AD603 Data Sheet Circuit - Figure 49, page 18


I am making an amateur radio transceiver.  I have based the IF strip of the receiver section of the transceiver on the circuit shown in the data sheet page 18 (figure 49).  My circuit is as shown in this figure except that instead of connecting pins 5 and 7 with a 2.5 k resistor, I used a wire link so as to decrease the gain of each AD603.  The amplifier section of this IF strip works very well and I am presently using it in a working prototype receiver.

I have a question regarding the AGC circuit shown in the data sheet, however.  I have been unable to get the AGC circuit shown in figure 49 to work.  I think what is happening is that Q2, a 2N3906, is biased so that it is saturated whereas transistor Q1, a 2N3904, conducts only on negative peaks of the amplifier output (The emitter voltage of Q1 averages at about 5 V, very nearly the same as the base voltage).  Consequently a constant voltage of about 9.5 volts is maintained on the capacitor marked Cav.  Since this voltage is the AGC voltage it is much higher than it should be for proper operation of the AD603 ICs in sequential gain mode.  Based on the data sheet information, its maximum value in this circuit should be 7 volts, I think.

I have made a different AGC circuit that works with my IF strip but I would still like to be able to try different AGC circuits for evaluation and, if possible, I would like to get a circuit like that shown in the data sheet (with suitable modifications) working.  I would certainly appreciate any suggestions as to how to modify the figure 49 AGC circuitry.

Thank you,

Allen Wootton 

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