bjts transistors all your need to know .ppt

transistors electronics

1. 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

2. 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

3. 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

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

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

7. 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

8. 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

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

10. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.10
Bipolar Transistor Characteristics
Behaviour can be described by the current gain, h
fe
or by the transconductance, g
m
of the device
21.4

11. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.11
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. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.12
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. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.13
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

14. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.14
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

15. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.15
Equivalent circuits for a bipolar transistor

16. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.16
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
FEI
B
Behaviour is characterised by the current gain or the
transconductance
21.5

17. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.17
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

18. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.18
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

19. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.19
Negative feedback amplifiers

20. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.20
Example – see Example 21.2 from course text
Determine the
quiescent output
voltage of this
circuit

21. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.21
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
CR
C = 10 - 2 mA 2.2 k = 5.6 V
V7.2
k 10k 72
k 10
01
21
2






RR
R
VV
CCB
mA 2
k 1
V0.2



E
E
E
R
V
I

22. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.22
A common-collector amplifier
–unity gain
–high input resistance
–low output resistance
–a very good
buffer amplifier

23. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.23
Other Bipolar Transistor Applications
A phase splitter
21.7

24. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.24
A voltage regulator

25. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.25
A logical switch

26. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 21.26
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