Bipolar Transistors

1. 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

2. 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

3. 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 terminaldetermines the currentbetween two others • as with a FET, abipolar transistorcan be used as a‘control device’

4. Notation • 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

5. Relationship between the collector current and the base current in a bipolar transistor • characteristic isapproximately linear • magnitude of collectorcurrent is generallymany times that of thebase current • the device providescurrent gain

6. Construction • two polarities:npn and pnp

7. 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

8. Now consider what happens when a positive voltageis 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 ismainly 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

9. Transistor action

10. 21.4 Bipolar Transistor Characteristics • Behaviour can be described by the current gain, hfe or by the transconductance, gm of the device

11. 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

12. 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

13. Output characteristics • region near to theorigin is the saturation region • this is normallyavoided in linearcircuits • slope of linesrepresents theoutput resistance

14. 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 gainhfeis given by ic / ib • transconductance gm is given approximately by gm  40IC  40 IE siemens

15. Equivalent circuits for a bipolar transistor

16. 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

17. 21.6 Bipolar Transistor Amplifiers • A simple transistor amplifier • RB is used to ‘bias’ thetransistor by injecting anappropriate base current • C is a coupling capacitorand is used to couple theAC signal while preventingexternal circuits fromaffecting the bias • this is an AC-coupled amplifier

18. 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

19. Negative feedback amplifiers

20. Example – see Example 21.2 from course text Determine thequiescent outputvoltage of thiscircuit

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

22. A common-collector amplifier • unity gain • high input resistance • low output resistance • a very goodbuffer amplifier

23. 21.7 Other Bipolar Transistor Applications • A phase splitter

24. A voltage regulator

25. A logical switch

26. 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