06-08-2013, 12:28 PM
COMMON EMITTER AMPLIFIER WITH SELF BIAS
Aim
To design and construct BJT Common Emitter Amplifier using voltage bias (self bias)
with and without bypassed emitter resistor.
To measure the gain and to plot the frequency response and to determine the Gain
Bandwidth product (GBW).
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 β. 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,
and at the same time, allow minimal ac signal degeneration, the bypass capacitor (Cbp) is placed
across R3. If Cbp is large enough, rapid signal variations will not change its charge materially and
no degeneration of the signal will occur.
Procedure
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