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1. Bipolar Junction Transistors
(BJT) as a switch

2. Introduction
• A transistor is a device that can be used as
either an amplifier or a switch. Transistor is
current controlling device.
Transistors can be either
npn or pnp type.

3. Introduction
• The three layers of BJT are called Emitter, Base and Collector
• Base is very thin compared to the other two layers
• Base is lightly doped. Emitter is heavily doped. Collector is moderately
doped
• NPN – Emitter and Collector are made of N-type semiconductors; Base is
P-type
• PNP – Emitter and Collector are P-type, Base is N-type
• Both types (NPN and PNP) are extensively used, either separately or in
the same circuit
• BJT has two junctions – Emitter-Base (EB) Junction
and Collector-Base (CB) Junction
• The device is called “bipolar junction transistor” because current is due
to motion of two types of charge carriers – free electrons & holes
• Transistor Analogous to two diodes connected back-to-back: EB diode
and CB diode

4. Transistor Structure
• In diodes there is one p-n junction.
• In Bipolar junction transistors (BJT), there are three
layers and two p-n junctions.
Note: Arrow Direction from P to N ( Like Diode)

5. Figure showing different transistor packages

6. Transistor Operation
• Operation of NPN transistor is discussed here
• For normal operation (amplifier application)
• – EB junction should be forward biased
• – CB junction should be reverse biased
• Depletion width at EB junction is narrow (forward biased)
• Depletion width at CB junction is wide (reverse biased)

7. Transistor- Normal Operation

8. • When EB junction is forward biased, free electrons from
emitter region drift towards base region
• Some free electrons combine with holes in the base to
form small base current
• Inside the base region (p-type), free electrons are
minority carriers. So most of the free electrons are swept
away into the collector region due to reverse biased CB
junction
• Three currents can be identified in BJT
1. Emitter current;
2. Base current;
3. Collector current

9. Current directions in NPN and PNP transistors:
For both varieties: ------------(1)

10. Transistor Configurations
• BJT has three terminals
• For two-port applications, one of the BJT terminals needs
to be made common between input and output
Accordingly three configurations exist:
– Common Base (CB) configuration
– Common Emitter (CE) configuration
– Common Collector (CC) configuration

11. Common Base ( CB ) configuration
Base is common between input and output
– Input voltage: VEB Input current: IE
– Output voltage: VCB Output current: IC
(Resistors are not shown here
for simplicity)

12. CB Input characteristics
– A plot of IE versus VEB for
various values of VCB
– It is similar to forward biased
diode characteristics
– As VCB is increased, IE increases
only slightly
– Note that second letter in the
suffix is B (for base)

13. CB Output characteristics
CB Output characteristics
– A plot of IC versus VCB for
various values of IE
– Three regions are identified:
Active,
Cutoff,
Saturation

14. Common Emitter configuration
(Resistors are omitted for simplicity)
Emitter is common between input and output
– Input voltage: VBE ; Input current: IB
– Output voltage: VCE; Output current: IC

15. CE input characteristics
• Plot of IB versus VBE for
various values of VCE
• Similar to diode
characteristics
• As VCE is increased, IB
decreases only slightly
• Note that second suffix is E
(for emitter)

16. CE output characteristics
– A plot of IC versus VCE
for various values of IB
– Three regions
identified:
Active,
Cut-off,
Saturation

17. – Active region:
• Linear region in the output characteristics
• E-B junction forward biased
• C-B junction reverse biased
• IC increases with IB
– Cut off region:
Region below IB=0 line (or IC=ICEO)
– Saturation Region:
Region to the left of the vertical line
VCE=VCE(sat)=0.3V ( for Silicon)

18. Transistor Biasing
• Applying external dc voltages to ensure that transistor
operates in the desired region
• Which is the desired region?
– For amplifier application, transistor should operate in
active region
– For switch application, it should operate in cut-off and
sat.

19. Quiescent point (Q-point)
• The point we get by plotting the dc values of IC , IB
and VCE (when ac input is zero) on the transistor
characteristics
Q-point is in the middle
of active region.

20. Voltage divider bias or Self bias
– Resistor RE connected between emitter
and ground
– Voltage-divider resistors R1 & R2
replace RB
– Circuit can be analyzed in two
methods:
• Exact method (using Thevenin’s
theorem)
• Approximation method
(neglecting base current)