This document discusses different types of transistors and their operating regions. It describes:
1) The two main types of transistors - bipolar junction transistors (BJT) and field effect transistors (FET) - and how they control current and voltage. BJTs use voltage to control current, while FETs use current to control voltage.
2) The two common types of BJTs - NPN and PNP - which differ in the order of doped semiconductor regions.
3) The three operating regions of BJTs - cutoff, active, and saturation. The active region is where BJTs function as amplifiers by controlling collector current with base current.
4) Key transistor parameters
2. 2
Transistors
•Transistors is a semiconductor which is used to control the flow of
voltage or current.
• The switch current can be controlled by either current or voltage
• Bipolar Junction Transistors (BJT) control current by voltage
• Field Effect Transistors (FET) control voltage by current
•They can be used either as switches or as amplifiers
6. 6
BJT α and β
•From the previous figure iE = iB + iC
•Define α = iC / iE
•Define β = iC / iB
•Then β = iC / (iE –iC) = α /(1- α)
•Then iC = α iE ; iB = (1-α) iE
•Typically β ≈ 100 for small signal BJTs (BJTs that
handle low power) operating in active region (region
where BJTs work as amplifiers)
7. 7
BJT in Active Region
Common Emitter(CE) Connection
• Called CE because emitter is common to both VBB and VCC
8. 8
BJT in Active Region
•Base Emitter junction is forward biased
•Base Collector junction is reverse biased
•For a particular iB, iC is independent of RCC
⇒transistor is acting as current controlled voltage source (iC is
controlled by iB, and iC = β iB)
• Since the base emitter junction is forward biased, from Shockley
equation
−
= 1exp
T
BE
CSC
V
V
Ii
9. 9
BJT in Active Region
•Normally the above equation is never used to calculate iC, iB
Since for all small signal transistors vBE ≈ 0.7. It is only useful
for deriving the small signal characteristics of the BJT.
•For example, for the CE connection, iB can be simply
calculated as,
BB
BEBB
B
R
VV
i
−
=
or by drawing load line on the base –emitter side
10. 10
BJT in Cutoff Region
•Under this condition iB= 0
•As a result iC becomes negligibly small
•Both base-emitter as well base-collector junctions may be reverse
biased
•Under this condition the BJT can be treated as an off switch
11. 11
BJT in Saturation Region
•Under this condition iC / iB < β in active region
•Both base emitter as well as base collector junctions are forward
biased
•VCE ≈ 0.2 V
•Under this condition the BJT can be treated as an on switch
12. 12
•A BJT can enter saturation in the following ways (refer to
the CE circuit)
•For a particular value of iB,if we keep on increasing RCC
•For a particular value of RCC,if we keep on increasing iB
•For a particular value of iB,if we replace the transistor
with one with higher β
BJT in Saturation Region
13. Early Effect and Early Voltage
As reverse-bias across collector-base junction increases, width of the
collector-base depletion layer increases and width of the base decreases
(base-width modulation).
In a practical BJT, output characteristics have a positive slope in forward-
active region; collector current is not independent of vCE.
Early effect: When output characteristics are extrapolated back to point of
zero iC, curves intersect (approximately) at a common point vCE = -VA which
lies between 15 V and 150 V. (VA is named the Early voltage)
Simplified equations (including Early effect)
13
iC
=IS
exp
vBE
VT
1+
vCE
VA
βF
=βFO
1+
vCE
VA
iB
=
IS
βFO
exp
vBE
VT
15. 15
BJT ‘Q’ Point (Bias Point)
•Q point means Quiescent or Operating point
• Very important for amplifiers because wrong ‘Q’ point
selection increases amplifier distortion
•Need to have a stable ‘Q’ point, meaning the the operating
point should not be sensitive to variation to temperature or
BJT β, which can vary widely
16. . 16
Input Characteristics
• Plot IB as f(VBE, VCE)
• As VCE increases, more
VBE required to turn the
BE on so that IB>0.
• Looks like a pn junction
volt-ampere
characteristic.
17. . 17
Output Characteristics
• Plot IC as f(VCE, IB)
• Cutoff region (off)
both BE and BC reverse
biased
• Active region
BE Forward biased
BC Reverse biased
• Saturation region (on)
both BE and BC forward
biased
VCC/RC
VCC
