact with the rear chassis and serves to keep the transistor temperature low in
order to insure reliability. A padding mica trimmer which has an excellent temperature
characteristic and causes little induction loss is also employed.
Low pass filter and SWR detector
Power amplifier output includes harmonic components and in passing through 2 Chebyshev section
and one standard section in order to get to the ANT terminal, harmonic components are attenuated
by about 70 dB. Cut-off frequency of this low-pass filter is set to about 180MHz, and so there is
very little loss in the 146-148 MHz band.
SWR
D10, D11 constitute an SWR detector. The closer it is to the ANT terminal the better the detector
functions, but because of diode rectification, harmonics are produced. An excellent compromise is
therefore made by inserting the detector between the low-pass filters. Standing waves that pass
through the SWR detector are rectified by D11 and supplied to the RF meter. Indication of the RF
meter for proper high power output is set to 4/5 of full-scale by R156.
APC circuit
Reflected waves are rectified by D10 and then amplified by Q20, 021 up to the level set by 89. By
raising Q22 source voltage, excitation level is lowered, and damage to the power amplifier
transistor due to mismatch is prevented.
