The class-A amplifier
The amplifier circuits in Figures given below will operate in class A. Weak-signal amplifiers, like the kind used in the 1st stage of a sensitive radio receiver, are always class-A. The term does not occur from inherent superiority of design or technique (it is not like saying grade-A eggs). It is just a name chosen by engineers so that they know operating conditions in bipolar transistor or FET.
A class-A amplifier is linear always. It means that the output waveform has the same shape as the input waveform.For class-A operation with the bipolar transistor, the bias should be such that, with no signal input, the device is near middle of straight-line portion of the IC vs EB (collector current vs. base voltage) curve. This is shown for NPN transistor in figure given below. For the PNP, reverse the polarity signs.Along with the JFET or MOSFET, the bias should be such that, with no signal input, the device is near the middle of straight line part of the ID vs EG .
Figure-- Various classes of amplifier operation for an NPN bipolar transistor.
voltage) curve. This is shown in the figure given below for an N-channel device. For P-channel, reverse polarity signs.
It is essential with class-A amplifiers that input signal not be too strong. Otherwise, during part of cycle, the base or gate voltage will be driven outside of straight-line part of the curve. When this happens, the output waveshape will not be a fair reproduction of the input waveshape; the amplifier will be nonlinear. This will create distortion of the signal. In the audio amplifier, the output might sound raspy or scratchy. In the radio frequency amplifier, output signal will contain a large amount of energy at the harmonic frequencies, the problem of harmonics, but, can be dealt with by means of resonant circuits in output. These circuits attenuate harmonic en- ergy, and allow amplifiers to be biased near, at, or past cutoff or pinchoff.