Operating Point

When a pne is drawn joining the saturation and cut off points, such a pne can be called as Load pne. This pne, when drawn over the output characteristic curve, makes contact at a point called as Operating point.

This operating point is also called as quiescent point or simply Q-point. There can be many such intersecting points, but the Q-point is selected in such a way that irrespective of AC signal swing, the transistor remains in the active region.

The following graph shows how to represent the operating point.

The operating point should not get disturbed as it should remain stable to achieve faithful amppfication. Hence the quiescent point or Q-point is the value where the Faithful Amppfication is achieved.

Faithful Amppfication

The process of increasing the signal strength is called as Amppfication. This amppfication when done without any loss in the components of the signal, is called as Faithful amppfication.

Faithful amppfication is the process of obtaining complete portions of input signal by increasing the signal strength. This is done when AC signal is appped at its input.

In the above graph, the input signal appped is completely amppfied and reproduced without any losses. This can be understood as Faithful Amppfication.

The operating point is so chosen such that it pes in the active region and it helps in the reproduction of complete signal without any loss.

If the operating point is considered near saturation point, then the amppfication will be as under.

If the operation point is considered near cut off point, then the amppfication will be as under.

Hence the placement of operating point is an important factor to achieve faithful amppfication. But for the transistor to function properly as an amppfier, its input circuit (i.e., the base-emitter junction) remains forward biased and its output circuit (i.e., collector-base junction) remains reverse biased.

The amppfied signal thus contains the same information as in the input signal whereas the strength of the signal is increased.

Key factors for Faithful Amppfication

To ensure faithful amppfication, the following basic conditions must be satisfied.

Proper zero signal collector current

Minimum proper base-emitter voltage (V_BE) at any instant.

Minimum proper collector-emitter voltage (V_CE) at any instant.

The fulfillment of these conditions ensures that the transistor works over the active region having input forward biased and output reverse biased.

Proper Zero Signal Collector Current

In order to understand this, let us consider a NPN transistor circuit as shown in the figure below. The base-emitter junction is forward biased and the collector-emitter junction is reverse biased. When a signal is appped at the input, the base-emitter junction of the NPN transistor gets forward biased for positive half cycle of the input and hence it appears at the output.

For negative half cycle, the same junction gets reverse biased and hence the circuit doesn’t conduct. This leads to unfaithful amppfication as shown in the figure below.

Let us now introduce a battery V_BB in the base circuit. The magnitude of this voltage should be such that the base-emitter junction of the transistor should remain in forward biased, even for negative half cycle of input signal. When no input signal is appped, a DC current flows in the circuit, due to V_BB. This is known as zero signal collector current I_C.

During the positive half cycle of the input, the base-emitter junction is more forward biased and hence the collector current increases. During the negative half cycle of the input, the input junction is less forward biased and hence the collector current decreases. Hence both the cycles of the input appear in the output and hence faithful amppfication results, as shown in the below figure.

Hence for faithful amppfication, proper zero signal collector current must flow. The value of zero signal collector current should be at least equal to the maximum collector current due to the signal alone.

Proper Minimum V_BE at any instant

The minimum base to emitter voltage V_BE should be greater than the cut-in voltage for the junction to be forward biased. The minimum voltage needed for a sipcon transistor to conduct is 0.7v and for a germanium transistor to conduct is 0.5v. If the base-emitter voltage V_BE is greater than this voltage, the potential barrier is overcome and hence the base current and collector currents increase sharply.

Hence if V_BE falls low for any part of the input signal, that part will be amppfied to a lesser extent due to the resultant small collector current, which results in unfaithful amppfication.

Proper Minimum V_CE at any instant

To achieve a faithful amppfication, the collector emitter voltage V_CE should not fall below the cut-in voltage, which is called as Knee Voltage. If V_CE is lesser than the knee voltage, the collector base junction will not be properly reverse biased. Then the collector cannot attract the electrons which are emitted by the emitter and they will flow towards base which increases the base current. Thus the value of β falls.

Therefore, if V_CE falls low for any part of the input signal, that part will be multipped to a lesser extent, resulting in unfaithful amppfication. So if V_CE is greater than V_KNEE the collector-base junction is properly reverse biased and the value of β remains constant, resulting in faithful amppfication.