16. IF-AMPLIFIERS

Tuned amplifiers are Class-A-amplifiers with a parallel-type-tuned-circuit as collector-resistor.

The parallel-tuned-circuit has a characteristics as shown in fig. 191. Its impedance is maximum at the resonant frequency.

As bigger the collector resistance is as bigger is the amplification of such a class-A-amplifier.

This is the reason why this amplifier amplifies best the resonant frequency.

fig. 191

It is used most often for two purposes:

- Radio frequency amplifiers
- Intermediate frequency amplifiers

Its special advantages are:

1. The output signal of the mixer-stage has to be amplified anyway, but the IF-filter makes it possible for the IF-amplifiers to amplify only signals with a frequency very near around the IF-frequencies. Therefore the selectivity of those amplifiers can be adjusted to very high degrees.

2. The IF-frequency is lower than the RF-and therefore we can easily avoid here problems which occur always at class-A-amplifiers with higher frequencies.

3. This kind of amplifier makes it especially simple to couple the next stage of the IF-amplifier exactly matching, because it is easy to provide the tuned circuit coil with a secondary coil which acts like a secondary coil in a transformer.

fig. 190, 192

SPECIALITIES IN IF-AMPLIFIERS

There are sometimes found some specialities with IF-amplifiers which should be mentioned and explained here.

- The capacitor found in fig. 192 parallel to the lower biasing resistor is introduced to the circuit in order to minimize the voltage-drop for the IF-signal via the loop: secondary coil of the tuned circuit (signal-source) base-emitter-junction and emitter-resistor-capacitor combination.

- There are sometimes found IF-amplifiers coupled by a second tuned circuit formed by the secondary coil of the first filter coil and an additional capacitor (as also shown in fig. 192). In this case the response of the two stages (here mainly the bandwidth which is amplified) can be adjusted by the tiny iron core between both coils.

- Anyway the tuned circuits of IF-amplifiers have to be adjusted so that they amplify with a maximum at IF-frequency. But don't try this alignment deliberately - the adjustment of a new radio is undertaken in the factory with very high quality equipment which you will not have for your “alignment”.

- Sometimes you find a special connection within these amplifiers:

An RC-series circuit connected from the base of the transistor to the output side of the amplifier.

This circuit is called a NEUTRALIZING CIRCUITS. It is necessary because there is always same capacitance between base and collector of the transistor. This capacitance will cause some feedback at higher frequencies which can cause very ugly oscillations of the amplification.

The neutralizing circuit cancels this effect.

fig. 193

CHECK YOURSELF:

1. RF- and IF-amplifiers both work on radiofrequency range. Explain the main difference between them.

2. Which kind of coupling is preferred between IF-amplifierstages, and why?

3. Why is the amplification of a tuned amplifier maximum just at a special frequency?

4. Under which condition would you try to realign an IF-frequency filter?