Test: Bias Compensation - Electrical Engineering (EE) MCQ

 Usually, the negative feedback is used to produce a stable operating point. But it reduces the voltage gain of the circuit. This sometimes is intolerable and should be avoided in some applications. So, the biasing techniques are used.

Compensation techniques refer to the use of temperature sensitive devices such as thermistors, diodes, transistors, sensistors etc to compensate variation in currents. Sometimes for excellent bias and thermal stabilization, both stabilization and compensation techniques are used.

For germanium transistor, changes in I_CO with temperature contribute more serious problem than for silicon transistor. On the other hand, in a silicon transistor, the changes of V_BE with temperature possesses significantly to the changes in I_C.

A diode is used as the compensation element used variation in V_BE and I_CO. The diode used is of the same material and type as that of transistor. Hence, the voltage across the diode has same temperature coefficient as V_BE of the transistor.

In this method, diode is used for the compensation in variation of I_CO. The diode used is of the same material and type as that of transistor. Hence, the reverse saturation current I_O of the diode will increase with temperature at the same rate as the transistor collector saturation current I_CO.

The thermistor has a negative temperature coefficient of resistance. It means, its resistance decreases exponentially with increasing T. The thermistor RT is used to minimize the increase in collector current.

The sensistor has a positive temperature coefficient of resistance. It is a temperature sensitive resistor. It is a heavily doped semiconductor. When voltage is decreased, the net forward emitter voltage decreases. As a result the collector current decreases.

: r_o=∆V_CE/∆I_C
=3/0.3m=10kΩ.

 The ratio of change in collector base voltage (∆V_CB) to resulting change in collector current (∆I_C) at constant emitter current (I_E) is defined as output resistance. This is denoted by r_o.

When no signal is applied, the ratio of collector current to emitter current is called dc alpha, αdc of a transistor. α_dc=-I_C/I_E. It is the measure of the quality of a transistor. Higher is the value of α, better is the transistor in the sense that collector current approaches the emitter current.