Transistor Biasing Basics

In order to establish a proper operating point for the transistor [collector current (I_C) and collector–emitter voltage (V_CE)], external components such as resistors and capacitors are used and DC voltages are applied to the transistor. This process is referred to as transistor biasing

Circuit used for transistor biasing is called a biasing circuit.

There are four conditions that should be met for a transistor to act as a faithful amplifier.

• The emitter-base junction should be forward-biased and the collector-base junction should be reverse-biased for all levels of input signal.
• The collector–emitter voltage (V_CE) should not fall below the knee voltage (V_CE(sat) for any part of the input signal, otherwise the collector–base junction is not properly reverse-biased.
• The value of collector current (I_C) when no signal is applied should be at least equal to the maximum collector current due to signal alone.
• The maximum ratings of the transistor (I_C(max), V_CE(max) and P_D(max)) should not be exceeded at any value of the input signal.

The DC collector current (I_C) and the collector–emitter voltage (VCE) when no input signal is applied is referred to as the operating point.

The operating point is referred to as the quiescent point (Q-point) as it is a fixed point on the output characteristics of the transistor.

Point D is located in the middle of the active region of the transistor characteristics. It will provide maximum linear gain and largest possible output voltage and current swing. It is the best suitable operating point.

Output characteristics with different operating points

Operating point A drives the transistor in the cut-off region. Point B would allow some positive and negative variations of the output signal but the peak-to-peak output voltage is limited due to the proximity of the operating point to the knee point (V_CE(sat)). Point C is too close to the P_D(max) curve of the transistor. Therefore, the output voltage swing in the positive direction is limited. Therefore, operating points A, B and C are not suitable.

DC analysis of a circuit refers to analyzing a circuit so as to establish the operating point in the absence of any input AC signal. For the purpose of DC analysis, the input and output capacitors are considered as open and it is assumed that all AC sources are zero.