in the order of milliwatts.
Power amplifiers develop watts or hundreds of
milliwatts of power. This distinction based on power levels is approximate. The
power levels of driver and power amplifiers depend on the equipment in which they
are used. A driver amplifier, as its name implies, is used to drive a succeeding
stage. Thus, the driver stage delivers power to another driver stage or to a power
amplifier.
Power amplifiers increase the signal power to the necessary level to
operate a device such as an antenna.
3-4.
SINGLE-ENDED AMPLIFIERS
a. Circuit Arrangements. Circuit arrangements for single-ended driver and power
amplifiers do not differ to any marked degree from class A preamplifiers. Drivers
and are carefully matched for power transfer. Transformer coupling is very useful
coupling (LC) is sometimes used to obtain a higher efficiency, but it has poor low-
b. Efficiency.
Efficiency is an important consideration at high power levels,
particularly for power amplifiers.
As long as we operate class A, efficiency is
poor. Both class B and class C operation provide greater efficiency than class A.
3-5.
PUSH-PULL AMPLIFIERS
Exact sound reproduction (fidelity) can be realized at higher efficiencies by a
push-pull circuit arrangement.
Improved efficiency and fidelity can be attained
with push-pull amplifiers operating class A. Just as in the case of electron-tube
push-pull operation, even-harmonic (nonsymmetrical) distortion is minimized.
If
the circuit is perfectly symmetrical, such distortion is completely eliminated.
Because of this fact, push-pull amplifiers can deliver greater power for an
allowable amount of distortion than can two single-ended amplifiers.
Moreover,
push-pull amplifiers can be operated class B and class AB since even-harmonic
distortion can be canceled.
Provided there is good circuit symmetry, excellent
linearity can be achieved with class AB operation, and fairly good linearity is
possible with class B operation.
a. Transformer Type. The class B transformer-coupled amplifier is the simplest
type. Note that the NPN transistor circuit shown in figure 3-2 resembles closely
the corresponding electron-tube class B push-pull amplifier.
Remember that PNP
transistors can be used instead, provided the bias polarities are reversed.
(1) Observe in figure 3-2 that there is no forward base-emitter bias; both the
base and emitter of Q1 and Q2 are at dc ground potential. Therefore, Q1
and Q2 are cut off; with no-signal input (the static condition) Ic1 = 0
and Ic2 = 0.
(2) A signal applied to the primary of the input transformer produces signals
at opposite ends of the center-tapped secondary 180 out of phase, as
shown. Therefore, when a negative alternation is applied to the base of
Q1, a positive alternation is applied to the base of Q2, and vice versa.
Because both Q1 and Q2 have 0 volt base-emitter bias, each will conduct
during positive alternations
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