Simple IF Amplifier

I wanted to replace the 150-kHz ceramic IF filter a Kenwood KT-880D used in narrow-IF mode with two Murata 110-kHz filters. I needed an amplifier to compensate for the additional loss and to isolate and properly terminate the filters.

Here's the circuit I came up with. Gain was 1.5 dB greater for the amplified 110s than for the unamplified 150 they replaced. Before modification, wide and narrow IF gains were identical, as were the muting and signal-meter thresholds. After modification the thresholds differed by just 1.5 dB. Current drain was 15 mA; you may see 1230 mA. The KT-880D provided 13 V to power the circuit.

At first I was leery of using a JFET because I was afraid its input capacitance might be too high for F1. But Fairchild J309 curves and a little calculation yield 15 pF for the amplifier under measured operating conditions. This is close enough to the 10-pF load Murata specifies. Capacitive loading effects on a 230-kHz filter are shown below.

Adjacent-channel selectivity improved from 17 dB for the single 150 to 40.5 dB for the 110s.


March 23, 201088108 MHz