ABSTRACT
The VAS is routinely (though usually unknowingly) linearised by applying local negative-feedback via the dominant-pole Miller capacitor Cdom, and this is a powerful argument against any other form of compensation. If VAS distortion still adds significantly to the amplifier total, then the local open-loop gain of the VAS stage can be raised to increase the local feedback factor. The obvious method is to raise the impedance at the VAS collector, and so the gain, by cascoding. However, if this is done without buffering the output stage loading will render the cascoding almost completely ineffective. Using a VAS-buffer eliminates this problem.
