Updated May 5,
here to go to our main page on amplifiers
Caution, this page is under construction!
New for July 2007! Here
we will give a brief explanation of the different classes of power
amplifiers, which mainly relate to bias point.
I-V curves of a transistor
The curves of a transistor are
shown in the following two figures. First is a field effect transistor.
Here the current source is voltage controlled, which means that
there is virtually no current on the input (gate) terminal.
In the case of a bipolar device
(such as an HBT) the device acts like a current controlled current
The bias point, also known as
the quiescent point "Q" is a function of the voltage (or
current) on the input terminal, and the voltage on the output terminal.
We've shown the bias points for class A, AB, B on the IV curve for
Class A small signal operation
is linear. Bias point is at halfway between saturated current and
pinch-off. When you hear he words "gain block" or "linear
amplifier", think Class A. The output signal uses 100% of the
input signal waveform.
In class B the transistor conducts
only in one half cycle of the signal. Thus for no excitation by an
input signal, the DC power consumed in Class B is ideally zero for
a FET-based amplifier, and very low for a bipolar amplifier. Theoretical
maximum efficiency for Class B is 78.5 %.
A "complimentary amplifier",
or push-pull amplifier uses opposite polarity (PNP and NPN) transistors
that operate class B yet still provide the full waveform. There
is distortion in that the transistors each have threshold voltages
to overcome, which can be reduced by bias circuit on the input.
Here the devices is biased somewhere
between linear (Class A) and Class B, perhaps at 25% of the maximum
current IMAX. This is by far the most popular bias point for a power
amplifier, offering a good compromise between gain, power and efficiency.
Quite often the gain of a solid state device is reduced the closer
you are to a zero-current bias point.
In practice, the AB bias point
drifts (or self-biases) under drive. Usually there is an appreciable
resistance between the input terminal and its power supply, so under
large-signal conditions, DC current starts to flow, either from
breakdown or forward conduction. In both cases the bias point will
shift in Class AB, the bias point is closer to breakdown than to...
This is similar to Class B but
a tuned circuit brings only the intended frequency out of the device.
Class D, E and F are considered
"switching amplifiers". We'll save that discussion for
another day, when someone wants to help us out!