Tab Electronics Guide to Understanding Electricity ... - Sciences Club

250 Chapter Eight
of Q4 and Q6 will be at about 18 volts also (minus a small drop across
their associated emitter resistors). If a positive-going AC signal is applied
to the input of this amplifier, transistors Q4 and Q6 will current-amplify
this signal. However, as soon as the AC signal goes into the negative
region, the signal applied to the base of Q4 will drop below 18 volts,
causing Q4 and Q6 to go into cutoff. Therefore, Q4 and Q6 are only
amplifying about half of the audio signal (i.e., the positive half-cycles of
the audio AC signal). Since transistors Q5 and Q7 are PNP devices, with
their associated collectors tied to circuit common, they are currentamplifying
the negative half-cycles of the audio AC signal, in a directly
inverse fashion as Q4 and Q6. Simply stated, all audio signal voltages
above 18 volts are amplified by Q4 and Q6, while all audio signal voltages
below 18 volts are amplified by Q5 and Q7. Consequently, the
entire amplified audio signal is summed at the positive plate of C4.
Note that resistor R7 is connected from the amplifier’s output back to
the “inverting” side of the input differential amplifier (i.e., the base of
Q2). R7 is a negative-feedback resistor. As you recall, the audio signal voltage
at the collector of Q3 is in phase with the audio input signal. Q4,
Q5, Q6, and Q7 are all connected as common-collector amplifiers (i.e.,
the input is applied to the bases with the output taken from the emitters).
Since common-collector amplifiers are noninverting, the audio signal
at the amplifier’s output remains in phase with the input signal.
Therefore, the noninverted output signal that R7 applies back to the
inverting input of the differential amplifier is negative feedback. Negative
feedback in this circuit establishes the voltage gain, increases linearity
(i.e., decreases distortion), and helps stabilize the quiescent voltage
levels. The voltage gain (A e
) of this amplifier is approximately equal to R7
divided by the parallel resistance of R5 and R6 (i.e., about 10).
Capacitor C4, like C1, is a coupling capacitor, serving to block the quiescent
18-volt DC level from being applied to the speaker. Note, however,
that the value of C4 is very large compared to the value of C1. This is
necessary because the impedance of most speakers is only about 8 ohms.
Therefore, in order to provide a time constant long enough to pass lowfrequency
audio signals, the capacity of C4 must be much greater.
Finally, resistor R13 and capacitor C3 form an output circuit that is
commonly referred to as a Zobel network. The purpose of a Zobel network
is to “counteract” the effect of typical speaker coil inductances,
which could have a destabilizing effect on the amplifier circuitry.
As you may have noted, the Fig. 8-8 amplifier consists of three basic
stages, commonly referred to as the input stage (Q1 and Q2), the voltage
amplifier stage (Q3), and the output stage (Q4, Q4, Q6, and Q7). Virtually