Amplifier architectures

This grandiose title simply refers to the large-scale structure of the amplifier;

i.e. the block diagram of the circuit one level below that representing it as

a single white block labelled Power Amplifier. Almost all solid-state

amplifiers have a three-stage architecture as described below, though they

vary in the detail of each stage.

The three-stage architecture

The vast majority of audio amplifiers use the conventional architecture,

shown in Figure 2.1. There are three stages, the first being a transconductance

stage (differential voltage in, current out) the second a

transimpedance stage (current in, voltage out) and the third a unity-voltagegain

output stage. The second stage clearly has to provide all the voltage gain

and I have therefore called it the voltage-amplifier stage or VAS. Other

authors have called it the pre-driver stage but I prefer to reserve this term for

the first transistors in output triples. This three-stage architecture has several

advantages, not least being that it is easy to arrange things so that interaction

between stages is negligible. For example, there is very little signal voltage

at the input to the second stage, due to its current-input (virtual-earth) nature,

and therefore very little on the first stage output; this minimises Miller phaseshift

and possible Early effect in the input devices.