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.
