Handbook for Sound Engineers

Amplifier Design 727

In Fig. 20-36 the batteries represent the positive and
negative rail supplies, Sp and Sn represent two groups of
several paralleled N channel power MOSFETs that
serve to connect to the positive and negative rail
supplies, respectively, and there are two matched induc-
tors as well as two matched diodes. An understanding of
the operation of this output stage and its associated
modulation technique may be had by examining three
conditions or modes of employment: quiescent mode
when no audio output is called for, positive mode when
a positive-going audio output is required, and negative
mode when a negative-going audio output is required.
Recall that one component signal of a pulse width or
switching modulator is a triangular waveform. The
fundamental frequency of the triangular waveform
employed in the associated modulator for this output
stage is 250 kHz so the full period is 4 microseconds
(4μs) and the half period is 2 microseconds (2μs). In
the quiescent mode when no audio output is required
the switches Sp and Sn are closed and opened in concert.
Both switches are closed for the first half-period or 2Ps
and both are open for the second half-period also of
2 Ps. The inductors in Fig. 20-36 have relatively large
self-inductance and quite small resistance such that their
time constants, being the ratio of L/R, are very much
larger than 2μs so that the current in each inductor
starts from zero and grows linearly in the clockwise
sense around the closed loop at the rate of VCC/L when
the switches are simultaneously closed. There is no
current in the diodes as they are reverse biased under

this condition. All during the growth period, the voltage

drop across the upper L/R series combination and the

lower L/R series combination are each equal to VCC

while the voltage between point A and ground remains

at zero. All during this time energy is being stored in the

magnetic fields associated with each inductor and this

stored energy reaches a maximum value when the

elapsed time reaches 2μs. When the elapsed time

reaches 2μs both switches are opened simultaneously

and the circuit effectively becomes the depiction

presented in Fig. 20-37.

When the switches are simultaneously opened, the

current begins to ramp down from the maximum value

achieved in the previous 2μs when the switches were

closed and the diodes that played no role in the previous

2 μs maintain circuit closure but now to the opposite

terminals of the batteries. In the next 2μs the collapsing

magnetic fields of the inductors continue to drive the

current with a linearly decreasing magnitude and energy

is being returned to the rail supplies. The energy

recovery is not quite complete as there is a small heat

loss in the imperfect inductors and diodes. During the

ramp down process, the voltage from point A to ground

again ideally remains at zero. Thus, in the quiescent

state with no audio signal present, the voltage at point A

remains at zero and there is no ripple arising from the

modulation process.

When a positive-going audio signal is present the

operation may be understood by reference to Fig. 20-38.

Figure 20-36. Simplified balanced current output stage.

D L R R L D

Point A

Vcc

Vcc

SN

SP

Figure 20-37. Equivalent decay circuit for current.

D L R R L D

Point A

Vcc

Vcc

SN

SP