20. DECOUPLING CIRCUITS

GENERALLY:

If you recollect the circuit of a simple Class A amplifier you can yourself very easily predict what would happen in this circuit if there is any change of supply voltage.

A change of supply voltage - even if it is very tiny - will cause a change at the centrepoint of the base-voltage divider. This causes a change of the base-emitter voltage at the transistor and this will cause an output signal.

As long as we observe only a single amplifierstage, we will not see a big problem connected with this effect because we use anyway a rather steady supply voltage and therefore we must not expect too strong changes of its values. But if the amplifier observed is an early stage of a radio receiver, followed by several amplifier stages, we will face much bigger problem because of two reasons:

1. the signal caused by the voltage changes before was very tiny but now it is amplified vey the next amplifierstages for up to several tenthousend times. This means: at the output of the receiver we will have considerable signal now.

2. Such a signal causes at the power amplifier a considerable big change of load current and this will cause - even at a stabilized power supply - at least a tiny voltage drop .... just the voltage drop which was the beginning of our consideration.

The effect which we derived here is a so called POSITIVE FEEDBACK and this causes always OSCILLATIONS - “MOTORBOATING” as the “Fundis” call it.

fig. 204

This is the reason why almost always all the stages except the power amplifier are DECOUPLED.

Sometimes some stages together sometimes each stage separately. Decoupling is achieved by nothing else than an ac-filter made up from an RC-series connection.

fig. 205