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have a switchable gain input amplifier, and it has an extra stage of crystal filtering. Only
the 10 dB Gain Input Buffer Amplifier will be described here. For descriptions of the other
circuits in A4A4, refer to descriptions of the corresponding circuits in A4A8.
Input Buffer Amplifier @
The 10 dB Input Buffer Amplifier (Figure 1) functions as a noninverting op amp.
Figure 1. A4A4 Input Buffer Amplifier, Equivalent Circuit
In the crystal mode (30 kHz, 10 kHz, and 3 kHz bandwidths), the amplifier includes Q7. The
biasing of the amplifier is independent of its RF (21.4 MHz) operation but is very critical for
its proper functioning. If a malfunction occurs, the dc bias should be checked first.
The current through Q14 is determined by the difference between two current sources, one
involving Q7 and the other involving QS. (See Figure 2.) The most convenient way to find
the current from each source is to measure the voltage across each emitter resistor (A 1 kS1
resistor should be used in series with the voltmeter probe tip to prevent the circuit from
oscillating and giving an erroneous reading). For Q7, the current through R90 must be
included. If results are inconsistent, the emitter resistor should be checked also. Check to see
that the BW5 line is at the voltage specified on the schematic.
In the LC mode (the four wider bandwidths), the BW5 line goes to approximately +14.8 V
and turns off the current source Q7. The current supplied by Q7 in the crystal mode is now
supplied through CR1 and R12 from the BW5 line. In the LC mode, the current through Q14
can be found by subtracting the current through R12 from the current through R4.
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