BIPOLAR JUNCTION TRANSISTOR CHARACTERISTICS

OBJECTIVES:

To become familiar with the theory of operation of bipolar junction transistors (BJT) and to examine the V-I characteristics of BJT's

INTRODUCTION:

The transistor type (NPN or PNP) can be determined using a multimeter. The test checks the polarity of the base-emitter and base-collector junctions. This test is to be performed at the beginning of the lab session. For the BJT, there are three regions of operation;

1. Active region: In this region, the base emitter junction is forward biased and the base-collector junction is reverse biased. This region is the normal transistor operation mode for amplification, and is characterized by the transistor current gain value, beta.
2. Cut-off region: In this region, both base-emitter and base-collector junctions are reverse biased and the transistor acts like an open switch. (IC = 0)
3. Saturation region: In this region, both base emitter and base-collector junctions are forward biased and the transistor acts like a closed switch. (VCE = 0)

In the active region of the transistor, a figure of merit has been defined to quantify the capability of the transistor to amplify the input signal. This parameter is defined as the ratio between IC to IB and typically called the β factor. Similarly an α factor is defined as the ratio between IC to IE. Thus;

β = IC / IB and α = IC / IE

It can be easily shown that β = α /(1 - α) and α = β /(β + 1). As a rule of thumb, the larger the value of β, the higher the gain obtainable from the transistor, i.e. the better the transistor. Typical values for β ranges from about 80 to 300 or higher.

LAB WORK

1. Determine Transistor Type utilizing DC Junction resistance of the transistor:
Verify the transistor type for each unit by checking the polarity of the base-emitter junction. Use a Fluke DMM in resistance mode. Tabulate your measured data. For the given transistor (2N3904), measure the forward and reverse bias resistance between Base and Emitter, Base and Collector, and Collector and Emitter. The lead connection of this transistor is as shown in Figure 1.

2. Figure 1 - Simplified Outline and Leads Connection of Transistor 2N3904
3. IC - VBE Characteristic of the bipolar junction transistor:
Connect the transistor test circuit as shown in Figure 2. Set The DC voltage (VB) to zero Volt and VCC to 10 V. Increase VB in 0.1 V steps and measure the DC voltage between the base and emitter (VBE), the DC current through the collector IC, and the current through the base IB. Tabulate your readings in clear table and plot the IC versus VBE. Make sure that you take enough data points to plot a typical characteristic of a BJT. Calculate β for every data point measured and tabulate the calculated values of β with the measured data. Plot β versus VBE.

4. Figure 2 - Test Circuit for Measuring the VBE versus IC Characteristic of the Bipolar Transistor
5. Measurement of IC versus VCE characteristic of the bipolar transistor:
Using the test circuit of Figure 2, adjust VB to generate 50 µA current into the base of the transistor. Vary VCC in order to change VCE. Pick reasonable steps for VCE (small steps at lower voltages; 0.1V for values between 0 and 1.0V, and larger steps at higher voltages; 1.0V for values above 1.0V). Measure VCE and IC.
Repeat the above measurement for IB = 100 µA, 150 µA and 200 µA. Plot a set of curves for IC versus VCE for constant IB.
From the measured data, determine the range of VCE over which IC is close to zero Amp.
Find the value of α from this set of measured data then calculate β. Compare the value of β obtained from this measurement with that obtained in the measurement performed in 2.