Transistor amplifier - Operating point, Working

Transistor amplifier

The important function of a transistor is the amplification. An amplifier is a circuit capable of magnifying the amplitude of weak signals. The important parameters of an amplifier are input impedance, output impedance, current gain and voltage gain. A good design of an amplifier circuit must possess high input impedance, low output impedance and high current gain.

Operating point

For the given values of the load resistance Rc and supply voltage Vcc, two points A (V_CC, 0) and B (0,Vcc/Rc) are located on the axes of V_CE and IC respectively, of the output characteristics         of the  transistor  (Fig  ).

Joining A and B, load line AB is obtained. The point of  intersection Q of  this line  in the active  region of  the  output  characteristics with a suitable value of the base current IB, such that the output voltage is symmetrical is called operating point or quiescent point for the amplifier. I_B(Q) is the input base current at the operating point. V_CE(Q) and I_C(Q) are the collector to emitter voltage and the collector current respectively at the operating point.

Working

A basic circuit of an amplifier in common emitter mode with NPN transistor is shown in Fig. The emitter-base junction is forward biased by a supply voltage V_BB. The input ac signal to be amplified is applied between base and emitter of the transistor. R_C is the load resistance.

The amplifying action of a transistor can be explained as follows. When the a.c. signal is not applied, the base current is available in small quantity in microamperes, which is represented by OP and the corresponding collector current in milliamperes is represented by PQ (Q is the operating point). When the ac signal voltage is applied, the potential difference between the base and emitter changes continuously. This results in increase of base current (I_B) from OP to OA, then decrease of base current from OA to OP (during positive half cycles of he input a.c. voltage) and then to PB and once again increase from OB to OP (during negative half cycle of the input a.c. voltage) for each cycle of the input signal voltage. This variation in base current is reflected in the collector current as shown in Fig. The collector current (I_C) increases from PQ to AA₁, falls from AA₁ to BB₁and again increases from BB₁ to PQ. Thus a variation in the base current in micro amperes produces a corresponding variation in the collector current in milliamperes. This produces a corresponding potential difference across R_C. The increase of potential difference across R_c makes a decrease in the output voltage.

Therefore, there is always a phase reversal of 180^o between the input and output voltages in CE amplifier.