Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.1 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 Chapter 21

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.2 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 21.1

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.3 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’ 21.2

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.4  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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.5  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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.6  Construction –two polarities: npn and pnp

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.7 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  V CE 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 21.3

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.8  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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.9  Transistor action

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT Bipolar Transistor Characteristics  Behaviour can be described by the current gain, h fe or by the transconductance, g m of the device 21.4

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  Transfer characteristics –can be described by either the current gain or by the transconductance –DC current gain h FE or  is given by I C / I B –AC current gain h fe is given by i c / i b –transconductance g m is given approximately by g m  40I C  40 I E siemens

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  Equivalent circuits for a bipolar transistor

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 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 I C = h FE I B  Behaviour is characterised by the current gain or the transconductance 21.5

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT Bipolar Transistor Amplifiers  A simple transistor amplifier –R B 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 21.6

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  AC-coupled amplifier –V B is set by the conduction voltage of the base-emitter junction and so is about 0.7 V –voltage across R B is thus V CC – 0.7 –this voltage divided by R B gives the base current I B –the collector current is then given by I C = h FE I B –the voltage drop across R C is given by I C R C –the quiescent output voltage is therefore V o = V CC - I C R C –output is determined by h FE which is very variable

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  Negative feedback amplifiers

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT Example – see Example 21.2 from course text Determine the quiescent output voltage of this circuit

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT Base current is small, so Emitter voltage V E = V B – V BE = 2.7 – 0.7 = 2.0 V Emitter current Since I B is small, collector current I C  I E = 2 mA Output voltage = V CC – I C R C = mA  2.2 k  = 5.6 V

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  A common-collector amplifier –unity gain –high input resistance –low output resistance –a very good buffer amplifier

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT Other Bipolar Transistor Applications  A phase splitter 21.7

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  A voltage regulator

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  A logical switch

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 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