IAN LANG ELECTRONICS
No 5.The Intermediate Frequency (IF) Amplifier.
The job of the IF amplifier is to take the still quite small output of the mixer and magnify (or amplify) it before demodulation. At this stage, although the signal has been converted into three separate frequencies, and we’ve filtered for one, it keeps the modulation that the signal frequency came in with. Demodulation works better when the signal is larger, and so we do that first. In doing this job we can also improve the selectivity of the receiver, that is the ability to capture one station on a certain frequency and exclude all others.
In AM Broadcasting there is a bandwidth of 9 kHz, and in the IF amplifier we can tune for this by using a filter. One such method is shown below in a drawing provided by the Radio Society of Great Britain:
The pass of the filter should be sharp but not too steep, allowing the IF (455 kHz) plus or minus 4.5 kHz to allow for the sidebands in AM in which all the information is carried. In our previous example we had a broadcast at 530 kHz and made the difference frequency 455 kHz by having the LO at 985 kHz. Now suppose we change to Absolute Radio at 1215 kHz. Because we have ganged the capacitors at the tank circuit and the LO, they track each other. Now the tank is resonant at 1215 kHz, and the LO oscillates at 1670. Once again we have an output frequency at 455 kHz. Our amplifier is amplifying the same frequency. Because of this, we can design this and the following stages to respond at its best at one frequency and one frequency only, and not have to compromise with a system working over a range of frequencies with the best performance in the centre. This is why a superhet system sounds much, much better than any other and can give a truly high fidelity even with an AM broadcast; with FM the quality of sound is even greater.
An intermediate frequency amplifier. A bipolar junction transistor (BJT) of NPN type works in class A with a tuned transformer as the collector load. Assuming the transistor has a gain of 100, a small varying input becomes a large varying output . The transformer is shielded to prevent outside interference and is varied by means of turning the iron “slug” thus varying the inductance rather than capacitance.