06-08-2013, 12:27 PM
COMMON EMITTER AMPLIFIER WITH FIXED BIAS
Aim
To design and construct BJT Common Emitter Amplifier using fixed bias .
To measure the gain and to plot the frequency response and to determine the Gain
Bandwidth product (GBW).
Demerits:
In this circuit, to keep IC independent of β the following condition must be met:
which is approximately the case if ( β + 1 )RE >> RB.
As β-value is fixed for a given transistor, this relation can be satisfied either by keeping
RE very large, or making RB very low.
If RE is of large value, high VCC is necessary. This increases cost as well as precautions
necessary while handling.
If RB is low, a separate low voltage supply should be used in the base circuit. Using two
supplies of different voltages is impractical.
In addition to the above, RE causes ac feedback which reduces the voltage gain of the
amplifier.
Usage: The feedback also increases the input impedance of the amplifier when seen from the
base, which can be advantageous. Due to the above disadvantages, this type of biasing circuit is
used only with careful consideration of the trade-offs involved.
Procedure
1) Connect the circuit as per the circuit diagram
2) Set Vin = 50mV in the signal generator. Keeping input voltage constant, vary the
frequency from 1Hz to 1MHzin regular steps.
3) Note down the corresponding output voltage.
4) Plot the graph: Gain in dB Vs Frequency in Hz.
5) Calculate the Bandwidth from the Frequency response graph
To plot the Frequency Response
1) The frequency response curve is plotted on a semi-log scale.
2) The mid frequency voltage gain is divided by √2 and these points are marked in the
frequency response curve.
3) The high frequency point is called the upper 3dB point.
4) The lower frequency point is called the lower 3dB point.
5) The difference between the upper 3dB point and the lower 3dB point in the frequency
scale gives the bandwidth of the amplifier.
6) From the plotted graph the bandwidth is obtained. (i.e) Bandwidth = fH - fL