Audio Engineering

316 Chapter 10

10.3.1 Two-Stage Amplifi er Architecture

In contrast, the architecture shown in Figure 10.2 is a two-stage amplifi er, with the
fi rst stage once again being more a transconductance stage, although now without a
guaranteed low impedance to accept its output current. The second stage combines VAS
and output stage in one block; it is inherent in this scheme that the VAS must double as
a phase splitter as well as a generator of raw gain. There are then two quite dissimilar
signal paths to the output, and it is not at all clear that trying to break this block down
further will assist a linearity analysis. The use of a phase-splitting stage harks back to
valve amplifi ers; where it was inescapable as a complementary valve technology has, so
far, eluded us.

Paradoxically, a two-stage amplifi er is likely to be more complex in its gain structure than
a three stage. The forward gain depends on the input stage gm , the input stage collector
load (because the input stage can no longer be assumed to be feeding a virtual earth),
and the gain of the output stage, which will be found to vary in a most unsettling manner
with bias and loading. Choosing the compensation is also more complex for a two-stage
amplifi er, as the VAS/phase splitter has a signifi cant signal voltage on its input and so
the usual pole-splitting mechanism that enhances Nyquist stability by increasing the pole

First stage,

input

subtractor and

gain

Second stage,

voltage

amplifier and

output



V

V



Figure 10.2 : The two-stage amplifi er structure. A voltage-amplifi er output follows the

same transconductance input stage.