HT66FV1x0 Integrated Audio Amplifier Application Guideline
AN0486EN V1.10
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June 8, 2021
IC
IC
IC
t
Load Line
Output
Waveform
Quiescent Point
Vi Input Waveform
Quiescent
Point
V
BE
V
CE
Class A
Class B Principles
The quiescent operating point of a Class B amplifier is located on the output characteristic curve
with a base current of zero, namely the cut-off point of the load line. In a Class B amplifier, a single
output transistor conducts only one half of the input signal cycle, that is, the conduction angle is
180 degrees. To amplify a complete signal, two transistors are used, one for the positive output
signal and the other for the negative output signal. A pseudo load, RL, is connected to the amplifier
output port, replacing the normal power receiving component, which is used to debug the power
amplifier circuit. After the debug has been successfully completed, a true load, a speaker is
connected. The efficiency of Class B amplifiers is much improved over Class A amplifiers.
However, at the crossover point from the on to off state of the two transistors, the distortion of Class
B is higher.
IC
IC
IC
t
t
Output
Waveform
Load Line
Quiescent
Point
Quiescet
Point
Vi Input Waveform
Crossover
Distortion
V
BE
V
CE
Class B
Class AB Principles
The Class AB design contains the advantages of both Class A and Class B. The quiescent operating
point of Class AB amplifier, which is lower than Class A but higher than Class B, is located between
the load line midpoint and cut-off point, resulting in efficiencies higher than Class A and distortion
lower than Class B. The conduction angle is in the range of 180~360 degrees. By biasing the two
transistors, when the signal is close to zero, both transistors are turned on with a relatively small
current, which is similar to Class A. In the case of a large signal, when one of the transistors turns
off the other must turn on. The two transistors are always turned on alternatively, which is similar
to Class B. The crossover distortion of Class B amplifiers is caused by the fact that neither transistor
can conduct when the input signal is in the range of -0.6V~0.6V. In a Class AB amplifier, a V
BB
bias is applied between the two transistors to reduce the crossover distortion.
Q1
V+
Q1
RL
V+
V-
Q2
