IF Amplifier

An intermediate frequency (IF) amplifier is a linear fixed-frequency tuned amplifier, found in the IF stage of a superheterodyne radio receiver. For AM radios, it is typically a common emitter Class-A amplifier which has the purpose of providing selective gain within a narrow frequency range. These types of amplifiers form the basis of narrow band radio frequency (RF) amplifiers with a bandpass response. A typical radio has multiple IF stages, where each stage consists of a single transistor circuit coupled by a tuned IF transformer. The primary winding of the transformer together with a capacitor form a parallel resonant circuit to provide a bandpass response to the amplification. In this circuit design example, we look at a typical IF stage using a transistor so you can make a homebrew IF amplifier of your own. In this article, we look at the basic theory to see how to calculate the operating frequency.

The intermediate frequency is the difference between the incoming carrier frequency and the oscillator frequency. We gang the oscillator frequency control with the tuning frequency control using a two stage variable tuning capacitor. This way, the oscillator frequency follows the carrier frequency and their difference remains the same all the time. For AM radios, the difference is usually 455 kHz, and therefore the IF amplifiers are designed to amplify a signal of this frequency, and reject everything else.

Circuit Diagram

The circuit diagram above shows an idealised RF amplifier of the type found in the IF stage of an AM radio. This amplifier has a bandpass response and it will amplify a signal with a frequency of approximately 720 kHz. I figured I would do a circuit that would be useful to crystal radio enthusiasts to play with. In the UK, if you live in the London area, then it is the frequency of BBC Radio 4 on the medium wave band.

In these types of circuits, the collector end usually has a tuned LC circuit, where the coil L is the primary winding of the IF transformer that would couple to the next IF stage. The primary coil should have a value of 10 mH, once set using the ferrite slug. The variable capacitor would be a simple trimmer with a range of 2 pF to 22 pF, which ideally is set to 4.88 pF. If you have an LCR meter then it helps a lot.

You could use the standard formula for calculating the resonance frequency of a parallel tuned circuit using the values set for inductance and capacitance just to verify, and you will get a resonance frequency of approximately 720461 Hz or thereabouts. If you had a spectrum analyser, then you would see that this transistor circuit amplifies signals within a narrow frequency range, and rejects everything outside its range.

AGC

An IF stage also typically has an automatic gain control (AGC) which adjusts the gain of the amplifier in response to the strength of the incoming signal. This way, greater amplification occurs for weak signals from distant locations, whilst stronger signals from a nearby location receive less amplification.

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