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**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 3Kâ„¦.

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**

- Current gain is less than unity and its magnitude decreases with the increase of load resistance R
,_{L} - Voltage gain A
is high for normal values of R_{V},_{L} - The input resistance R
is the lowest of all the three configurations, and_{i } - The output resistance R
is the highest of all the three configurations._{o }

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

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

**Characteristics of Common Collector Amplifier**

- For low R
(< 10 kâ„¦), the current gain A_{L}is high and almost equal to that of a CE amplifier._{i } - The voltage gain A
is less than unity._{V } - The input resistance is the highest of all the three configurations.
- 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**

- The current gain A
is high for R_{i }< 10 kâ„¦._{L} - The voltage gain is high for normal values of load resistance R
_{L.} - The input resistance R
is medium._{i} - The output resistance R
is moderately high._{o}

*Applications*: CE amplifier is widely used for amplification.

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