Bipolar Transistor BJT – Pitt

 

 

Bipolar Transistor Basics

In the Diode tutorials we saw that simple diodes are made up from two pieces of semiconductor material, either

silicon or germanium to form a simple PN-junction and we also learnt about their properties and characteristics. If we

now join together two individual signal diodes back-to-back, this will give us two PN-junctions connected together in

series that share a common P or N terminal. The fusion of these two diodes produces a three layer, two junction,

three terminal device forming the basis of a Bipolar Transistor, or BJT for short.

Transistors are three terminal active devices made from different semiconductor materials that can act as either an

insulator or a conductor by the application of a small signal voltage. The transistor’s ability to change between these

two states enables it to have two basic functions: “switching” (digital electronics) or “amplification” (analogue

electronics). Then bipolar transistors have the ability to operate within three different regions:

• 1. Active Region – the transistor operates as an amplifier and Ic = β.Ib
•
• 2. Saturation – the transistor is “fully-ON” operating as a switch and Ic = I(saturation)
•
• 3. Cut-off – the transistor is “fully-OFF” operating as a switch and Ic = 0

Typical Bipolar Transistor

The word Transistor is an acronym, and is a combination of the words Transfer Varistor used to describe their

mode of operation way back in their early days of development. There are two basic types of bipolar transistor

construction, NPN and PNP, which basically describes the physical arrangement of the P-type and N-type

semiconductor materials from which they are made.

The Bipolar Transistor basic construction consists of two PN-junctions producing three connecting terminals with

each terminal being given a name to identify it from the other two. These three terminals are known and labelled as

the Emitter ( E ), the Base ( B ) and the Collector ( C ) respectively.

Bipolar Transistors are current regulating devices that control the amount of current flowing through them in

proportion to the amount of biasing voltage applied to their base terminal acting like a current-controlled switch. The

principle of operation of the two transistor types NPN and PNP, is exactly the same the only difference being in their

biasing and the polarity of the power supply for each type.

Bipolar Transistor Construction

The construction and circuit symbols for both the NPN and PNP bipolar transistor are given above with the arrow in

the circuit symbol always showing the direction of “conventional current flow” between the base terminal and its

emitter terminal. The direction of the arrow always points from the positive P-type region to the negative N-type

region for both transistor types, exactly the same as for the standard diode symbol.

Bipolar Transistor Configurations

As the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an

electronic circuit with one terminal being common to both the input and output. Each method of connection

responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each

circuit arrangement.

• 1. Common Base Configuration – has Voltage Gain but no Current Gain.
•
• 2. Common Emitter Configuration – has both Current and Voltage Gain.
•
• 3. Common Collector Configuration – has Current Gain but no Voltage Gain.

The Common Base (CB) Configuration

As its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to

both the input signal AND the output signal with the input signal being applied between the base and the emitter

terminals. The corresponding output signal is taken from between the base and the collector terminals as shown with

the base terminal grounded or connected to a fixed reference voltage point. The input current flowing into the emitter

is quite large as its the sum of both the base current and collector current respectively therefore, the collector current

output is less than the emitter current input resulting in a current gain for this type of circuit of “1” (unity) or less, in

other words the common base configuration “attenuates” the input signal.

The Common Base Transistor Circuit

This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Vout

are in-phase. This type of transistor arrangement is not very common due to its unusually high voltage gain

characteristics. Its output characteristics represent that of a forward biased diode while the input characteristics

represent that of an illuminated photo-diode. Also this type of bipolar transistor configuration has a high ratio of output

to input resistance or more importantly “load” resistance (RL) to “input” resistance (Rin) giving it a value of

“Resistance Gain”. Then the voltage gain (Av for a common base configuration is therefore given as:

Common Base Voltage Gain

The common base circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or

radio frequency (Rf) amplifiers due to its very good high frequency response.

The Common Emitter (CE) Configuration

In the Common Emitter or grounded emitter configuration, the input signal is applied between the base, while the

output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly

used circuit for transistor based amplifiers and which represents the “normal” method of bipolar transistor connection.

The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar

transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward-biased

PN-junction, while the output impedance is HIGH as it is taken from a reverse-biased PN-junction.

The Common Emitter Amplifier Circuit

In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the

transistor as the emitter current is given as Ie = Ic + Ib. Also, as the load resistance (RL) is connected in series with

the collector, the current gain of the common emitter transistor configuration is quite large as it is the ratio of Ic/Ib and

is given the Greek symbol of Beta, (β). As the emitter current for a common emitter configuration is defined as

Ie = Ic + Ib, the ratio of Ic/Ie is called Alpha, given the Greek symbol of α. Note: that the value of Alpha will always

be less than unity.

Since the electrical relationship between these three currents, Ib, Ic and Ie is determined by the physical construction

of the transistor itself, any small change in the base current (Ib), will result in a much larger change in the collector

current (Ic). Then, small changes in current flowing in the base will thus control the current in the emitter-collector

circuit. Typically, Beta has a value between 20 and 200 for most general purpose transistors.

By combining the expressions for both Alpha, α and Beta, β the mathematical relationship between these

parameters and therefore the current gain of the transistor can be given as:

Where: “Ic” is the current flowing into the collector terminal, “Ib” is the current flowing into the base terminal and “Ie”

is the current flowing out of the emitter terminal.

Then to summarise, this type of bipolar transistor configuration has a greater input impedance, current and power

gain than that of the common base configuration but its voltage gain is much lower. The common emitter
configuration is an inverting amplifier circuit resulting in the output signal being 180o out-of-phase with the input

voltage signal.

The Common Collector (CC) Configuration

In the Common Collector or grounded collector configuration, the collector is now common through the supply. The

input signal is connected directly to the base, while the output is taken from the emitter load as shown. This type of

configuration is commonly known as a Voltage Follower or Emitter Follower circuit. The emitter follower

configuration is very useful for impedance matching applications because of the very high input impedance, in the

region of hundreds of thousands of Ohms while having a relatively low output impedance.

The Common Collector Transistor Circuit

The common emitter configuration has a current gain approximately equal to the β value of the transistor itself. In the

common collector configuration the load resistance is situated in series with the emitter so its current is equal to that

of the emitter current. As the emitter current is the combination of the collector AND the base current combined, the

load resistance in this type of transistor configuration also has both the collector current and the input current of the

base flowing through it. Then the current gain of the circuit is given as:

The Common Collector Current Gain

This type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin and Vout are in-

phase. It has a voltage gain that is always less than “1” (unity). The load resistance of the common collector transistor

receives both the base and collector currents giving a large current gain (as with the common emitter configuration)

therefore, providing good current amplification with very little voltage gain.

Bipolar Transistor Summary

Then to summarise, the behaviour of the bipolar transistor in each one of the above circuit configurations is very

different and produces different circuit characteristics with regards to input impedance, output impedance and gain

whether this is voltage gain, current gain or power gain and this is summarised in the table below.

Bipolar Transistor Characteristics

The static characteristics for a Bipolar Transistor can be divided into the following three main groups.

Common Base –
ΔVEB / ΔIE
Common Emitter – ΔVBE / ΔIB

Input Characteristics:-

Output Characteristics:-

Transfer Characteristics:- Common Base –

ΔVC / ΔIC
Common Base –
Common Emitter – ΔVC / ΔIC

ΔIC / ΔIE
Common Emitter – ΔIC / ΔIB

with the characteristics of the different transistor configurations given in the following table:

Characteristic

Input Impedance

Output Impedance

Phase Angle

Voltage Gain

Current Gain

Power Gain

Common
Base

Low

Very High
0o

High

Low

Low

Common
Emitter

Medium

High
180o

Medium

Medium

Common
Collector

High

Low
0o

Low

High

Very High

Medium

In the next tutorial about Bipolar Transistors, we will look at the NPN Transistor in more detail when used in the

common emitter configuration as an amplifier as this is the most widely used configuration due to its flexibility and

high gain. We will also plot the output characteristics curves commonly associated with amplifier circuits as a function

of the collector current to the base current.

The NPN Transistor

In the previous tutorial we saw that the standard Bipolar Transistor or BJT, comes in two basic forms. An NPN

(Negative-Positive-Negative) type and a PNP (Positive-Negative-Positive) type, with the most commonly used

transistor type being the NPN Transistor. We also learnt that the transistor junctions can be biased in one of three

different ways – Common Base, Common Emitter and Common Collector. In this tutorial we will look more closely

at the “Common Emitter” configuration using NPN Transistors with an example of the construction of a NPN

transistor along with the transistors current flow characteristics is given below.

An NPN Transistor Configuration

Note: Conventional current flow.

We know that the transistor is a “current” operated device (Beta model) and that a large current ( Ic ) flows freely

through the device between the collector and the emitter terminals when the transistor is switched “fully-ON”.

However, this only happens when a small biasing current ( Ib ) is flowing into the base terminal of the transistor at the

same time thus allowing the Base to act as a sort of current control input. The transistor current in an NPN transistor

is the ratio of these two currents ( Ic/Ib ), called the DC Current Gain of the device and is given the symbol of hfe or

nowadays Beta, ( β ). The value of β can be large up to 200 for standard transistors, and it is this large ratio between

Ic and Ib that makes the NPN transistor a useful amplifying device when used in its active region as Ib provides the

input and Ic provides the output. Note that Beta has no units as it is a ratio.

Also, the current gain of the transistor from the Collector terminal to the Emitter terminal, Ic/Ie, is called Alpha, ( α ),

and is a function of the transistor itself (electrons diffusing across the junction). As the emitter current Ie is the

product of a very small base current plus a very large collector current, the value of alpha α, is very close to unity,

and for a typical low-power signal transistor this value ranges from about 0.950 to 0.999

α and β Relationship in a NPN Transistor