HASMUKH GOSWAMI COLLAGE OF HASMUKH GOSWAMI COLLAGE OF ENGINEERING BRANCH :- electronics & communication TOPIC :- Bipolar Transistors Vegda Jignesh : 15024311016

Bipolar Transistors Introduction An Overview of Bipolar Transistors Chapter 21 Bipolar Transistors Introduction An Overview of Bipolar Transistors Bipolar Transistor Operation Bipolar Transistor Characteristics Summary of Bipolar Transistor Characteristics Bipolar Transistor Amplifiers Other Bipolar Transistor Applications

21.1 Introduction Bipolar transistors are one of the main ‘building-blocks’ in electronic systems They are used in both analogue and digital circuits They incorporate two pn junctions and are sometimes known as bipolar junction transistors or BJTs Here will refer to them simply as bipolar transistors

An Overview of Bipolar Transistors 21.2 An Overview of Bipolar Transistors While control in a FET is due to an electric field, control in a bipolar transistor is generally considered to be due to an electric current current into one terminal determines the current between two others as with a FET, a bipolar transistor can be used as a ‘control device’

Notation bipolar transistors are 3 terminal devices collector (c) base (b) emitter (e) the base is the control input diagram illustrates the notation used for labelling voltages and currents

Relationship between the collector current and the base current in a bipolar transistor characteristic is approximately linear magnitude of collector current is generally many times that of the base current the device provides current gain

Construction two polarities: npn and pnp

Bipolar Transistor Operation 21.3 Bipolar Transistor Operation We will consider npn transistors pnp devices are similar but with different polarities of voltage and currents when using npn transistors collector is normally more positive than the emitter VCE might be a few volts device resembles two back-to-back diodes – but has very different characteristics with the base open-circuit negligible current flows from the collector to the emitter

Now consider what happens when a positive voltage is applied to the base (with respect to the emitter) this forward biases the base-emitter junction the base region is light doped and very thin because it is likely doped, the current produced is mainly electrons flowing from the emitter to the base because the base region is thin, most of the electrons entering the base get swept across the base-collector junction into the collector this produces a collector current that is much larger than the base current – this gives current amplification

Transistor action

Bipolar Transistor Characteristics 21.4 Bipolar Transistor Characteristics Behaviour can be described by the current gain, hfe or by the transconductance, gm of the device

Transistor configurations transistors can be used in a number of configurations most common is as shown emitter terminal is common to input and output circuits this is a common-emitter configuration we will look at the characteristics of the device in this configuration

Input characteristics the input takes the form of a forward-biased pn junction the input characteristics are therefore similar to those of a semiconductor diode

Output characteristics region near to the origin is the saturation region this is normally avoided in linear circuits slope of lines represents the output resistance

Transfer characteristics can be described by either the current gain or by the transconductance DC current gain hFE or  is given by IC / IB AC current gain hfe is given by ic / ib transconductance gm is given approximately by gm  40IC  40 IE siemens

Equivalent circuits for a bipolar transistor

Summary of Bipolar Transistor Characteristics 21.5 Summary of Bipolar Transistor Characteristics Bipolar transistors have three terminals: collector, base and emitter The base is the control input Two polarities of device: npn and pnp The collector current is controlled by the base voltage/current IC = hFEIB Behaviour is characterised by the current gain or the transconductance

Bipolar Transistor Amplifiers 21.6 Bipolar Transistor Amplifiers A simple transistor amplifier RB is used to ‘bias’ the transistor by injecting an appropriate base current C is a coupling capacitor and is used to couple the AC signal while preventing external circuits from affecting the bias this is an AC-coupled amplifier

AC-coupled amplifier VB is set by the conduction voltage of the base-emitter junction and so is about 0.7 V voltage across RB is thus VCC – 0.7 this voltage divided by RB gives the base current IB the collector current is then given by IC = hFEIB the voltage drop across RC is given by IC RC the quiescent output voltage is therefore Vo = VCC - IC RC output is determined by hFE which is very variable

Negative feedback amplifiers

Example – see Example 21.2 from course text Determine the quiescent output voltage of this circuit

Base current is small, so Emitter voltage VE = VB – VBE = 2.7 – 0.7 = 2.0 V Emitter current Since IB is small, collector current IC  IE = 2 mA Output voltage = VCC – ICRC = 10 - 2 mA 2.2 k = 5.6 V

A common-collector amplifier unity gain high input resistance low output resistance a very good buffer amplifier

Other Bipolar Transistor Applications 21.7 Other Bipolar Transistor Applications A phase splitter

A voltage regulator

A logical switch

Key Points Bipolar transistors are widely used in both analogue and digital circuits They can be considered as either voltage-controlled or current-controlled devices Their characteristics may be described by their gain or by their transconductance Feedback can be used to overcome problems of variability The majority of circuits use transistors in a common-emitter configuration where the input is applied to the base and the output is taken from the collector Common-collector circuits make good buffer amplifiers Bipolar transistors are used in a wide range of applications