NEC2207 Experiment 3 - Common Collector Amplifier PDF

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OBJECTIVESAt the end of this experiment, the learner should be able to:1. Compute the DC and AC parameters for a Common Collector (CC)Amplifier with VDBC.2. Build a CC amplifier and measure the DC and AC parameters.3. Analyze the faults and predict the results in a CC Amplifier.BACKGROUND INFORMATIO...

Description

NEC322

Experiment No. 3

Elect 2 Lab.

OBJECTIVES At the end of this experiment, the learner should be able to:

1. Compute the DC and AC parameters for a Common Collector (CC) Amplifier with VDBC. 2. Build a CC amplifier and measure the DC and AC parameters. 3. Analyze the faults and predict the results in a CC Amplifier.

BACKGROUND INFORMATION 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 3.1(a). The ac output voltage almost perfectly duplicates the input voltage waveform. Although this means that the voltage gain is approximately equal to 1, the current gain is not; hence, the emitterfollower 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 a CC amplifier.

1

Figure 3.1 Voltage divider bias can be used as illustrated in Figure 3.1(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 3.1(b). This techniques is common to power amplifiers with push-pull output stages (Experiment No. 4) but can not be used if the emitter follower is capacitively coupled. The procedure for finding the DC parameters with VDBC is similar to CE amplifier. The steps are illustrated in lecture Module 2 The AC parameters for the amplifier can now be analyzed. The equivalent AC circuit is illustrated in Figure 3.2. The steps for the analysis are:. 1. Replace all capacitors with a short circuit. Compute the ac resistance of the emitter r’e from the equation EquaEquati on 3.1

r 'e =

25 mV IE

2. Compute the amplifier’s voltage gain. 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 is based on the voltage divider equation.

Equation 3.2

3. Compute the total input resistance seen by the ac signal:

Equation 3.3

4. Compute the amplifiers power gain. In this case, we are interested only in the power delivered to the load resistor. The output power is V 2/R .outThe Linput power is Vin2 / Rin(total). Since the voltage gain is approximately equal to 1, the power gain can be expressed as a ratio of Rin(total) to RL.

Equation 3.4

It is emphasized that the previous equations were developed for a particular configuration of the CC amplifier; 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.

Figure 3.1

MATERIALS 2 – 1 k 1 – 10 k 1 – 33 k 1 – 2N3904 NPN transistor

1 – 1 µF 1 – 10 µF 1 – 10k potentiometer

PROCEDURE 1.

Measure and record the resistance of the resistors listed in Table 3.1

Table 3.1

Resisto r

Listed Value

R1 R2 RE RL

10 k 33 k 1 k 1 k

Measured Value 10 k 33 k 1 k 1 k

Figure 3.3

2. Compute the dc parameters listed in Table 3.2 for the CC amplifier as shown in Figure 3.3. Compute VCE by subtracting VE from VCC. Enter your computed values in Table 3.2 Table 3.2

DC Parameter

Computed Value

Measured Value

VB

7.674 V

7.565 V

VE IE VCE

6.648 V

6.648 V

6.65 mA

NA

3.352 V

3.350 V

Table 3.3

AC Parameter

Computed Value

Measured Value

Vb Vc r’e AV

1 Vpp 0.993 Vpp 3.76  0.993 7.01 k 13.92

1 Vpp 0.985 Vpp NA 0.985 7.01 k 13.92

Rin(total) Ap

3. Construct the circuit shown in Figure 3.3. The signal generator should be turned off. Measure and record the DC voltages listed in Table 3.2. Your measured and computed values should agree within 10%. 4. Compute the ac parameters listed in Table 3.3 Assume V b is the same as the source voltage Vs. Use the methods in the background information to compute the remaining values. 5. Turn on the signal generator and adjust Vs for a 1.0 Vpp signal 1 kHz. Use the oscilloscope to set the proper voltage and check the frequency. Measure the ac signal voltage at the transistor’s emitter, V out, and determine the voltage gain, AV. Measure Rin(total) using the method employed for the CE amplifier. Use the measured Rin(total) and RL to determine the measured power gain as described in the Background Information. 6. Use the 2-channel oscilloscope, compare the input and output waveforms. What is the phase relationship between vin and vout? 7. Replace RL with a 10 k variable resistor set to 1 k. 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 cutof clipping because the transistor is turned off. If the positive peaks are

NEC322

Experiment No. 3

Elect 2 Lab.

clipped, this is called saturation clipping because the transistor is fully conducting. What types of clipping is first observed? Performing the procedure in number 7, which is replacing the 10 kto 1 kthen connecting the  oscilloscope probe to the emitter. Raising the signal amplitude, it can be seen that the negative peaks are clipped, so the types of clipping first observed is cut- of clipping because the transistor is turned of.

8. Vary RL while observing the output waveform. Describe your observation.

Varying RL while observing the output waveform, it can be observed that there were no changes in increasing, just change upon decreasing so it means that the negative value was clipped of.

9. Test the effect of VCC on the clipping level by varying the power supply voltage. Describe your observation.

In testing the efect of VCC on the clipping level by varying the power supply voltage, it can be observed that there was no distortion in the waveform or it was simply a normal waveform.

7

NEC322

Experiment No. 3

V B =V CC V B =10

(

R2 R2 + R1

)

Re (r 'e + Re ) 500 500 AV = = 503.76 (3.76+500) A V =0.993 AV =

( 10 33k +k33 k )

V B =7.674 V V CE =V CC −V E V CE =10 V −6.648 V V CE =3.352V

I e=

Ve

=

AV V b

Re Re (0.993 )(1) I e= 500 I e=1.986 mA

β dc =163.73 I B=40.372 µF I C =β I B −5

I C =(163.73 )( 4.0372×1 0 ) I C =6.61 mA

I E =I C + I B I E =6.61mA + 40.372 µA I E =6.65 mA

Elect 2 Lab.

β ac =163.33 R¿(base)=β ac R e

R¿(base)=(163.33)( 500) R¿(base)=81.67 k R¿(total)=R1∨| R2|∨R¿(base) R¿(total)=10 k ∨|33 k|∨81.67 k R¿(total)=7.01 k

25 mV IE 25 mV r 'e = 6.65 mA r 'e =3.759 ∨3.76

I e=1.986 mA V¿ 1V = I¿= R¿ ( total) 7.01 k I ¿ =142.653 µA

Re =R E | | R L

A

r 'e =

Ie

1.986 mA 8

QUESTIONS AND PROBLEMS 1. Compare the input resistance of the CC amplifier in this experiment with the CE amplifier. What is the major contributing factor to their differences? The major contributing factor to the diferences between the CC amplifier and CE amplifier was the place or location of load resistance. For the CC amplifier it was placed on the collector while the for the CE amplifier, the load resistance was placed on the emitter.

2. Compare the phase you observed between the input and output voltage for the CC and CE configuration

The phase where I had been observed the input and output voltage for the CC configuration and CE configuration was for the CC voltage gain is less than one. However, for the CE voltage gain was high.

3. In step 8, you observed the effect of clipping as RL was varied. What type of clipping occurs when RL was made very small?

Based on the procedure number 8, it can be observed that the efect of clipping as RL was varied. The type of clipping occurs when RL was made very small was the Cut-of clipping.

4. What effect does an increase in Vcc have on: (a).saturation clipping (b). cut-off clipping

The efect of an increase in VCC have on the saturation clipping and cut-of clipping were depends on the RL and AC voltage value. Hence, there were still no changes between the two.

5. The emitter-follower can be used to drive a low impedance load such as a loudspeaker. What characteristics of the emitter follower makes this effective?

The emitter-follower, EF, also called commoncollector, CC, amplifier provides nearly unity voltage gain, and current gain, which can be large, and low output resistance. Emitter-follower amplifiers are commonly used as output stages that are capable of driving low impedance loads due to their current gains and low output resistances.

6. Assume that the circuit in Figure 3.3 has a shorted capacitor, C2. What effect would this have on the dc emitter voltage, VE?

When assuming that the circuit in Figure 3.3 has a shorted capacitor, there would be no efect on the dc emitter voltage. The emitter bias capacitor that is C2 is shown in the figure gives a short circuit to the ac signal about the resistance of emitter which keeps the emitter at ac ground which we discussed above. With the bypass capacitor, the gain value of the amplifier is extreme and equals to the RC/r’e.

EXPERIMENTAL DISCUSSION This experiment entitled, “The Common Collector _ Amplifier”, it is 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 diferent impedance levels. _

_

Through the use of multisim, we measured the following _ needed value for the computations and input all of these in the _ tables above. Based on the experiment, the common collector or grounded collector configuration is generally used where_a _ high impedance input source needs to be connected to a low _ impedance output load requiring a high current gain. It can _ _ provide a reasonable level of voltage gain but sufers 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.

_ _ It is used primarily as an impedance bufer 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. _ _

CONCLUSION Through conducting this experiment with the use of multisim software, we were able to meet the following objective of this experiment such as to compute the DC and AC parameters for a Common Collector (CC) Amplifier with VDBC; Build a CC amplifier and measure the DC and AC parameters; and Analyze the faults and predict the results in a CC Amplifier. According to the Electronics Tutorials (n.d.), The Common Collector Amplifier is another type of bipolar junction transistor, (BJT) configuration where the input signal is applied to the base terminal and the output signal taken from the emitter terminal. Thus the collector terminal is common to both the input and output circuits. This type of configuration is called Common Collector, (CC) because the collector terminal is efectively “grounded” or “earthed” through the power supply. In many ways the common collector (CC) configuration is the opposite of the common emitter (CE) configuration, as the connected load resistor is moved from the usual collector terminal, labeled RC, to the emitter terminal where it’s is labeled RE. In Common Collector Amplifier, Input is applied to B-C Junction and Output is taken from E-C terminal, here Collector terminal is common for both input and output. It is also known as Voltage Follower and it is moderately used amplifier circuit because it has a good current gain but the voltage gain is unity. And it has very high input impendence very low output impendence making it ideal for being used as voltage bufer amplifier. It is used for driving heavy loads because the current gain is very high and it_ can drive any high resistive circuit.

REFERENCES (APA Format): Books Spring,A. (2013). Analog Electronics. Common-Base and Emitter Follower (Common Collector) Transistor Amplifiers. Dokuz Eylul University. Department of Electrical and Electronics Engineering. Fiore,J.(2016).Electrical Engineering.Common Amplifier.Mohawk Valley Community College.

Collector

Internet Source Electronics Tutorial. (n.d.). Common Collector Amplifier. Retrieved March 5, 2021 from https://www.electronicstutorials.ws/amplifier/common-collector-amplifier.html Mercer, D. (January 2018). Class A NPN Emitter-Follower Amplifier. Retrieved March 5, 2021 from https://wiki.analog.com/university/courses/engineering_dis covery/lab_11

Instructor’s Initial: Date Performed: 3/05/2021

REFERENCE: Floyd, Thomas, Electronic Devices 9th Edition Buchla, David, Experiments in Electronics and Electric Circuit Fundamentals 4th Edition...