The given circuit is a CB amplifier. (a) The dc operating point or Q-point is midway between cutoff and saturation points. Cutoff point: IC = 0, VCE = 10 V Saturation point: IC = 2 mA, VCE = 0.2 V Q-point: IC = 1 mA, VCE = 5 V (b) Maximum unclipped signal is the distance between Q-point and either cutoff or saturation point. Maximum peak-to-peak signal = Saturation point - Cutoff point = 0.2 V - 10 V = 9.8 V (c) For no clipping, the ac signal amplitude should be less than half of the maximum

1. Chapter 4
Bipolar Junction Transistors (BJTs)

2. • Transistor is a combination of two words i.e. transfer
and resistor. It is because a transistor is basically a
resistor that amplifies electrical impulses as they are
transferred through it from its input to output
terminal.
• There are two basic types of transistors :
– The bipolar junction transistor (BJT)
– The field-effect transistor (FET)
• The bipolar junction transistor is used in two broad
areas of electronics
– as a linear amplifier to boost an electrical signal
– as an electronic switch.
Transistor

3. • Two back-to-back P-N junctions manufactured in a single
piece of a semiconductor crystal.
• These two junctions give rise to three regions called
emitter, base and collector.
• The emitter, base and collector are provided with terminals
which are labelled as E, B and C. The two junctions are :
emitter-base (E/B) junction and collector-base (C/B) junction.
• Emitter
– Heavily doped than any of the other regions
– Supplies majority charge carries (either electrons or holes) to the base.
• Base
– Forms the middle section .
– Very thin (10–6 m) as compared C&E
– Very lightly-doped.
• Collector
– Collects majority charge carriers from
the emitter and passing through the base.
Architecture of a Transistor

4. • For Proper Working (FR Biasing) of a Transistor
emitter-base junction is always forward-biased
 collector-base junction is always reverse-biased.
BJT Biasing

5. • An integrated circuit (IC) consists of
transistors, resistors, diodes and capacitors
combined together in one wafer-thin chip of
silicon. This one wafer-thin chip is called a
microchip. The microchip is only a few
millimeters square with a thickness of 0.5 mm.
IC?

6. • PNP
– Collector and base negative with respect to emitter
• Collector more negative than base
– N is same as in middle of PNP
• NPN
– Collector and base Positive with respect to emitter
• Collector more negative than base
– P is same as in middle of NPN
Important Biasing Rule

7. Transistor Currents
Applying Kirchhoff's Current Law, we have

8. DC Analysis of a Transistor
The four basic guideposts about all
transistor circuits are :
1.Conventional current flows along the
arrow whereas electrons flow against it;
2.E/B junction is always forward-biased;
3.C/B junction is always reverse-biased;
4.IE = IB + IC.
Summing Up

9. • Three types of circuit connections (called
configurations) for operating a transistor.
1. Common-base (CB),
2. Common-emitter (CE),
3. Common-collector (CC).
Also called as grounded-base, grounded-emitter,
and grounded-collector.
Transistor Circuit Configurations

10. • In this configuration, emitter current IE is the input current
and collector current IC is the output current. The ratio of
the collector current to the emitter current is called dc
alpha (αdc) of a transistor.
• The negative sign is due to the fact that current IE flows into
the transistor whereas IC flows out of it. Hence, IE is taken
as positive and IC as negative.
CB Configuration

11. • The α of a transistor is a measure of the quality of a
transistor
• Higher the value of α, better the transistor in the sense that
collector current more closely equals the emitter current.
• Its value ranges Its value ranges from 0.95 to 0.999.
• Obviously, it applies only to CB configuration of a transistor.
CB Configuration (DC α)

12. • Incidentally, there is also an a.c. α for a transistor. It refers
to the ratio of change in collector current to the change in
emitter current.
• It is also, known as short-circuit gain of a transistor and is
written as – hfb. It may be noted that upper case subscript
‘FB’ indicates dc value whereas lower case subscript ‘fb’
indicates ac value.
• For all practical purposes,
CB Configuration (AC α)

13. • Following current readings are obtained in a transistor
connected in CB configuration : IE = 2 mA and IB = 0.002 mA.
Compute the values of α and IC.
Example

14. • Input between the base and emitter
• Output signal from the collector and emitter circuit.
• IB is input current
• IC is output current
• The ratio of the d.c.
collector current to dc
base current is called
dc beta (βdc) or just β of the
transistor.
• Βdc Value approx 500
• βac = ΔIC / ΔIB.
• The flow of various currents in a CE configuration both for PNP
and NPN transistor.
CE Configuration

15. Relation Between α and β

16. • Input signal between base and collector
• Output signal from emitter-collector circuit.
• IB is input current
• IE is output current.
• The current gain is
CC Configuration

17. Relations Between Transistor Currents

18. Comparison of Amplifier Configurations
The basic building blocks of transistor amplifiers are single-stage
common-base, common emitter and common-collector circuits. The
choice of configuration and type of transistor for a given application
will depend largely upon the desired input and output impedances,
voltage, current and power gains and frequency response.

19. Amplifier Classification Based on Biasing Conditions

20. Different Ways of Drawing Transistor Circuits

21. Different Ways of Drawing Transistor Circuits

22. Common Base Formulas

23. Common Emitter Formulas

24. Common Collector Formulas

25. BJT Switches
•BJTs are used as electronic switches.
•A given load can be turned ON or OFF by a small control signal.
•This control signal might be the one appearing at the output of an
electronic device.
•Signal Switches BJT ON and OFF and BJT turns Load ON and OFF.
•Two Levels
•Cuttoff as an open
•Saturation as a short-circuit.

26. DC Load Lines
For drawing the dc load line of a transistor, one need to know only its cut-off and saturation
points. It is a straight line jointing these two points. For the CE circuit of Fig. 58.28, the load line
is drawn in Fig. 58.1. A is the cut-off point and B is the saturation point. The voltage equation of
the collector-emitter is

27. Q-Point
It is a point on the dc load line, which represents the values of IC and VCE that exist in a
transistor circuit when no input signal is applied. It is also known as the dc operating point or
working point. The best position for this point is midway between cut-off and saturation
points where VCE= (½) VCC (like point D in Fig. 58.1).

28. Q-Point (Example)
Example 58.2. In the CB circuit of Fig. 58.3 (a), find
(a) dc operating point and dc load line (b) maximum peak-to-peak unclipped signal
(c) the approximate value of ac source voltage that will cause clipping.