Drexel University

ECE-E302, Electronic Devices

Lab VII: Bipolar Junction Transistor

Bipolar Junction Transistor

## Objectives

To obtain the volt-ampere characteristic curves of a Bipolar Junction Transistor (BJT) and to demonstrate that the transistor is capable of producing amplification when biased in the active region. Typical gain parameters of the given transistor are calculated.

## Theory

Bipolar Junction Transistor:

A bipolar junction transistor (BJT) is widely used in discrete circuits as well as in IC design, both analog and digital. Its main applications are in amplification of small signals, and in switching digital logic signals. In a BJT, both majority carriers and minority carriers play a role in the operation of the transistor, hence the term bipolar.

The circuit symbol of the NPN transistor with current and voltage polarities marked is shown in Figure 1.

Figure 1. Circuit symbol of a NPN BJT

The characteristic curves of BJT are not only helpful in studying the behavior of the transistor but also in determining the region of operation for the transistor. As was done with a diode, we can draw a load line on the above curves to determine the operating point of the transistor. The intersection of the load line with the curves gives the operating point, referred to as the Q-point. The regions of interest in a transistor are the amplifying region, cutoff region and the saturation region. The last two are extensively used when the transistor is used in digital circuits. These three regions are defined as follows:

CUTOFF : both the emitter and collector junction are reverse biased

SATURATION : both the emitter and collector junctions are forward biased

ACTIVE : emitter junction is forward biased, collector junction is reverse biased

In the active region, the collector current is independent of the value of the collector voltage and hence the transistor behaves as an ideal current source where the current is determined by VBE.

The collector current is dependent on the base current as,

where b is called the dc common emitter current gain of the transistor, defined as

Another parameter of interest in the active region is a, the current transfer ratio, or common base current gain, which is defined by

The current gain parameters a and b are related by:

When the transistor is biased in the active region it operates as an amplifier. The biasing problem is that of establishing a constant DC current in the emitter (or the collector) which is insensitive to variations in temperature, value of b and so on. This is equivalent to designing the transistor circuit so that the Q-point is in the middle of the DC load line. Changing the biasing resistors has the effect of shifting the Q-point along the DC load line, moving the transistor into the regions of cutoff or saturation. The amplification property can be graphically interpreted as seen in Figure 2.

Figure 2. Amplification of a small signal

Writing the output loop equation from Figure 4, we have

For b large,

Using equations 5 and 7, we can write:

or

This is the (straight line) equation of the load line on a plot of IC vs VCE, where VCC is the power supply voltage. The axis crossings can be found by setting IC=0 or VCE = 0.

Figure 3. BJT IV curves and load line

The analysis suggests that small sinusoidal signals, vbe, superimposed on the DC voltage VBE, will give a sinusoidal collector current, iC, superimposed on the DC current IC at the Q-point. Depending upon the configuration of the resistors in the collector, the emitter, and the load, there will be an ideal Q-point for a maximum distortion-free output signal amplitude. Determining these resistor requires constructing an ac loadline. These topics will be covered in Electronics II.

PROCEDURE - Bipolar Transistors

a) Obtain the volt-ampere characteristics of the given BJT using a curve tracer. Be sure to record the part number of the transistor in your notebook.

b)  Construct the circuit of Figure 4

c)  Record the ammeter and voltmeter readings.

d)  Superimpose the dc load line on the characteristic curves and check whether the transistor is operating at the Q-point.

e)  Modify your circuit as shown in Figure 5, with Rc = 13 kΩ and R2 = 2 kΩ. Apply a small (~100 mV) sinusoidal input at 5 kHz. Look at the input and output across the load resistor (RL) on the scope. Obtain a printout of these waveforms.

Figure 4. Bipolar transistor bias circuit

Figure 5. Bipolar transistor amplifier

(f) Increase the amplitude of the sine wave as much as possible before distortion occurs. Obtain a printout of a distorted output. Record the level of the input signal.

(g) Decrease the bias resistor R2 to 500 Ω. Apply the small signal as in step (e). Has the output waveform changed? Obtain printout.

(h) Increase the amplitude of the sine wave as much as possible before distortion occurs. Obtain a printout of a distorted output. Record the level of the input signal.