Submitted to - Valencia Collegefd.valenciacollege.edu/file/mejaz/Sample Lab Report - 05... · 2020. 4. 28. · Q2N3904 R1 1K R2 18K R3 18K 0 Vcc 10V C2 10uF RL 100K Q2 Q2N3904 RE2

EET 4301L – Electronics II

Summer 2014

Experiment # 2

BJT Emitter Follower

Performed By:

Masood Ejaz

Submitted to:

Dr. XXXX YYYY

Department of Electrical & Computer Engineering Technology

Division of Engineering, Computer Programming, & Technology

Valencia College

Introduction and Purpose:

BJT Emitter Follower, which is also called BJT Common Collector amplifier, acts as a voltage

buffer when used between a source and a load. Overall voltage gain of the amplifier is very

close to one but it provides a large current gain, thus fulfilling the requirements for a variety of

loads without producing pronounced loading effect. In this experiment a BJT Emitter Follower

circuit was built and examined for DC biasing points, small-signal voltage gain, and small-signal

input output resistances. Next, a cascaded circuit with BJT Common Emitter amplifier followed

by a BJT Emitter Follower was built and examined for the overall voltage gain for different loads

to confirm the buffering characteristics of the Emitter Follower circuit.

List of Parts:

1. BJT 2N3904 (2)

2. Resistors: 18K (2), 10K (4), 47K (1), 470 (2), 4.7K (1), 220 (1), 100K (1) , 1K (3),

potentiometer 10K

3. Capacitors: 10F (3), 100F (1)

Procedure and Results:

All of the important procedural steps [1] are briefly given before their respective results.

1. BJT Emitter Follower circuit, as shown in figure 1, was built and DC bias points were

measured.

IB IC IE DC VBE VCE VE

25uA 4.225mA 4.25mA 169 0.7V 5.75V 4.25V

As it can be seen that output voltage (VE) is biased roughly around the center of the DC

limits (0-10V)

Figure 1: BJT Emitter Follower

2. A sinusoidal source is introduced, as shown in figure 2, and output voltage is measured for

two very different loads; 220 and 100K

Figure 2: BJT Emitter Follower with sinusoidal source and load

Q1

Q2N3904

R1

1K

R2

18K

R3

18K

0

Vcc

10V

Q1

Q2N3904

R1

1K

R2

18K

R3

18K

0

Vcc

10V

V1

FREQ = 10KHzVAMPL = 100mV

VOFF = 0

AC =

C1

10uF

C2

10uF

RL

100K

V

V

RL = 220

Figure 3: Output for RL = 220: (a) Lab Output (b) Simulation Output

RL = 100K

Figure 4: Output for RL = 100K: (a) Lab Output (b) Simulation Output

(a) Oscilloscope

(b) PSpice

(b) Oscilloscope

(b) PSpice

Observe that the output voltage for 220 is a little smaller than the input voltage but for

100Kit is almost equal to the input voltage. This shows that variation of load does not

bring any drastic change in the output voltage; hence, it works well as a voltage buffer.

3. Next, small-signal input and output resistances for the circuit were measured. To measure

the small-signal input resistance, a variable resistor was inserted between the input source

and DC-blocking capacitor C1 and value of the resistor was changed until the output voltage

became half of the voltage observed in step 2. The corresponding value of the variable

resistor is the approximate value of the input resistor. Likewise to measure the output

resistance, a variable resistor was connected in parallel to the load and its value was

changed until output became half of the value observed in step 2. The corresponding value

of the variable resistor is the approximate value of the output resistor. For both

measurements, a load of 100K was used.

rin (observed) = 8.5K

(a)

(b)

Figure 5: (a) Circuit to measure the small-signal input resistance (b) Input (blue) and output

(red) waveforms [Note: Waveform should be the one saved from the lab]

Q1

Q2N3904

R1

1K

R2

18K

R3

18K

0

Vcc

10V

V1

FREQ = 10KHzVAMPL = 100mV

VOFF = 0

AC =

C1

10uF

C2

10uF

RL

100K

R4

8.5K

V

V

rout (observed) = 7 (approximately)

(a)

(b)

Figure 6: (a) Circuit to measure the small-signal input resistance (b) Input (blue) and output

(red) waveforms

4. Next, a BJT Common Emitter amplifier was built, as shown in figure 7, and output was

measured for the two loads used earlier. Input voltage used was a sinusoid with 25mV peak

and 10KHz frequency.

Figure 7: BJT Common Emitter Amplifier

Q1

Q2N3904

R1

1K

R2

18K

R3

18K

0

Vcc

10V

V1

FREQ = 10KHzVAMPL = 100mV

VOFF = 0

AC =

C1

10uF

C2

10uF

RL

100KR47

V

V

Q1

Q2N3904

RE2470

R1

47K

R2

10K

0

Vcc

10V

V1

FREQ = 10KHzVAMPL = 25mV

VOFF = 0

AC =

C1

10uF

C2

10uF

RC

4.7K

C3

100u

RL

100K

RE1

470

V

V

Output from Lab Oscilloscope

RL = 100K

Figure 8: Output for RL = 100K: (a) Lab Output (b) Simulation Output. Input (blue) and output

(red) waveforms

RL = 100K

Figure 9: Output for RL = 220: (a) Lab Output (b) Simulation Output. Input (blue) and output

(red) waveforms

(a) Oscilloscope

(b) PSpice

(b) Oscilloscope

(c) PSpice

Observe from figure 8 and figure 9 how drastically the value of the output voltage and

hence the overall gain of the system has dropped down once the load is changed from a

very high value (100K) to a very low value (220). In fact for 220 resistor, it is no longer

gain, it is attenuation. This is because of the output loading of the circuit as the common

emitter circuit has relatively large small-signal output resistance that produces a significant

drop once a small load is connected and a large current is flowing through it [2].

5. The last step is to connect BJT Emitter Follower that was designed in step 1 between the

Common Emitter Amplifier and the two loads and observe that how it produced a voltage

buffer between the source and the load to kept almost a constant output voltage.

Figure 10: BJT Common Emitter Amplifier Followed by Emitter Follower

RL = 100K

Figure 11: Output for RL = 100K: (a) Lab Output (b) Simulation Output. Red waveform is output

and Blue is input

Q1

Q2N3904

R1

1K

R2

18K

R3

18K

0

Vcc

10V

C2

10uF

RL

100K

Q2

Q2N3904

RE2470

R4

47K

R5

10K

V1

FREQ = 10KHzVAMPL = 25mV

VOFF = 0

AC =

C3

10uF

C4

10uF

RC

4.7K

C5

100u

RE1

470

V

V

(a) Oscilloscope

(b) PSpice

RL = 220

Figure 12: Output for RL = 220: (a) Lab Output (b) Simulation Output. Red waveform is output

and Blue is input

Output voltage as observed for RL = 100Kwas 150mV (peak) and RL = 220was 140mV

(peak). Waveforms are shown in figure 11 and figure 12. Notice that the voltage drop is

extremely small as compared to the output of BJT Common Emitter circuit where output for

the two loads was drastically different.

Discussion:

As learned in the biasing theory, it is quite important for the amplifier to be biased roughly in

the center of the output range such that the output amplified voltage can swing maximally and

equally above and below the biasing voltage. This was done in step # 1 of the procedure. For an

amplifier to have a gain of unity that does not change with the variation of load, small-signal

output resistance of the circuit should be considerably low such that if this amplifier is used as a

buffer between another amplifier and different loads, small signal output resistance will appear

in series with the load and voltage drop across the small-signal output resistance will be

negligible compared to the drop in load; hence constant gain and output voltage can be

maintained, as carried out in the last step of the procedure. This is due to the fact that the

small-signal output resistance of the emitter follower turned out to be only 7 in the

experiment.

(b) Oscilloscope

(b) PSpice

Conclusion:

The lab clarified the theory of emitter follower circuit and its use as a voltage buffer between a

source and a load. It was observed that the voltage gain of the emitter follower circuit is

roughly one and it makes an excellent voltage buffer such that the source voltage (Common

Emitter amplifier in this case) connected to the input of the emitter follower circuit provides a

constant voltage for a variety of loads connected to the output of the emitter follower circuit.

References:

1. Joe Brown, “BJT Emitter Follower,” in Microelectronic Circuits, 4th ed., Ed. New York:

McGraw-Hill, 2006, pp. 24-28.

2. Electronics.com, ‘BJT Common Emitter Amplifier’, 2014. [Online]. Available:

http://wwwelectronics.com/BJTCommonEmitter. [Accessed: 23- Jun- 2014].

Don’t forget to go over the next two pages for important information

Things to Observe and Remember When Writing a Lab Report

1. Nowhere in the report, I, we, us, you etc., i.e. first or second person active form is used. Lab

reports are recommended to be written in passive form.

2. All circuits built in the lab should be reproduced in Multisim or PSpice for the lab report.

This you have to do in Prelab. Prelab results are in the lab report just for the comparison

purpose against the actual results.

3. All important observations and answers to the questions asked in the lab book should be

either highlighted or written with a different color to make them stand out. There is no need

to create a separate question/answer section if you are answering all the questions under

Results or Discussion section. If some of the questions do not fit anywhere in the lab report,

create a separate section before Conclusion and answer them there. Make sure to write the

original question before you write its answer.

4. Observe that Discussion actually shows the characteristics of the circuits that were built in

the lab from theoretical and practical point of views. Theoretical discussion of the subject

matter shows your understanding about the expected results and practical discussion using

your results shows that if you were able to achieve your expected results or not. If not then

what were the reasons that you didn’t get your results. Although, discussion for the sample

report is brief but it does encompass all the important theoretical as well as practical points.

5. Conclusion emphasizes on the most important things learned or proved in the experiment. It

could be a very brief summary of your discussion highlighting most important points.

6. All figures should be labeled. Labeling of figures must go under the figure and label should

explain the figures in an efficient way.

7. All tables should have labeling on top of the table. Labeling should describe the purpose of

table.

8. All the formulae/equations/expressions should be written with equation editor, i.e.

professionally.

9. Make sure to capture all the waveforms from oscilloscope to include in your lab report under

Results section.

10. No hand-written/scanned work is allowed in the lab report unless it is extremely difficult to

reproduce it professionally for the lab report or your have received approval from your

instructor before writing the report.

11. Don’t forget references in IEEE format! Reference numbers should appear in the text in

square brackets for corresponding references from the list.