Prelab for Lab 8

In this lab we will investigate how a bipolar junction transistor (BJT) can be used to amplify signals.

1. First, use your multi meter to measure the base-emitter and base-collector junctions of your transistor (Figure 1 and 2). Put the meter in ``Diode'' mode (a picture of a diode shows),the meter indicates the voltage for 1 mA current when forward biased and 0 if reversed biased. Test the method on a diode then on a 2N3904 transistor. Record your readings. This is a useful way to check for bad transistors. This is better than using the Ohm-Meter for measuring the forward and reverse resistances of the junctions, as the measuring voltage and current are not known.

2. The basic principle of a BJT is that the base-emitter voltage v_BE controls the collector current i_C, when the transistor is biased in its `active' mode (b-e forward biased, c-b reverse biased).

• Construct the circuit (Figure 3). Lay your circuit out neatly as it looks on the schematic, between the bus lines for V_CC and ground.
• Connect a sine wave to v_S and display v_BE and v_C in on your oscilloscope.
• Adjust input sine wave, v_S (the d.c. bias (offset) and amplitude) so that v_BE varies from about 0.0 to about 0.73 volts. Be sure to measure v_BE at the base of the transistor. Make v_BE big enough that v_O goes from 10 V to 0 V.
• Both v_BE and v_O will look like clipped sine waves. Where do they clip?

3. Note that v_C = V_CC - i_C R_C. Use this to determine where the transistor is in cutoff (i_C = 0) and where it is in saturation (v_CE less than 0.7). (In between, the transistor is in its active region).

• Display v_C vs. v_BE in the x-y mode. Using this curve, plot i_C vs. v_BE. THIS CURVE SHOULD LOOK like figure 4.12 in Sedra and Smith. Note the three regions on the plot
• For what value of v_BE does the transistor start to turn `on'? (Collector current start to flow?) Does this make sense?
• When v_BE is too large, the collector current will drop v_C below v_B, thus forward-biasing the collector-base junction and driving the transistor into saturation. For what values of v_BE and i_C does this occur?

4. The transfer characteristic is linear if v_BE is restricted to small changes about a bias point, as in Figure 4.24.

• Reduce the p-p signal amplitude and change the offset until only a small part of the transfer curve going from about 3.5 mA to 6.5 mA is left (fiddle with the signal generator until the output is a sine wave that goes between 6.5 V and 3.5 V. Start with an input amplitude of 0.1V and offset of 0.5v). Now you should find that v_C changes by 3 V p-p about the bias point at I_C = 5 mA.
• Sketch the operating range on the plot of part 3 above.
• In the time domain, display v_BE and v_C on your scope. What is the voltage gain v_c / v_be for the small signals? (Be sure to measure the `input' v_be after the 10 kOhm resistor.)
• How does the gain compare with theoretical value found in the prelab?

5. How could the gain be made larger? (There are two ways.)

6. Increase v_BE and note the distorted nature of v_C. Increase v_BE further until the output just "clips" on top and bottom. (Readjust the d.c. level so that the clipping is symmetrical).

• Sketch a clipped waveform.
• Note the d.c. levels of the clipping, and whether the clipping is due to saturation or cutoff.

7. The output voltage can be linearized over its entire range by using the input signal to control the base current rather than the base voltage. This is because the collector current is proportional to the base current, i.e. i_C = beta i_B. We will show this in the following way:

First we will bias the transistor in its active state by supplying a current to the base through R_B from +V_CC (see Figure 4). This provides a d.c. base current I_B = (V_CC - 0.7V)/R_B, which gets amplified by the transistor to give a d.c. collector current I_C = beta I_B. Because beta varies from transistor to transistor you will have to select the value of R_B to obtain the desired I_C.

• The desired current for I_C is 5 mA
• Begin by assuming beta=100 and calculate R_B.
• Use this R_Bin the circuit, measure I_C, and then get a better estimate of beta.
• Repeat this step until I_C = 5 mA +- 0.5 mA.

• What value of R_B did you need, and what value of beta does this imply?

8. Now add a signal component to the base current through a coupling capacitor, as shown in Figure 5, which does not affect the d.c. bias. Note that the input voltage has been converted to a current by resistor R_S.

• Increase v_s until the output clips, and back off until the clipping just stops.
• What is the maximum output voltage swing
• Is the output linearly related to the input. v_s? (Adjust the scope -position, sensitivity, invert- until the input and output line up. If they line up and the output is a bigger copy of the input they are linearly related.)
• What is the voltage gain, v_out / v_s?

9. EXTRA CREDIT. The theoretical signal gain is determined from the expressions v_out = - i_c R_C, i_c = beta i_b, and i_b = v_in / R_S. where all the currents and voltages are changes from their bias values caused by the signal.

• Solve these equations for the signal gain v_out / v_in.
• Use the measured value of the gain to estimate beta again.
• How does this value of beta compare with the value determined in part 7?