LAB 6: BJT AMPLIFIER

LEARNING OUTCOMES:

In this lab, students design and implement single-stage BJT amplifiers and observe

amplitude and frequency response. Breadboard and the Analog Discovery Kit are to be

used. The students will use various tools and functions from the Waveform software to

perform measurement and plotting amplifier response. The students will compare the gain

and frequency response of the BJT amplifier and the MOSFET amplifier in Lab 5.

MATERIAL AND EQUIPMENT

Material Equipment

2N2222 npn BJT transistor Breadboard

Capacitors Multimeter

Resistors Analog Discovery

2N2907 pnp BJT transistor Digilent Waveform software

Curve tracer

PRE-LAB

Look at the handout and be familiar with the CE amplifier. You need to know how to

calculate the DC voltage, current and small-signal gain. Design an amplifier with a voltage

gain of -4, a collector current of 5 mA, bias the collector voltage to provide a maximum

output voltage swing (i.e., ½ of Vcc). Use a Vcc of +5 V from the power supply of the

Analog Discovery unit.

PROCEDURES

1. NPN transistor characteristics

Obtain 2N2222 Si npn from the lab instructor or from the technician in room 2C97. Measure

their βDC at VCE =5V using the Tektronix curve tracers. Use a scale of 0 - 10 mA for IC and 0

- 10 V for VCE. The step current for the base is 10uA. You need to capture the transistor

characteristics to your lab report. Label βDC on the curves. The normal βDC of 2N2222 is

between 100-200. Also use the information in Lab 5 BJT IV curve.

2. Common-Emitter amplifier

Construct the circuit shown in Figure 1. You can use either 1 uF for capacitor CB and 4.7 uF

for the capacitor CC.

A. Measure the DC currents and voltages: IB, IC, IE, VC, VB, VE. Remember to measure one

at a time and NO input AC signal. In your report, make a table and list the results from

your experiment and hand calculations. In your hand calculation, use β determined from

your IV curve. Assume VBE = 0.7V.

2

B. At this time, do NOT connect the load resistor RL yet. Generate an input signal from

AWG1 (100 mVpp, 1 KHz) and send the signal to the input of the amplifier. Observe the

input and output voltage waveforms using two probes of an oscilloscope (Channel 1 and

Channel 2). First observe the waveforms of the input signals that are before and after CB.

Note that how DC voltage at the base is preserved by using coupling capacitor. Then

observe the waveforms of signals Vc and Vo that are before and after CC. The waveform

at Vc has both AC and DC components and waveform at Vo is a pure AC signal because

of the coupling capacitor CC. Capture the waveforms at Vi, VB, Vc and Vo copy them to

your lab report. Find the voltage gain from the waveforms (Vo/Vi).

Figure 1: CE amplifier

C. Perform hand calculations for the output voltage and voltage gain. List the results from

your experiment, hand calculation as a table in your lab report.

D. Connect the load resistor RL = 1K to the amplifier. Read the output voltage from the

oscilloscope and find the voltage gain.

E. Connect a polarized capacitor CE =22uF to the circuit as shown in Figure 1 so that it is in

parallel with RE . At this time, the emitter is AC shorted to ground. Observe the output

waveform at Vc and Vo and copy it to your lab report. Explain why the waveforms are

distorted in your report. Once you have finished this step, recover the circuit connection

according to Figure 1 by removing CE and RL.

F. At the last step, you need to adjust the frequency of the input signal from the signal

generator. First you need to reduce the frequency and observe the voltage gain. The gain

will reduce after certain frequency. Find the frequency, fL, when the voltage gain

decreases to 70%. Then you need to increase the frequency and also find the input signal

frequency, fH, when voltage gain decreases to 70%. The amplifier bandwidth is defined as

fH-fL. Document the frequencies in your report.

G. Use “Sweep” function in the Arbitrary Waveform Generator to sweep the frequency of

the input signal and observe the response of the amplifier. Describe the response of the

amplifier.

H. Compare the results with the n-channel MOSFET amplifier in Lab 4.

I. Optional: Repeat the steps using 2N2907 (pnp) transistor. Comment on the differences

between npn and pnp amplifiers.

2N2222

+5V

To Channel 2 of

the oscilloscope RC

R2 RE

CC

CB

CE

To Channel 1 of the

oscilloscope

~ AWG1

R1 VC

VO VB

Vi RL

3

Scope sample npn BJT (2N2222) amplifier:

R1=10K, R2=3.9K, Rc=560, Re=150, 1KHz, 100mV, Vb=1.35V. Ve=0.709V, Vc=2.463V

4

Scope sample pnp BJT (2N2907) amplifier:

(R1=10K, R2=27K, RE=100, RC=560)

(VB=3.85V, VC=2.6V, VE=4.54V)

2N2907

+5V

To Channel 2 of

the oscilloscope

RE

R2 RC

CC

CB

CE To Channel 1 of the

oscilloscope

~ AWG1

R1

VC VO

VB

Vi

RL

5

REFERENCES

1. Sedra/Smith, “Microelectronic Circuits,” 6th

Edition, Oxford University Press, 2010.

2. Mark N. Horenstein, “Instructor’s Resource Manual, Microelectronic Circuit and

Devices,” 2nd

Edition, Prentice Hall, 1996.

3. “A Complete Analog Design Kit for the Price of a Textbook,” Diligent,

http://www.digilentinc.com/Products/Detail.cfm?NavPath=2,842,1018&Prod=ANALOG

-DISCOVERY

4. n-channel MOSFET specification: http://www.vishay.com/docs/91127/sihfd110.pdf

5. p-channel MOSFET specification: http://www.micrel.com/_PDF/mic94050.pdf

6. n-channel MOSFET http://www.onsemi.com/pub_link/Collateral/BS107-D.PDF

7. 2N2222 npn BJT specification: http://www.onsemi.com/pub_link/Collateral/P2N2222A-

D.PDF

8. 2N2907 pnp BJT specification:

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERAT

URE/DATASHEET/CD00003088.pdf