IF Amplifier Circuit Design Example

Here is an example of a real life IF amplifier circuit using transistors and slug-tuned IF transformers. This is the first of the three IF stages in a vintage Zenith Transoceanic Radio and as you can see, to the uninitiated, it can get complicated straightaway.

In this IF amplifier design, they use the same IF stages for both AM and FM bands, thereby minimising the number of transistors required. To achieve this, they use a single transistor to drive two sets of IF transformer coils, and in the circuit diagram above you can see that T3 has a bandpass of 10.7-megacycles, hence this is tuned for the FM band. However, T4 and T5 are for 455-kilocycle reception, which is for the AM band.

T3 has a 40-pF capacitor in parallel with coil L28 thus forming a parallel tuned circuit for 10.7 MHz bandpass. T4 has a 1000 pF capacitor in parallel with coil L30 forming a parallel tuned circuit for 455 kHz bandpass.

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