Question

Operating point of a transistor is
A. Zero signal value of ${V_{CC}}$ and ${I_o}$
B. Zero signal value of ${I_C}$ .
C. Zero signal value of ${V_{CC}}$.
D. Zero signal value of ${I_C}$ and ${V_{CE}}$

Answer 1

Hint- Operating point is the point where the load line meets with the output characteristics of a transistor. The operating point is chosen such that that the transistor works in the active region and the transistor gives faithful amplification. For the proper working of a transistor to give faithful amplification the input should be forward biased and output should be reverse biased.

Complete step by step solution:
When a line is drawn connecting the saturation point and the cutoff point of a transistor such a line is termed as the load line. If we draw this line on the output characteristics of a transistor it will make contact with a point on the characteristic curve, this point is called the operating point. The operating point should be chosen such that the transistor will remain in the active region.

Operating point should give faithful amplification for the transistor. Faithful amplification is the process of increasing the signal strength without any loss in the components of the signal.
For a transistor to work properly the base emitter junction should be forward biased and the collector base junction should be reverse biased. So, for faithful amplification there are certain conditions which must be satisfied they are
(a)zero signal collector current
(b)minimum collector emitter voltage.
(c)minimum base emitter voltage.
If these conditions are satisfied then the transistor will work in the active region and the input will be forward biased then output will be reverse biased.
Consider the transistor circuit given below. In the circuit the input is forward biased and output is reverse biased. When the signal is supplied for the positive half cycle of the signal the biasing condition remains undisturbed but during the negative half cycle the input gets reverse-biased. So, we need to add another voltage in the base emitter circuit such that the input remains forward biased even for the negative cycle of the input signal. Due to this voltage applied in the input even when there is no signal applied, there will be an output current in the collector. This output current is known as the zero-signal collector current.

For faithful implication one more condition is required that is the collector emitter voltage should not fall below the knee voltage. If the voltage of the collector emitter falls below knee voltage then the collector base will not be reverse biased and hence faithful amplification will not take place.
So, the conditions for the operating point to be stable is that there should be zero signal collector current and collector emitter voltage.

So, the correct answer is option D.

Note: Remember that a transistor works only when there is proper biasing in the input and the output. The condition for proper working of the transistor is that the base emitter junction should be forward biased and the collector emitter junction should be reverse biased.