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Electronics I Laboratory Manual

Bipolar Junction Transistors

A transistor is a three-terminal semiconductor device that is extremely versatile.  In the modern world of today very seldom you will find an electronic device that in some way or another does not use a transistor.  Without the transistor the computers of today, the telecommunication devices, our defense systems, the entertainment systems, industrial processes, etc. could not be possible the way we know them.

There are many types of transistors available.  The main difference between them is in the way we construct them.  In this experiment we will study the first useful transistor constructed in the 1950s by engineers at Bell Laboratories.  They called this type of transistor the bipolar junction transistor or BJT.  It is built by joining together three pieces of semiconductor materials.  Because basically there are two types of semiconductor materials – the N- and the P-type, as you must know! – we can have two types of BJTs.  If we sandwich a piece of P-type material between two pieces of N-type material we produce the NPN BJT.  If, on the other hand, we put a piece of N-type material between two pieces of P-type material we have a PNP BJT transistor.  The current and voltage polarities of the PNP are opposite to the NPN’s polarities.  Otherwise, both transistor types behave alike, although the NPN is the most commonly used BJT.


To understand the behavior of the transistor, you must know the relationships between the transistor voltages and currents.  As you know, two-terminal devices like diodes and resistors are described with one voltage and one current.

Three-terminal devices, on the other hand, are described with three voltages and three currents.  Each terminal of a BJT is assigned a name, and the currents are named after the terminals where they are flowing.  The three voltages are the voltages between any two terminals.  The terminal’s names are:  Emitter, Collector, and Base1.  The currents are referred to as IE, the emitter current; IC, the collector current; and IB, the base current.  The voltages are VBE, VCE, and VCB.  Figure 8.l shows the construction layout, the symbol and the direction and polarities of currents and voltages in a NPN transistor.  For a PNP transistor these currents and voltages have opposite polarities.


1The reader should review these ideas from main textbook of the course.

A transistor can operate basically in three different states:  (1) Cut-Off, in which the transistor has no current output; (2) Active, in which the output collector current (IC) is controlled by the base current (IB); and (3) Saturated, in which the transistor collector current reaches a maximum value.  In this state an increase in the base current has no effect on the collector current.

                                                                                                                                                   

The Current Gain β:

The transistor can produce a large output (IC) with very small input (IB).  Because of this – the output current is much larger than the input current – the transistor is a current amplifier.  The factor by which the output current (IC) is bigger than the controlling or input current (IB) is called the DCcurrent gain of this amplifier and is represented by the Greek letter beta (β).  Its value normally is given by the manufacturer and it can be found by the approximate formula

 

(8.1)

This is a very important parameter of the transistor.  The student should be aware of its importance in order to understand the general behavior of transistors.

                                                                                                                                                   

Transistor Operation

A transistor can be operated in any of the three states indicated above.  Different equations apply for the calculation of the currents for each region.  In what follows we present a short summary of the relations between the currents.

Active
A transistor is active when the output current (IC) is controlled by the input current (IB).  This operating mode is useful when designing voltage and current amplifiers, as you will see in future experiments. 

The relationship between the currents is given by

 

IE  =  IC  +  IB

(8.2)


 

(8.3)

and by using Eq. 8.3, we can re-write the emitter current as

 

(8.4)

Eqs. 8.32 and 8.4 show that the collector and emitter currents are functions of the input current IB.  We say that the controlling current is the base currents and that the transistor is a current controlled device.

Saturation
Saturation is the operating mode where the transistor in a particular circuit will produce the maximum current for the circuit.  The value of the maximum current depends on the circuit parameters.  However, in any case the transistor is saturated when the collector-emitter voltage3 is close to zero (≈ 0.2 V).  Thus we can express the saturation condition as follows:

VCE  ≈  0

IC is maximum

Saturation, we say, is the condition where you have maximum current and minimum output voltage in the transistor.


2In reality the collector current is expressed as

 

IC  =  βIB  +  ICEO

(8.5)

where ICEO is the emitter leakage current.  This current is very small, so by using the approximation given by Eq. 8.3 we do not introduce big errors in the calculations.


3This is for the case of the Common-emitter configuration.

Cut-off 
This condition is the opposite of saturation.  When the transistor is in cut-off the output current is zero (or close to zero.)  When IC is zero, the output voltage is maximum (normally equal to the source bias voltage).  Thus, we can say that for cut-off we have

 VCE  =  VCC 

 

IC  =  0

 

where VCC is the bias voltage.

Common-emitter

When the transistor is connected as shown in Figure 8.2 we have the configuration known as Common-emitter.  The name is derived from the fact that the emitter is grounded.  Notice that we need two power supplies to completely bias the transistor.  This fact makes this arrangement very expensive and the implementation of the circuit requires a big space.  This circuit is the standard common-emitter bias, also known as the common-emitter fixed-bias circuit.

Another way to connect the transistor in a common-emitter configuration is by biasing the input (base-emitter) and the output (collector-emitter) junctions with the same power supply.  In this case only one source is needed making the implementation less expensive and more compact.  Because there is only one power supply that feeds the input and the output loops this configuration is also known as common-emitter self bias circuit.  This is a very popular transistor connection.  Many amplifier circuits make use of this configuration.  Figure 8.3 shows the self-bias circuit.

For the rest of this experiment we will study the fixed-bias configuration (with two sources) because it is more suitable for the general purpose of the experiment.  The self-bias (and others) configuration will be studies in the next experiment.

Voltages

Assuming that the transistor is active we calculate the important voltages of this circuit as follows:  First, we calculate the base current using the following equation derived from the solution of the mesh equation of the base loop4

 

(8.6)

where Vbe is approximately equal to 0.7 V.

The other two currents can be found with Eqs. 8.3 and 8.4 above.

Next, we determine the terminal voltages VC, VE, and VB

 

(8.7)


 

(8.8)

and

 

Vb  =  Vbe  +  Ve

(8.9)

or

 

Vb  =  Vcc  -  IBRb

(8.10)

And, finally the transistor voltageVce and Vcb

 

Vce  =  Vc  -  Ve

(8.11)


 

Vcb  =  Vc  -  Vb

(8.12)

In this experiment we will develop some understanding of the characteristics of transistors by studying this configuration.


4The student should derive this equation.

Characteristic Curves

The characteristic curve – or IV curve – of devices, as you should remember, is the relationship between the current and the voltage of the device.  A device with two terminals has only one characteristic curve.  A diode is an example of such a device.  A transistor, on the other hand, has three terminals.  The transistor, because of this feature, has an infinite number of characteristic curves.  We say that the transistor has a family of curves.  To complicate things a little bit more, transistors have two families of curves: one for the input (the base-emitter junction) and one for the output (the collector-emitter junction).  In this experiment we will concentrate in the output characteristic curves of transistors.

Figure 8.4 shows a typical set of output characteristic curves for a transistor.  (This is the set of curves of a 2N2222A npn silicon transistor.)  Notice that these are the plots of the collector current (Ic) versus the collector-to-emitter voltage (Vce).  Each curve is drawn for a particular constant value of the base current (Ib).

The student will have the opportunity to reproduce these curves in this experiment.

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