Bias stability, Fixed bias, Collector-to-base bias, Voltage divider bias, Emitter bias
Bias stability
One of the basic problems with transistor amplifiers is establishing and maintaining the proper values of quiescent current and voltage in the circuit.
This is accomplished by selecting the proper circuit-biasing conditions and ensuring these conditions are maintained despite variations in ambient (surrounding) temperature, which cause changes in amplification and even distortion (an unwanted change in a signal).
Thus a need arises for a method to properly bias the transistor amplifier and at the same time stabilize its dc operating point (the no signal values of collector voltage and collector current).
Types of bias circuit
The following discussion treats four common biasing circuits used with bipolar transistors:
Fixed bias
Collector-to-base bias
Voltage divider bias
Emitter bias
Self Bias: A portion of the output is fed back to the input 180o out of phase. This negative feedback will reduce overall amplifier gain.
Fixed Bias: Uses resistor in parallel with Transistor emitter-base junction.
Combination Bias: This form of bias stabilization uses a combination of the emitter resistor form and a voltage divider. It is designed to compensate for both temperature effects as well as minor fluctuations in supply (bias) voltage.
Emitter Resister Bias: As temperature increases, current flow will increase. This will result in an increased voltage drop across the emitter resistor which opposes the potential on the emitter of the transistor.