Where is an IF amplifier used and why?

Answer 1

IF stands for intermediate frequency, an IF amplifier would typically be used in radio circuits. To help explain this, lets start with a model of a superheterodyne AM receiver.

All the carrier frequencies the radio is capable of receiving comes through the internal ferrite antenna. The signal may be weak, so there is an RF amplifier to boost the signal. That signal is still far from what we need. Sure we could try to employ a detector stage to filter the amplitude shifts from the carrier, but the quality would be poor. Transistors don't work as well at very high frequencies. Plus, if the receiver is tunable, it can be difficult to tune all the filters, amplifiers, and detectors in unison. And selectivity can be a challenge. Higher frequency tuners take up more bandwidth, making it harder to tune closer signals. But there is a way to work around all of these challenges.

What if you could convert all the other carrier frequencies to a single, lower frequency regardless of where the radio's dial is set, and then have the rest of the circuits tuned to work with that lower frequency? So that requires employing a local oscillator. Then you can use a dual gang variable capacitor to tune both the RF frequency and the local oscillator frequency. Then the RF frequency and the local oscillator frequency are both mixed together using the heterodyne principle to form a third frequency called the Intermediate Frequency, or IF. Actually 2 new frequencies are created (the sum and difference of the two original frequencies), and filters are used to allow only one of those through, and in this case 455 KHz is standard for an AM radio (and 10.7 MHz for FM). Then you would amplify this new frequency using an IF amplifier. There may be an additional filter after the IF amplifier stage to keep it steady at the standard IF frequency. Then from there is the detector stage which is extracts the information carried on the IF from the IF, leaving the audio frequency. There may be a preamp stage at this point, and it may receive reverse bias from the RF stage, allowing the preamp to compensate for different signal levels. Then from there is the driver amplifier stage which is what drives the speakers.

So the IF amplifier is used to help convert the RF frequencies to a single intermediate frequency while keeping the information that is modulated on the carrier. That simplifies the circuitry, allows for less expensive components to produce quality output, and provides better selectivity.

This method is used in other places too, such as with satellite and digital television. In the case of satellite, there is no way to use ordinary coax cable unless an intermediate frequency is used, hence the dual LNB converter which converts a 10.7-12.75 GHz signal into a 950-2150 MHz signal. Then from there, the set top box converts it to 480 MHz.

Also, the music instrument called a theremin uses a local oscillator to make the tone more stable and allow the performer (thereminist) to have more control of the sound produced.

Answer 2

IF stands for intermediate frequency, an IF amplifier would typically be used in radio circuits. To help explain this, lets start with a model of a superheterodyne AM receiver.

All the carrier frequencies the radio is capable or receiving comes through the internal ferrite antenna. The signal may be weak, so there is an RF amplifier to boost the signal. That signal is still far from what we need. Sure we could try to employ a detector stage to filter the amplitude shifts from the carrier, but the quality would be poor. Transistors don't work as well at very high frequencies. Plus, if the receiver is tunable, it can be difficult to tune all the filters, amplifiers, and detectors in unison. And selecitivity can be a challenge. Higher frequency tuners take up more bandwidth, making it harder to tune closer signals. But there is a way to work around all of these challenges.

What if you could convert all the other carrier frequencies to a single, lower frequency regardless of where the radio's dial is set, and then have the rest of the circuits tuned to work with that lower frequency? So that requires employing a local oscillator. Then you can use a dual gang variable capacitor to tune both the RF frequency and the local oscillator frequency. Then the RF frequency and the local oscillator frequency are both mixed together using the heterodyne principle to form a third frequency called the Intermediate Frequency, or IF. Actually 2 new frequencies are created (the sum and difference of the two original frequencies), and filters are used to allow only one of those through, and in this case 455 KHz is standard for an AM radio (and 10.7 MHz for FM). Then you would amplify this new frequency using an IF amplifier. There may be an additional filter after the IF amplifier stage to keep it steady at the standard IF frequency. Then from there is the detector stage which is extracts the information carried on the IF from the IF, leaving the audio frequency. There may be a preamp stage at this point, and it may receive reverse bias from the RF stage, allowing the preamp to compensate for different signal levels. Then from there is the driver amplifier stage which is what drives the speakers.

So the IF amplifier is used to help convert the RF frequencies to a single intermediate frequency while keeping the information that is modulated on the carrier. That simplifies the circuitry, allows for less expensive components to produce quality output, and provides better selectivity.

This method is used in other places too, such as with satellite and digital television. In the case of satellite, there is no way to use ordinary coax cable unless an intermediate frequency is used, hence the dual LNB converter which converts a 10.7-12.75 GHz signal into a 950-2150 MHz signal. Then from there, the set top box converts it to 480 MHz.

Also, the music instrument called a theremin uses a local oscillator to make the tone more stable and allow the performer (thereminist) to have more control of the sound produced.