Audio Amplifi er Performance 315voltage in, current out), the second a transimpedance stage (current in, voltage out), and
the third a unity-voltage-gain output stage. The second stage clearly has to provide all
the voltage gain and I have therefore called it the voltage-amplifi er stage or VAS. Other
authors have called it the predriver stage but I prefer to reserve this term for the fi rst
transistors in output triples. This three-stage architecture has several advantages, not least
being that it is easy to arrange things so that the 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 fi rst stage output; this
minimizes Miller phase shift and possible early effect in the input devices.
Similarly, the compensation capacitor reduces the second stage output impedance so
that the nonlinear loading on it due to the input impedance of the third stage generates
less distortion than might be expected. The conventional three-stage structure, familiar
though it may be, holds several elegant mechanisms such as this. Since the amount of
linearizing global NFB available depends on amplifi er open-loop gain, how the stages
contribute to this is of great interest. The three-stage architecture always has a unity-gain
output stage—unless you really want to make life diffi cult for yourself—and so the total
forward gain is simply the product of the transconductance of the input stage and the
transimpedance of the VAS, the latter being determined solely by the Miller capacitor
Cdom , except at very low frequencies. Typically, the closed-loop gain will be between 2 0
and 30 dB. The NFB factor at 20 kHz will be 25 to 40 dB, increasing at 6 dB per octave
with falling frequency until it reaches the dominant pole frequency P1, when it fl attens
out. What matters for the control of distortion is the amount of NFB available, rather than
the open-loop bandwidth, to which it has no direct relationship. In my Electronics World
Class-B design, the input stage gm is about 9 mA/V, and Cdom is 100 pF, giving an NFB
factor of 31 dB at 20 kHz. In other designs I have used as little as 26 dB (at 20 kHz) with
good results.
Compensating a three-stage amplifi er is relatively simple; since the pole at the VAS is
already dominant, it can be easily increased to lower the HF NFB factor to a safe level.
The local NFB working on the VAS through Cdom has an extremely valuable linearizing
effect.
The conventional three-stage structure represents at least 99% of the solid-state amplifi ers
built, and I make no apology for devoting much of this book to its behavior. I doubt if
I have exhausted its subtleties.