Simple IF Amplifier
I wanted to install two Murata 110-kHz ceramic filters in place of the single 150-kHz filter a Kenwood KT-880D used in narrow IF mode. 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 is 1.5 dB greater for the amplified
110s than for the unamplified 150 they replace. Before modification, wide and
narrow IF gains were identical, as were the muting and signal-meter
thresholds. After modification the thresholds differ by just 1.5 dB. Current
drain is 15 ma; you may see 12-30 ma. The KT-880D provides 13 volts 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 with the 110s.
More is here.
Updated February 12, 2008
