Common Collector Amplifier

Aim:

To study the common collector amplifier and find it’s Bandwidth.

Components:

Name	Quantity
Transistor BC107	1
Resistor 33 K, 3.3 K, 2.2K, 1K, 330K	1, 1, 1, 1, 1
Capacitor 10 F	2

Equipment:

Theory:

In common collector amplifier as the collector resistance is made to zero, the collector is at AC ground that is the reason for which the circuit is also called as grounded-collector amplifier or this configuration has voltage gain close to unity and hence a change in base voltage appears as an equal change across the load at the emitter, hence the name emitter follower is given to this circuit. In other words emitter follows the input signal. 

This circuit performs the function of impedance transformation over a wide range of frequencies with voltage gain close to unity. In addition to that, the emitter follower increases the output level of the signal. Since the output voltage across the emitter load can never exceed the input voltage to base as emitter-base junction would become back biased. Common collector state has a low output resistance, the circuit suitable to serve as buffer or isolating amplifier or couple to a load with large current demands.

Characteristics of CC Amplifier:

1. Higher current gain.
2. Voltage gain of approximately unity.
3. No current or voltage phase shift.
4. Large input resistance.
5. Small output resistance.

Circuit Diagram:

Procedure:

1. Connect the circuit as shown in the circuit diagram.
2. Set source voltage V_s = 50mV (say) at 1 KHz frequency using the function generator.
3. Keeping input voltage constant vary the frequency from 50 Hz to 1 MHz in regular steps and note down the corresponding output voltage.
4. Plot the graph: gain (dB) verses Frequency on a semi log graph sheet.
5. Calculate the bandwidth from the graph.

Expected waveform:

In the usual application, mid band frequency range are defined as those frequencies at which the response has fallen to 3dB below the maximum gain (|A| max). These are shown as f_L and f_H and are called as the 3dB frequencies are simply the lower and higher cut off frequencies respectively. The difference between higher cut off and lower cut off frequency is referred to as bandwidth ( f_H - f_L ).

Observation tables:

V_s = 50mV

Result:

Common Collector Amplifier is studied and its Bandwidth is calculated.

Viva Questions:

1. Why CC amplifier is known as emitter follower?

Ans: The common collector junction transistor amplifier is commonly called an emitter follower. The voltage gain of an emitter follower is just a little less than one since the emitter voltage is constrained at the diode drop of about 0.6 volts below the base . Its function is not voltage gain but current or power gain and impedance matching.

2. Mention the applications of CC amplifier?

Ans: The low output impedance allows a source with a large output impedance to drive a small load impedance; it functions as a voltage buffer. This configuration is commonly used in the output stages of class-B and class-AB amplifiers and  impedance matching

3. What are the differences between CE,CB and CC amplifier?

Ans:

1. In CC configuration we use to get the low output impedance where as in ce we use to get the high output impedence.
2. In CC amplifier we use to have the voltage gain equal to unity where as in ce amplifier we use to have the high voltage gain.
3. In CC amplifier there is high power gain which is used for impedance matching where as in ce amplifier due to the high voltage gain the impedance matching is less impossible.

4. Mention the characteristics of CC amplifier?

Ans: At low frequencies and using a simplified hybrid-pi model, the following small-signal characteristics can be derived. (Parameter   and the parallel lines indicate components in parallel.)

	Definition	Expression	Approximate expression	Conditions
Current gain
Voltage gain
Input resistance
Output resistance

5. What is gain BW product?

Ans: The gain–bandwidth product (designated as GBWP, GBW, GBP or GB) for an amplifier is the product of the amplifier's bandwidth and the gain at which the bandwidth is measured.

For devices such as operational amplifiers that are designed to have a simple one-pole frequency response, the gain–bandwidth product is nearly independent of the gain at which it is measured; in such devices the gain–bandwidth product will also be equal to the unity-gain bandwidth of the amplifier