728 Chapter 20
The three sketches in the top portion of the figure
describe, respectively, the operation of the positive
switch, the modulator behavior, and the operation of the
negative switch while the final sketch at the bottom
displays the effective voltage presented to the output
filter of the BCA. Observe that the positive switch is
turned on when the triangle waveform first crosses the
negative error voltage value and stays on until the
triangle waveform again crosses the negative error
voltage value. Note also that the duration of the on-time
of the positive switch is 2.4μs. Observe also that the
negative switch is turned on when the triangle wave-
form first crosses the positive error voltage value and
stays on until the triangle waveform again crosses the
positive error voltage value. The duration of the on-time
of the negative switch is 1.6μs. When both switches are
off there is no output and when both switches are on
simultaneously there is no output. There are two periods
when the positive switch is on while the negative switch
is off during which output is generated. Thus, the funda-
mental ripple frequency of the output is twice that of the
fundamental frequency of the triangle waveform or
500 kHz. The switches themselves operate at only the
fundamental frequency of the triangle waveform or
250 kHz. Another very important observation is that the
on time of the two switches adds to 4μs, which is the
period of the triangle waveform. This is always true for
this type of modulation technique. If an even larger
output had been required, the negative error voltage line
would have been lower while the positive error voltage
line would have been higher. In such an instance, the on
period of the positive switch would be increased while
that of the negative switch would be correspondingly
reduced. The two positive output pulses would still have
amplitudes of VCC but each would endure for a longer
period such that the average value following the output
filter would be correspondingly larger.
When a negativegoing audio signal is present, refer-
ence must be made to Fig. 20-39.A study of Fig. 20-39 indicates that now the negative
switch is turned on when the triangle waveform first
crosses the negative error voltage value and remains on
until the triangle waveform again crosses the negative
error voltage line. In brief, the role of the two switches
has now been reversed. The period of the negative
switch is now 2.4μs while the period of the positive
switch is now1.6μs. The output pulses as a conse-
quence are now negative. The other general properties
remain the same.
The output of the BCA under all three modes of
operation—quiescence, positive audio output, and nega-
tive audio output—is further clarified by viewing the
equivalent circuit displayed in Fig. 20-40.
When the three-position switch in Fig. 20-40 is in
position two as shown in the drawing, the output of theFigure 20-38. Modulation, switching, and effective voltage
waveforms when the instantaneous audio signal calls for a
positive 20% of full-scale output.
Sp on
off51
–1–5
on
offSnVCCEffective voltageModulation+Verror- Verror
Vtrt—Mst—MsFigure 20-39. Modulation, switching, and output wave-
forms when the instantaneous audio signal calls for a nega-
tive 20% of full-scale output.Sp on
off51
–1–5
on
offSnVCCEffective voltageModulation+Verror- Verror
Vtrt—Mst—Ms