Ich kann dein ASC nicht sehen, aber:
Ein Class-AB Verstärker besteht aus mehr als den Endstufentransistoren
(die auch nicht unbedingt als Emitterfolger ausgeführt sein müssten).
Die Spannungsverstärkung macht die mittlere Stufe, die optimalerweise
gegen eine Konstantstromquelle arbeitet). Wie schreibt Fouglas Self;
The Three-Stage Amplifi er Architecture
The vast majority of audio amplifi ers use the conventional
architecture, shown in Figure 2.1 , and
so it is dealt with fi rst. There are three stages, the fi rst being a
transconductance stage (differential
voltage 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 thepre-driver 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 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. They will be fully revealed in later
chapters. Since the amount of
linearizing global negative feedback (NFB) available depends upon
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
- 20 and - 30 dB. The NFB factor at 20 kHz will be 25 – 40 dB,
increasing at 6 dB/octave with falling frequency until it reaches the
dominant pole frequency P 1, 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.
