TRANSISTOR BIAS STABILITY

BJT: Bipolar Junction Transistors

The Bipolar transistor amplifiers should be properly biased to operate correctly. In the circuits made with the individual devices (discrete circuits), biasing networks consisting of resistors are employed commonly.  Much more elaborate biasing arrangements are used in the integrated circuits, for instance, band gap voltage references and current mirrors.

Bias circuit requirements:

For the analog circuit operation, Q-point is placed so the transistor stays in the active mode (does not shift to the operation in saturation region or cut-off region) when input is applied. For digital operation, Q-point is placed so the transistor does the contrary - switches from "on" to the "off" state. Q-point is established around the center of active region of transistor characteristic to allow similar signal swings in the negative and positive directions. The Q-point should be stable. In particular, it should be insensitive to variations in transistor parameters (for instance, it should not shift if the transistor is replaced by another transistor of the same type), variations in the temperature, variations in the power supply voltage and so forth. The circuit should be practical: easily implemented and cost effective.

Load line:

The concept of load line is very significant in understanding the working of the transistor. It can be described as the locus of the operating points on output characteristics of transistor. It is the line on which the operating point moves when the AC signal is applied to the transistor.

The DC load line provides the value of Ic and VCE corresponding to the zero signal conditions. The AC load line gives the value of IC and VCE when an AC signal is applied. AC load line is steeper than the DC  load  line  but  two  intersect  at  the  Q-point determined  by  biasing  DC voltages  and currents. The AC load line takes into account AC load resistance while the DC load line considers only the DC load resistance.

Q Point:

Operating point of the device, also called as quiescent point, bias point, or Q-point, is the point on the output characteristics that shows the DC collector-emitter voltage (Vce) and the collector current (Ic) with no input signal applied. The term is commonly used in connection with devices like transistors.

For bipolar junction transistors the bias point is chosen to keep the transistor operating in the active mode, by using a several of circuit techniques, establishing Q-point DC voltage and the current.  A small  signal  is  then  applied  on  top  of Q-point bias  voltage,  thus  either modulating or switching the current, depending upon purpose of the circuit.

The quiescent point of the operation is typically around the middle of DC load line. The process of obtaining certain DC collector current at the certain DC collector voltage by setting up the operating point is called as biasing.

After the establishment of operating point, when the input signal is applied, output signal should not move the transistor either to the saturation or to cut-off. However, this unwanted shift still can occur, because of following reasons:

1. Parameters of transistors are dependent on the junction temperature. As the junction temperature increases, leakage current as a result of minority charge carriers (I_CBO) increases. As the I_CBO increases, I_CEO also increases, causing increase in the collector current I_C. This generates heat at collector junction. This process keeps on repeating, and, finally, the Q-point can shift into the saturation region. Sometimes, excess heat generated at junction can even burn the transistor. This is popularly known as thermal runaway.

2.   When the transistor is replaced by another of the same type, the Q-point can shift, because of changes in parameters of transistor, like current gain (β) which varies a little for every unique transistor.

To get rid of the shift of Q-point, bias stabilization is quite essential. The various biasing circuits can be taken in use for this particular purpose.

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