Input Impedance of CE Amplifier

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Input Impedance of CE Amplifier

When one CE amplifier is being used to drive another, the input impedance of the second amplifier will serve as the load resistance of the first.

Therefore, in order to calculate the voltage gain (Av) of the first amplifier stage correctly, we must calculate the input impedance of the second stage. The input impedance of an amplifier can be find by using the ac equivalent circuit of the amplifier as shown in Fig. 1.

Zin = R1 || R2 | | Zin (base)

Where,

Zin = input impedance of the amplifier

Z_{in (base)}= input impedance of transistor base

Now

Z_{in (base)}= *β r_e′ The input impedance [Z_in] is always less than the input impedance of the base [Z_in(base)].

Voltage Gain Stability

One important consideration for an amplifier is the stability of its voltage gain. An amplifier should have voltage gain values that are stable so that the output of the circuit is predictable under all normal conditions. In a standard CE amplifier, the entire d.c. emitter resistance R_E is bypas by the bypass emitter capacitor CE. Therefore, the total a.c. emitter resistance is r′_e. The voltage gain of such an amplifier at no-load is given by ;

Voltage gain, Av =R_C/r′_e where r′_e =25mV/I_E

The voltage gain of a standard CE amplifier is quite large. However, the drawback of the circuit is that its voltage gain changes with emitter current IE, temperature variations and transistor replacement. For example, if emitter current IE increases, the a.c. emitter resistance r_′e decreases. This changes the voltage gain of the amplifier. Similarly, when the temperature varies or when a transistor is replace, the a.c. current gain β changes.

This will also result in the change in voltage gain. In order to stabilise the voltage gain, the emitter resistance RE is partially bypass by CE. Such an amplifier is call a swamped amplifier.

Fig. 2 (i) shows the emitter leg of a standard CE amplifier while Fig. 2 (ii) shows the emitter leg of swamped amplifier. In swamped amplifier, the resistance RE is split into two parts viz. R_E1 and R_E2. Only R_E2 is bypass by CE while R_E1 is not.