Audio Engineering

Audio Amplifi er Performance 325

device off anyway, when current fl ows. The need for drivers, with their extra junction
drops, also complicates things.

A new extension of this technique is to redesign the translinear loop so that 1/ In  1 / Ip 
constant; this is known as harmonic-mean AB operation.^22 It is too early to say whether
this technique (assuming it can be made to work outside an IC) will be of use in reducing
crossover distortion and thus improving amplifi er performance.

10.4.17 Nested Differentiating Feedback Loops

This is a most ingenious, but conceptually complex technique for signifi cantly increasing
the amount of NFB that can be applied to an amplifi er (see Cherry^23 ).

10.5 AC- and DC-Coupled Amplifi ers ..........................................................................

All power amplifi ers are either AC coupled or DC coupled. The fi rst kind have a single
supply rail, with the output biased to be halfway between this rail and ground to give the

V

V

Re1

Re2

Out

VAS

I

Vbias

Figure 10.5 : A conventional double emitter–follower output stage with emitter

resistors Re shown.