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

2. 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

3. 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

4. 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’

5. 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

6. 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

7. Construction
two polarities: npn and pnp

8. 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

9. 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

10. Transistor action

11. 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

12. 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

13. 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

14. 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

15. 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

16. Equivalent circuits for a bipolar transistor

17. 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

18. 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

19. 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

20. Negative feedback amplifiers

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

22. 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 = mA 2.2 k = 5.6 V

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

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

25. A voltage regulator

26. A logical switch

27. 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