EE 321 Lab


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 vBE controls the collector current iC, when the transistor is biased in its `active' mode (b-e forward biased, c-b reverse biased).

3. Note that vC = VCC - iC RC. Use this to determine where the transistor is in cutoff (iC = 0) and where it is in saturation (vCE less than 0.7). (In between, the transistor is in its active region).

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

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

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

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. iC = beta iB. 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 RB from +VCC (see Figure 4). This provides a d.c. base current IB = (VCC - 0.7V)/RB, which gets amplified by the transistor to give a d.c. collector current IC = beta IB. Because beta varies from transistor to transistor you will have to select the value of RB to obtain the desired IC.

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 RS.

9. EXTRA CREDIT. The theoretical signal gain is determined from the expressions vout = - ic RC, ic = beta ib, and ib = vin / RS. where all the currents and voltages are changes from their bias values caused by the signal.

© Copyright 1996 New Mexico Institute of Mining and Technology