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Applied Electronics : Laboratory Instructions:

Experiment Number 04

Study on frequency response analysis of BJT Common Emitter Amplifier.

Aim: To design and construct BJT Common Emitter Amplifier (self bias) To measure the gain and to

plot the frequency response.

Circuit Diagram:

Apparatus required:

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Applied Electronics : Laboratory Instructions:

Theory:

Voltage divider bias (Self bias)

A combination of fixed and self-bias can be used to improve stability and at the same time overcome

some of the disadvantages of the other two biasing methods. One of the most widely used combination-bias systems is the voltage-divider type. The voltage divider is formed using external resistors R1 and R2.

The voltage across R2 forward biases the emitter junction. By proper selection of resistors R1 and R2, the

operating point of the transistor can be made independent of. In this circuit, the voltage divider holds

the base voltage fixed independent of base current provided the divider current is large compared to the

base current. However, even with a fixed base voltage, collector current varies with temperature (for

example) so an emitter resistor is added to stabilize the Q-point, however, to provide long-term or DC

thermal stability.

MeritsUnlike above circuits, only one dc supply is necessary.

Operating point is almost independent of variation.

Operating point stabilized against shift in temperature.

Design of RC and RE

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Applied Electronics : Laboratory Instructions:

Design of R1 and R2:

[

Design of Input capacitor:

Experiment Procedure:

Connect the circuit as per the circuit diagram Set Vin = 50mV in the signal generator. Keeping input voltage constant, vary the frequency from 1Hz

to 1MHzin regular steps. Note down the corresponding output voltage. Plot the graph: Gain in dB Vs Frequency in Hz. Calculate the Bandwidth from the Frequency response graphTo plot the Frequency Response

The frequency response curve is plotted on a semi-log scale. The mid frequency voltage gain is divided by 2 and these points are marked in the frequency

response curve.

The high frequency point is called the upper 3dB point. The lower frequency point is called the lower 3dB point. The difference between the upper 3dB point and the lower 3dB point in the frequency scale gives the

bandwidth of the amplifier.

From the plotted graph the bandwidth is obtained. (i.e) Bandwidth = F H - FL

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Applied Electronics : Laboratory Instructions:

Model Graph:

Tabulation for Frequency response:

Frequency Vo Gain AV = V0 / Vi Gain in dB= 20 Log AV

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Applied Electronics : Laboratory Instructions:

Results:

1. AMAX=

2. AMAX/ 2 =

3. FH =

4. FL =

5. Bandwidth = FH - FL =

Signature of authorization: