TABLE OF CONTENTS Distortion 4 is that which results from the loading of the Voltage Amplifier Stage (VAS) by the non-linear input impedance of a Class-B output stage. This was looked at in Chapter 4 from the point of view of the VAS, where it was shown that since the VAS provides all the voltage gain, its collector impedance tends to be high. This renders it vulnerable to non-linear loading unless it is buffered or otherwise protected.
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.
As explained in Chapter 4, the VAS collector impedance, while high at LF compared with other circuit nodes, falls with frequency as soon as Cdom takes effect, and so Distortion 4 is usually only visible at LF. It is also often masked by the increase in output stage distortion above dominant-pole frequency P1 as the amount of global NFB reduces.
The fall in VAS impedance with frequency is demonstrated in...
