lab report

Introduction to Bipolar Junction Transistors (BJTs)

Laboratory Report for EENG 3306

College of Engineering and Computer Science

Department of Electrical Engineering

University of Texas at Tyler

Houston, TX

December 8, 2014

Mugisha Omary

Group Members

Hamza Ahmad

Shamir Mohammed

I. Project description

The purpose of this lab is to take measurement of the common-emitter characteristics (collector current IC vs collector-to-emitter voltage VCE of small-signal NPN and PNP bipolar transistors and also simulate IC vs VCE characteristics of 2N4401 and 2N3906 transistors.

A BJT is a semiconductor device that uses a small current to control a larger current. This property makes it essentially a current amplifier. In this lab the student will build a simple test circuit to evaluate a transistor’s current and voltage relationships and then use this data to determine the transistors DC value and plot the collector characteristic curve.

II. Theoretical background

A BJT is a three terminal two – junction semiconductor device in which the

conduction is due to both the charge carrier. Hence it is a bipolar device and it

amplifier the sine waveform as they are transferred from input to output. BJT is

classified into two types – NPN or PNP. A NPN transistor consists of two N

types in between which a layer of P is sandwiched. The transistor consists of

three terminal emitter, collector and base. The emitter layer is the source of the

charge carriers and it is heartily doped with a moderate cross sectional area.

The collector collects the charge carries and hence moderate doping and large

cross sectional area. The base region acts a path for the movement of the

charge carriers. In order to reduce the recombination of holes and electrons the

base region is lightly doped and is of hollow cross sectional area. Normally the

transistor operates with the EB junction forward biased. In transistor, the current is same in both junctions, which indicates that there is a transfer of resistance between the two junctions. One to this fact the transistor is known as transfer resistance of transistor.

The symbol of an NPN BJT. The symbol is "not pointing in."

The symbol of a PNP BJT. The symbol "points in proudly."

When a transistor’s base current (IB) is set to a certain value and left unchanged while the collector current is swept through a range of values and IC and VCE are recorded and then graphed, a collector characteristic curve is produced for that particular IB. If IB is now changed, and again the collector current is swept through a range of values, and IC and VCE are plotted, another collector characteristic curve for this different IB value is produced. Repeating this process for several IB values results in a family of curves referred to as the transistors collector characteristic curves. Figure 2 shows the characteristics for a notional transistor.

Figure 1. Transistor Collector Characteristic Curves

The flow of the current in the circuit is shown below.

Figure 1- Transistor Current

III. Methods and materials

Equipment

1- Curve Tracer

1- 2N4401 Transistor

1- 2N3906 Transistor

1- Oscilloscope

1- Power Supply

1-Breadboard

Wires bundles

Experimental procedure

We used the curve tracer and connected it to 2N4401 installed on the breadboard for measuring common-emitter IC vs VCE characteristics of the NPN transistor, secondly we used the circuit with the oscilloscope to measure the IC vs VCE characteristics of the 2N4401 transistor for base currents from 0 to 100µA in increments of 20µA and then we imported the results in excel and produced a graph as shown in figure 5 of the lab handout.

For the PNP common emitter , we used the circuit with the oscilloscope to measure the IC vs VCE characteristics of the 2N3906 transistor for base currents from 0 to -100µA in increments of -20µA, Adjusted the amplitude of the triangle-wave generator to obtain a minimal VCE of at least -20V. We imported these results into excel and produced a graph similar to that shown in figure 5 of the lab handout.

Figure 2-Oscilloscope connections to the curve tracer

Figure 3- Signal generator connections to the curve tracer

Figure 4- Connections for an NPN BJT

Figure 5- Configuration and connections for a PNP transistor

IV. Results

Measurement of NPN common-emitter graph created using excel is shown below

Figure 6-NPN graph

To determine the current gain β (ratio of IC to IB ) at VCE=5V and IB=40µA

β=IC/IB=7mA/40µA, β=175

At VCE=15V, IB=80µA, IC=17mA

β=17mA/80µA, β=212.5

Early voltage of the transistor is

Measurement of PNP common-emitter graph created using excel is shown below

Current gain at VCE= -5v and IB= 40µA, IC= -7mA

β= -7mA/40µA=-175

Current gain at VCE=-15V, IB=80µA, IC= -17mA

β=IC/IB, β=-17mA/80µA, β=-212.5

The Early voltage of the transistor

V. Discussion

A precision half-wave rectifier and voltage amplifier with a voltage gain of ±10V/V and an output range of ±23V and ±30mA. This amplifier is driven by an external signal generator and provides a unipolar voltage VCE (positive-going for NPN or negative-going for PNP, selectable with the Polarity switch) to the transistor under test. The external signal generator is set to produce a triangle wave with no dc offset at a frequency of 50Hz. The amplitude of the triangle wave determines the maximal value of |VCE|.

A transresistance amplifier whose output voltage is proportional to IC with sensitivity of −5mA/V.

A voltage-controlled current source (VCCS) that provides base current IB to the transistor under test. The transconductance of the VCCS is ±50µA/V. The polarity of the base current is determined by the Polarity switch (positive for NPN, negative for PNP).

VI. Conclusions

A bipolar junction transistor (BJT) is a transistor that relies on the contact of two types of semiconductor for its operation. After performing this lab and constructing the circuit in the laboratory procedure we were able to measure IC vs VCE for an NPN transistor and we were able to simulate the characteristics of the 2N4401 NPN BJTs. When the experiment was repeated for the PNP common-emitter the same results were obtained except that the graph is in the third quadrant because the IC and VCE have negative values. At any given base current IB , the graphs of IC vs VCE were plotted and they all appear to have the same shape, meaning as VCE increases , IC also increases.