Lab5

Physics 340

Laboratory #5

BIPOLAR TRANSISTORS: THE COMMON EMITTER CONFIGURATION Textbook Readings: Diefenderfer and Holton, Chapter 8

1. Objectives This will be the first experiment in which you use an active element to provide voltage, current, and power gains greater than 1. The bipolar transistor can be used in a common emitter configuration to provide these gains. In this experiment you will use an npn silicon transistor, 2N4123, in a circuit like the one below. As you may already know, the collector resistor R C is used with V to set the load line. The base resistor R B is used to set the operating point. Before using the transistor, however, you must measure its characteristics. Once these are known for your transistor you can then assemble a common-emitter amplifier and compare measured gains with theoretical ones.

Transistor viewed from the top with legs sticking down.

top C B E

C B E

Name: __________________________________________ Partner: __________________________________________

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2. C-E Characteristic Curves A. Set up the following circuit using your DMM to measure VCE. You can use one power supply to supply both input and output.

DMM (20 ma)

1 MΩ Ω

10 k Ω

µa

B. Vary the 1 M Ω and 10 k Ω resistance boxes so you can vary I B from 5-80 µA and V CE from 0-10 volts. You probably will not be able to reach VCE = 0.0V, take measurements at as low a value as you can obtain below 0.1 V. Be careful to switch in more resistance before switching out a resistance. You must have some resistance to limit the current! C. Record IC (ma) for the following combinations of IB and VCE. If you use resistance boxes, you might also want to record your settings. Be careful to use consistent meter settings for all measurements. Values of Ic as a function of BI and VCE IB VCE

10 µA

20 µA

30 µA

0.0

0.1 0.2 0.4 0.6 1.0 2.0 4.0 6.0 8.0 10.0

2

40 µA

50 µA

80 µA

Physics 340

Laboratory #5

D. Make a quick graph in lab of Ic versus VCE for each choice of IB. This set of curves will be important for what follows. So, make sure you have sufficient data and that your curves lookOK before you continue. Include the quick plot in your lab copy and a proper plot of this data in your final write -up.

QUESTION 1 

dI c If we define β as dI B 

(sometimes β is defined as just






V CE

Ic ), what is the range of β values you observe IB

when VCE = 5 volts? What values of β (sometimes called hFE) would you calculate from your characteristic curves if VCE = 0.5 volts? You should show comparisons between the different definitions forβ, and discuss any differences.

QUESTION 2 How do your values ofβ agree with those in the data sheets for a 2N4123?

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Laboratory #5

3. The Load Line and Operating Point A. Assume that you will operate this transistor as an amplifier with V = 10 volts and Rc = 2.7 k Ω as shown below.

On the graph on which you plotted your measured transistor characteristics, plot the load line for this amplifier. Be sure to include this in your final plot as well. If your load -line does not cross more than one of your characteristic curves, you may need to use a lower value of R C, consult the Instructor/TA in this event (do not go below 1 kΩ). B. Once the load line is determined for the amplifier a point on the load line is chosen as the operating point. Usually this point is chosen so that VCE is about midway between 0and +V. This will allow for the largest possible excursion for VCE when the input current changes. Using the above criterion, indicate on your load line an operating point.

Question 3 What value of IB is necessary to set such an operating point? Fr om your previous data curves, what is the value of IC at your chosen operating point?

Question 4 What is the calculated value of RB necessary to set the above operating point?

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Laboratory #5

C. Set up the circuit using RB calculated above. Measure VCE, IC, and IB. VCE = ______________ IC = ______________ IB = ______________

Question 5 Compare your measured value of V CE with the value you expect at the operating point. How well do they agree?

Question 6 Assume that the amplifier has a load line as given in your graph and thatBI is a sinusoidal signal with a peak to peak value of 40 µA. Sketch the output potential, VCE, if the operating point was chosen so VCE was originally 5 volts. Make another sketch of the output potential (same IB input) assuming that the original VCE was 1 volt. Be careful with these graphs and ask questions if you do not understand what is expected, students often do these plots incorrectly. You should make these sketches on separate rgaph paper.

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Physics 340

Laboratory #5

4. The Small Signal Common Emitter Amplifier You will now use the common emitter amplifier you studied to amplify a small amplitude sinusoidal signal. Add the additional components shown below to complete the amplifier.

RC = 4.7 kΩ Rg = 100 kΩ 20 µF 20 µF

D. Set the output of the signal generator to 1 kHz, 1 Vpt, or as low a signal level as you obtain if you cannot get to 1 V pt. You can pull out gently on the amplitude control knob on the signal generator to get a finer adjustment on the output. Connect your signal generator to the transistor base through the 100 k Ω resistor and the 20 µF capacitor as pictured in the figure above. The 100 k Ω precision resistor is used to convert the signal generator voltage to an AC current source. Since the 100 k Ω is in series with the amplifier and 100 k Ω is much greater than the impedance of the amplifier, the AC current input, BI, is given by

IB ≈

V sig.gen. Rg

The electrolytic capacitors on both the input and output are intended merely to block DC level. Pay attention to the polarity of these capacitors and make sure they are discharged before you handle them. E. Using the oscilloscope, look at the AC voltages at the base and collector (on the right side of the output capacitor). Call these VBE and VCE, respectively. Record these values, along withbIin the following table. Use peak-trough values for all quantities, IB, VCE, VBE, etc. You likely will find the input signal is VERY noisy. A light touch with one of your fingers on the face of one of your resistor boxes or perhaps one of the cables may quiet the circuit to reveal the low level sinusoidal input. Record a screen image of your oscilloscope with both VBE and V CE with as low a noise level on the V BE and V CE as you can obtain. Ask the Instructor or TA if you have trouble. Include the screen image in your write-up. IB VBE VCE

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Question 7 From your screen image of VCE, VBE, what is the phase relation between the input potential, BVE, and the output potential, VCE?

F.

Only one more quantity has to be found before you can compute the voltage and current gains of your amplifier. The output current, IC, can be computed by realizing that any point ofconstant potential can be considered as ground as far as AC signals are concerned. Therefore, the AC equivalent for the amplifier circuit is as follows:

The output signal (AC) current is, therefore, given simply by

IC =

V CE RC

To calculate IC accurately you must measure RC as best you can. Remember that the color code only tells you the approximate value. RemoveRC from your circuit and measure its resistance with your meters. RC, measured IC

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Physics 340

Laboratory #5

G. Compute the small signal gains as in the following table. You must be careful in your measurements of Vce and Vbe and be sure to not drive your amplifier beyond its limits.

Experimental

Type of gain AC Voltage gain (Av)

Equation

V CE V BE

Experimental

AC Current gain (Ai) 

Theoretical (derived in text) Assume rBE = 3 kΩ

AC Voltage gain (Av)

IC IB 

rms

- β RC r BE

QUESTION 8 How does the Experimental voltage gain compare to the Theoretical value)?

QUESTION 9 What is the power gain of your amplifier (Hint: P = IV)?

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Value