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. 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)
4. Transistor Operation
• Operation of NPN transistor
• 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)
6. • 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
7. 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
8. Common Base ( CB ) configuration
Base is common between input and output
– Input voltage: VEB Input current: IE
– Output voltage: VCB Output current: IC
9. 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
10. Input resistance ri
Voltage amplification factor AV
Note: Both can be determined from the CB input characteristics
11. CB Output characteristics
CB Output characteristics
– A plot of IC versus VCB for
various values of IE
– Three regions are identified:
Active,
Cutoff,
Saturation
12. Active region:
• E-B junction forward biased
• C-B junction reverse biased
• IC is positive, VCB is positive
• IC increases with IE
Cut off Region:
• When IE = 0, IC = ICBO
• ICBO is collector to base current with emitter open
• – Below this line we have cut-off region
• – Here both junctions are reverse biased
Saturation Region:
• Region to the left of y-axis (VCB negative)
• Here both junctions are forward biased
• IC decreases exponentially, and eventually changes direction
13. • Output resistance ro
•Current amplification factor AI or αac
Note: Both can be measured from output characteristics
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. •Input resistance ri •Output resistance ro
•Voltage gain AV •Current gain AI or βac
Note: All these parameters can be determined from CE characteristics
