Simplified Common Emitter Hybrid Model - Electrical Engineering (EE) PDF Download

Simplified common emitter hybrid model:

In most practical cases it is appropriate to obtain approximate values of A _V , A _i etc rather than calculating exact values. How the circuit can be modified without greatly reducing the accuracy. Fig. 4 shows the CE amplifier equivalent circuit in terms of h-parameters Since 1 / h_oe in parallel with R_L is approximately equal to R_L if 1 / h_oe >> R_L then h_oe may be neglected. Under these conditions,

I_c = h_fe I_B .

h_re V_c = h_re I_c R_L = h_re h_fe I_b R_L .
 

                                                                Fig. 4

Since h_fe.h_re = 0.01(approximately), this voltage may be neglected in comparison with h_ie I_b drop across h_ie provided R_L is not very large. If load resistance R_L is small than h_oe and h_re can be neglected.
 

Output impedance seems to be infinite. When V_s = 0, and an external voltage is applied at the output we find I_b = 0, I _C = 0. True value depends upon R_S and lies between 40 K and 80K.

On the same lines, the calculations for CC and CB can be done.

CE amplifier with an emitter resistor:

The voltage gain of a CE stage depends upon h_fe. This transistor parameter depends upon temperature, ageing and the operating point. Moreover, h_fe may vary widely from device to device, even for same type of transistor. To stabilize voltage gain A_V of each stage, it should be independent of h_fe. A simple and effective way is to connect an emitter resistor R_e as shown in fig. 5. The resistor provides negative feedback and provide stabilization.
                   

An approximate analysis of the circuit can be made using the simplified model

Subject to above approximation A _V is completely stable. The output resistance is infinite for the approximate model

 Comparison of Transistor Amplifier Configuration

The characteristics of three configurations are summarized in Table. Here the quantities A_i, A_v, R_i, R_o and A_P are calculated for a typical transistor whose h-parameters are given in table .The values of R_L and R_s are taken as 3KΩ.

Table: Performance schedule of three transistor configurations.
 

The values of current gain, voltage gain, input impedance and output impedance calculated as a function of load and source impedances.

Characteristics of Common Base Amplifier

1. Current gain is less than unity and its magnitude decreases with the increase of load resistance R_L,
2. Voltage gain A_V is high for normal values of R_L,
3.  The input resistance R_i is the lowest of all the three configurations, and
4.  The output resistance R_o is the highest of all the three configurations.

Applications: The CB amplifier is not commonly used for amplification purpose. It is used for

1. Matching a very low impedance source
2. As a non inverting amplifier to voltage gain exceeding unity.
3.  For driving a high impedance load.
4. As a constant current source.

 Characteristics of Common Collector Amplifier

1. For low R_L (< 10 kΩ), the current gain A_i is high and almost equal to that of a CE amplifier.
2. The voltage gain A_V  is less than unity.
3.  The input resistance is the highest of all the three configurations.
4. The output resistance is the lowest of all the three configurations.

Applications: The CC amplifier is widely used as a buffer stage between a high impedance source and a low impedance load.

Characteristics of Common Emitter Amplifier

1. The current gain A_i is high for R_L < 10 kΩ.
2. The voltage gain is high for normal values of load resistance R_L.
3.  The input resistance R_i is medium.
4. The output resistance R_o is moderately high.

Applications: CE amplifier is widely used for amplification.