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Laboratory Exercise No. 5.2

EE 01 (Electronic Circuits: Devices and Analysis)
Small Signal Amplifier; Common-Collector
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I. Introduction

Along with the common emitter and common base configurations, the common
collector amplifier is one of the three fundamental transistor amplifier topologies.
The base of a common collector amplifier, sometimes referred to as an emitter
follower circuit, is applied with the input signal, and the collector and emitter are
linked to a supply voltage and ground, respectively. The emitter provides the output.
The term "emitter follower" refers to a device in which the emitter voltage roughly
"follows" the base voltage with a voltage loss of 0.7 volts. As a result of the voltage
drop across the base-emitter junction, the output voltage is nearly identical to the
input voltage but has a little smaller amplitude.
A common collector amplifier's principal benefit is its high input impedance, which
makes it perfect for driving low impedance loads like speakers. It is appropriate for
use in audio applications since it also offers a high voltage gain.
The fact that it has a low current gain, however, means that it is less able to
amplify signals with high current levels. Furthermore, since the emitter follower
design doesn't offer phase inversion, it can't function as an inverting amplifier.

II. Learning Objectives:

After performing this experiment, you will be able to:
 Compute the dc and ac parameters for a common-collector amplifier with voltage divider
bias.
 Build a common-collector amplifier and measure the dc and ac parameters.
 Predict the effect of faults in a common-collector amplifier.
 III. Materials
Resistors:
Two 1.0 kΩ, one 10 kΩ, one 33 kΩ
Capacitors:
One 10 kΩ variable resistor
One 1.0 µF, one 10 µF
One 2N3904 NPN transistor (or equivalent)
For Further Investigation:
Materials from Experiment 38

IV. Theory

The common-collector (CC) amplifier (also called the emitter-follower) has the input signal applied to
the base and the output signal is taken from the emitter as shown in Figure 39-1(a). The ac output
voltage almost perfectly duplicates the input voltage waveform. Although this means the voltage gain is
approximately 1, the current gain is not; hence, the emitter-follower can deliver increased signal power
to a load. The CC amplifier is characterized by a high input impedance and a low output impedance. This
is the most important characteristic of the emitter-follower.

Voltage divider bias can be used as illustrated in Figure 39-1(a). Frequently, however, a CC
amplifier is used immediately following a voltage amplifier, and bias may be obtained through a
dc path connected to the previous stage, as illustrated in Figure 39-1(b). This technique is
common in power amplifiers with push-pull output stages (Experiment 41) but cannot be used if
the emitter-follower is capacitively coupled.
 The procedure for finding the dc parameters with voltage divider bias is similar to the
procedure described in Experiment 38 for the common-emitter amplifier. The steps for the circuit
illustrated in Figure 39-1(a) are:
1. Mentally remove capacitors from the circuit since they appear open to dc. This causes
the load resistor, RL, to be removed. Solve for the base voltage, VB, by applying the
voltage divider rule to R1, and R₂.'
2. Subtract the 0.7 V forward-bias drop across the base-emitter diode from V B, to obtain the
emitter voltage, VE.
3. The dc current in the emitter circuit is found by applying Ohm's law to R E. The collector
current is nearly equal to the emitter current, and the collector voltage is equal to VCC

The ac parameters for the amplifier can now be analyzed. The equivalent ac circuit is
illustrated in Figure 39-2. The analysis steps are:

 Mentally replace all capacitors with a short. Compute the ac resistance of the emitter, r e,
from the equation:

 Compute the amplifier's voltage gain. Voltage gain is the ratio of the output voltage
divided by the input voltage. The input voltage is applied across r e, and the ac emitter
resistance, whereas the output voltage is taken only across the ac emitter resistance.
Thus, the voltage gain is based on the voltage divider equation:

 Compute the total input resistance seen by the ac signal:

 Compute the amplifier's power gain. In this case, we are interested only in the power
delivered to the load resistor. The output power is Vout2/RL. The input power is Vin2/Rin(T).
Since the voltage gain is approximately 1, the power gain can be expressed as a ratio of
Rin(T) to RL:
 It is emphasized that the previous equations were developed for a particular
configuration of the emitter-follower circuit; that is, one with "stiff" voltage divider bias
and a separate load resistor. The formulas are valid only for the circuits for which they
were derived. You should not assume that these equations are valid for other
configurations.

V. Procedure
1. Measure and record the resistance of the resistors listed in Table 39-1.

2. Compute the dc parameters listed in Table 39-2 for the emitter-follower amplifier shown in
Figure 39-3. Compute VCE by subtracting VE from VCC. Enter your computed values in Table 39-
2.
3. Construct the amplifier shown in Figure 39-3. The signal generator should be turned
off. Measure and record the de voltages listed in Table 39-2. Your measured and
computed values should agree within 10%.
4. Compute the ac parameters listed in Table 39-3. Assume Vb, is the same as the
source voltage, VS. Use the procedure outlined in the Summary of Theory to
compute the remaining values.
5. Turn on the signal generator and set VS for 1.0 VPP at 1.0 kHz. Use the oscilloscope
to set the proper voltage and check the frequency. Measure the ac signal voltage at
the transistor's emitter, Vout, and determine the voltage gain, AV. Measure Rin(T) using
the method employed for the CE amplifier. (See Experiment 38, step 6.) Use the
measured Rin(T) and RL, to determine the measured power gain as described in the
Summary of Theory.
6. Use a two-channel oscilloscope, compare the input and output waveforms. What is
the phase relationship between Vin and Vout?
7. Table 39-4 lists some possible troubles with the CC amplifier. For each trouble listed,
predict the effect on the dc voltages. Then insert the trouble into the circuit and test
your prediction. Insert the open collector and open emitter troubles by removing the
transistor lead and measuring the voltages at the circuit. For each fault, describe the
effect on the ac output waveform (clipped, no output, etc.).

8. Replace RL with a 10 kΩ variable resistor set to 1.0 kΩ. Connect an oscilloscope

probe to the emitter. Raise the signal amplitude until you just begin to observe
clipping. If the negative peaks are clipped, this is called cutoff clipping because the
transistor is turned off. If the positive peaks are clipped, this is called saturation
clipping because the transistor is fully conducting. What types of clipping is first
observed?
9. Vary RL while observing the output waveform. Describe your observations.
10. Test the effect of VCC on the clipping level by varying the power supply voltage.
Describe your observations.

Conclusion:
 :
VI. Evaluation and Review Questions
1. Compare the input resistance of the common-collector amplifier in this
experiment with the common-emitter amplifier in Experiment 38. What is the
major factor contributing to their differences?

2. Compare the phase you observed between the input and output voltage for the
common-collector and common-emitter configurations.

3. In step 9, you observed the effect of clipping as RL was varied. What type of
clipping occurs when RL is made very small?

4. What effect does an increase in VCC have on:

(a) saturation clipping
(b) cutoff clipping

5. The emitter-follower can be used to drive a low-impedance load such as a

loudspeaker. What characteristic of the emitter-follower makes this effective?

6. Assume that the circuit in Figure 39-3 had a shorted capacitor, C₂. What effect
would this have on the dc emitter voltage, V ?

For Further Investigation:

The common-collector circuit can be biased from the collector of a common-

emitter circuit as illustrated in Figure 39-1(b). Construct the circuit shown using the
common-emitter amplifier from Experiment 38 by removing the load resistor and the
coupling capacitor. Compute and measure the dc and ac parameters for the circuit. How
does the addition of the common-collector amplifier affect the overall gain? Summarize
your results in a laboratory report.