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Common Emitter Amplifier

Sayed Taher Zewari

ECE 334- 201

Lab No. 7

09/28/00

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Common Emitter Amplifier

I. IntroductionThe purpose of this is to investigate the operation of a common-emitter NPN transistor amplifier.

II. Background InformationAmplifiers: Amplifier is a circuit that is capable of increasing the magnitude level of a signal. Amplifiersare divided into three groups based on the configuration. The configurations of a single stage transistoramplifier are:

Common Emitter Amplifier (CE)Common Collector Amplifier (CC)

Common Base Amplifier (CB)

The Common Emitter configuration:

This configuration of the amplifier is the most used of the three. It gives both voltage gain and currentgain greater than unity and the input and output impedances can have medium values compared to the

other configurations. D

The Common Base configuration:

In this configuration, the current gain is less than unity and the voltage gain is high around that of thecommon emitter configuration gain. This configuration has the lowest input impedance and the highest

output impedance of the three configurations mentioned above. It has few applications because there isnot advantage at low and medium frequencies of the common base configuration. The main advantage ofthe common base configuration is that it has a better performance in high frequencies.

The Common Collector configuration:

In this configuration, the current gain is high around that of the common emitter configuration and the

voltage gain is less than unity. The input impedance is the highest and the output impedance is the lowestof the three configurations mentioned above. It is also called emitter follower and has wide applications

as a buffer between high impedance and low impedance stages.

Input Resistance: The input resistance is the resistance seen by the current source or voltage source thatdrives the circuit.Output Resistance: is an indication of a source's ability to drive load impedance. An ideal voltage

source has zero output resistance, and an ideal current source has infinite output resistance.

III. Materials UsedThe following materials were used in this lab: four resistors of 100 k, 47 k, 1.258 k and 32k. DCvoltage sources of 12 V, multimeters, banana to alligator wires, resistor substitution box, function

generator, oscilloscope, and the bread board on the Heathkit Trainer.

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IV. Procedure1. Determine the values of , VT, gm, r, and RB, Rc.2. Once the above values determined, using a thevenin equivalent circuit find the value of Re.3. When the value of Rc, Re determined, setup the following circuit:

R1

100k

V4

FREQ = 1000

VAMPL = .010

VOFF = 0

0

V2

12Vdc0

V

C6

200u

C4

200u

Re

3300

C7

200u

0

Rc

1650

R2

47k

R3

1kVQ1

Q2N2222

4. Supply dc voltage of 12 V.5. Using function generator, supply an input voltage. Select any desired voltage level and frequency.6. Use the capacitors, as indicated in the circuit, to isolate the amplifier circuit.7. Measure the gain that is the Av = Vo / Vi.Input Resistance:

8. Connect a variable resistance in series with the circuit and voltage supply.9. Adjust the variable resistor until the output is half of that of the input.10.Measure the resistance of the variable resistor. That is the input resistance.Output Resistance:

11.Connect a variable resistance in parallel with the circuit12.Adjust the variable resistor until the output is half of that of the input.13.Measure the resistance of the variable resistor. That is the output resistor.

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V. ResultsRe = 3299.84 Rc = 1650 for Ic = 0.75 mA gain = 50

Re = 3299.84 Rc = 1650 for Ic = 1 mA gain = 37

Input resistance: 5.2 k This is usually close to the value of r.Output resistance: 1.3 k This is close to the value of Rc.

VI. ConclusionThe data above indicates that the determining factor in performance of the amplifier is the value of the

resistor Rc. If Rc is low the gain is low; and when it is high the gain is also high. When the couplingcapacitor is removed the output dramatically drops below that of the input. Its gain is obviously zero.When the coupling capacitor is removed and the load is directly connected to the collector, the gain drops

significantly.