Experiment No. 9

DESIGN AND CHARACTERISTICS OF COMMON BASE AND COMMON

COLLECTOR AMPLIFIERS

1. Objective: The objective of this experiment is to explore the basic applications of the bipolar junction

transistor (BJT). A common base and a common collector amplifier will be designed and tested.

The frequency response will be measured and the DC voltages will be compared to calculated

values. In this experiment, transistor type 2N3904 is used.

2. Introduction: Common emitter amplifiers are the most widely used form of BJT in amplifier configurations. It is mostly used to provide reasonably high voltage gain as well as some power gain. However, another two amplifier configurations can also be used to provide some power gain, or to provide simple interface between various circuits with different impedance levels. The common-base amplifier can provide a reasonable level of voltage gain but suffers from low input impedance and a current gain of less than one. However, this circuit is used in high-frequency applications because its terminal characteristics at high frequencies are better than those of a common-emitter configuration using the same transistor. The low input impedance of the common base amplifier will limit its use to specialized RF applications. The common collector configuration, also typically known as emitter follower, is used primarily as an impedance buffer stage to prevent a low impedance load from loading a stage with relatively high output impedance. The emitter follower has a high input impedance and low output impedance, which is almost equal to the emitter resistance. The voltage gain of the emitter follower is less than one, but it has reasonably high current gain. Unlike the common emitter amplifier, the output voltage of the common collector amplifier is in phase with the input voltage.

3. Pre-Lab work:

Design the circuit shown in Figures 1 and 2 by calculating the values of R1, R2, R3 and R4.

2N3904 NPN transistor is to be used. In this design the bias voltage between emitter and

collector should be 5V DC. The quiescent collector current is 1mA. The design should provide

maximum possible voltage swing at the amplifier output.

Figure 1- Circuit diagram of the common base BJT amplifier configuration.

Figure 2- Circuit diagram of the common emitter amplifier configuration.

4. Lab work:

a. Connect the common base amplifier circuit you designed. Set the values of capacitors C1, C2, and C3 to 1µF each. Make sure the positive polarity of these capacitors are connected to the higher positive voltage in the circuit. Set RL to be 1 kΩ and the supply voltage to 15V DC, as shown in Figure 3. Measure the DC bias voltages on the base, emitter and the collector. Calculate the collector current. Compare the measured voltages with the design intent and calculation. Tabulate the measured versus the calculated bias voltages and current.

b. Measure the frequency response of the amplifier starting from 100 Hz. Change the test

frequency to cover the upper cut-off frequency of the amplifier. Through out the measurement of the frequency response, apply low input signal levels (in the order of few milli-Volts) to ensure that the out put signal is not distorted. Monitor both input and output waveforms on the oscilloscope.

Insert R5 = 200Ω between the source and the common base amplifier input to prevent excessive loading of the signal source by the fairly low input resistance of the amplifier. b.1 Plot the obtained frequency response. On the measured plot clearly indicate the lower and upper cut-off frequencies of the amplifier. b.2 What is the mid-band gain of the amplifier? c.3 Calculate the amplifier bandwidth.

Figure 3. Common base amplifier circuit under test.

c. If you know that the output (internal) resistance of the signal source is 50Ω at 1kHz

frequency, can you make some measurement to determine the input resistance of the common base amplifier at 1kHz frequency? Discuss your idea with the lab supervisor before doing the measurement.

d. Connect the common collector amplifier circuit you designed. Set the values of capacitors C1, C2, and C3 to 1µF each. Set RL to be 1 kΩ and the supply voltage to 15V DC, as shown in Figure 4. Measure the DC bias voltages on the base, emitter and the collector. Calculate the collector current. Compare the measured voltages with the design intent and calculation. Tabulate the measured versus the calculated bias voltages and current.

e. Measure the frequency response of the amplifier starting from 100 Hz. change the test

frequency to cover the upper cut-off frequency of the amplifier. Through out the measurement of the frequency response, apply low input signal levels (in the order of few milli-Volts) to ensure that the out put signal is not distorted. Monitor both input and output waveforms on the oscilloscope.

d.1 Plot the obtained frequency response. On the measured plot clearly indicate the lower and upper cut-off frequencies of the amplifier. d.2 What is the mid-band gain of the amplifier? d.3 Calculate the amplifier bandwidth.

5. Report:

Figure 4 - Emitter follower circuit under test.

In addition to reporting the data obtained in this experiment;

a. Comment on the differences between the intended bias voltages for the two amplifiers and the measured voltages. Why there is a difference? How can you reduce this difference when you calculate the bias resistors?

b. Compare the voltage gain and bandwidths for the three amplifier configurations, namely,

the common emitter, common collector and common base. Tabulate the numbers obtained through out this experiment and the previous experiment to quantify the comparison.