Question

Which transistor bias circuit provides good Q-point stability with a single-polarity supply voltage?

(a) Base bias

(b) Collector feedback bias

(c) Voltage divider bias

(d) Emitter bias

I have been asked this question in class test.

This intriguing question comes from Input Bias and Offset Currents of the Bipolar Differential Amplifier in section Differential and Multistage Amplifiers of Electronic Devices & Circuits

Answer 1

Correct choice is (c) Voltage divider bias

The explanation is: When the transistor starts operating, temperature at the junction increases. Hence IC increases. As a result of which, Ie increases. Due to this increase in Ie, voltage drop across Re increases. This reduces the forward voltage across the emitter. Thus Ib reduces.

We know that, for any value of ICO,  IC = β * Ib +(1+ β) ICO

where  ICO =reverse saturation current   (increases with temperature)

            β = gain

Therefore, β  and ICO increases and at the same time there is decrease in Ib. Hence above equation confirms that IC can be maintained within limits. Thus the circuit is more thermally stable and the operating point is more stable.