BIPOLAR JUNCTION TRANSISTOR -COMMON COLLECTOR AMPLIFIER

AIM:

To design a BJT common collector amplifier for the following specifications .To observe frequency response and cathode ray oscilloscope waveforms.

APPARATUS:

NI Multisim software version 1.1

S.No Equipment Value/Type Tolerance Quantity

1 BJT BC107BP - 1

2 AC Voltage 5mVpk,1kHz,0 - 1

3 DC Voltage 12V - 1

4 DC Ground - - 1

5 Cathode Ray

oscillscope

XSC1 5% 1

6 Resistor 100kohm 5% 1

10kohm 5% 2

47kohm 5% 1

7 Capacitor 0.1uF 5% 1

THEORY:

In electronics, a common-collector amplifier (also known as emitter follower )is one of three basic single stage bipolar junction transistor (BJT) amplifier topologies ,typically used as voltage buffer. In this circuit the base terminal of the transistor serves as the input. The emitter is the output and the collector is common to both(for example it may be tied to ground reference or a power supply rail),hence its name.

The circuit can be explained by viewing the transistor as being under the control of negative feedback. From this view point, a common collector stage amplifier is an amplifier with full series negative feedback. In this configuration, the entire output voltage Vout is placed contrary and in series with the input voltage Vin. Thus the two voltages are subtracted according to KVL and their difference is applied to the base emitter junction. The transistor monitors Vdiff and adjusts its emitter voltage almost equal to the input voltage by passing the according collector current through the emitter resistor Re. As a result the output voltage follows the input voltage variations from VBEO up to V+ ; hence the name ,emitter follower. Intuitvley ,this behavior can be also understood by realizing that the base-emitter voltage in the bipolar transistor is very insensitive to bias changes in base voltage is transmitted directly to the emitter. It depends slightly on various disturbances. Since the transistor react to these disturbances and restore the equilibrium. It never saturates even if the input voltage reaches the positive rail.

The common collector circuit can be shown mathematically to have a voltage gain of almost unity:

AV=Vout/Vin =1

PROCEDURE:

1. Rig up the circuit using multisim software. Find out the D.C conditions.2. Place multimeters across the transistor and find out the operating point.(the operating point

must be in the active region).3. Verify the results using cathode ray oscilloscope as shown beside.

Simulate->analysis->oscilloscope

4. Verify the results using AC analysis as shown beside

Simulate->analysis->AC Analysis

5. Plot the frequency response by entering the input output net names which are connected to the CRO.

6. With the help of traces find out the bandwidth of the output waveform from the frequency response curve.

PRECAUTIONS:

1. Ensure that the tolerances are considered for passive elements in the circuit .2. Make changes in the circuit only when the program is not in execution mode.3. Do save the program before executing it.

RESULT:

BJT common collector amplifier is designed for given specifications.

Frequency response is plotted and bandwidth is observed . Waveforms in the cathode ray oscilloscope are observed.