Download Transistors-II

TRANSISTOR
BY: Sai Kiran Reddy
Dwarampudi
Recall p-n junction
W
+
P
N
N
P
W
-
+
-
Vappl < 0
Vappl > 0
Forward bias, + on P, - on N
(Shrink W, Vbi)
Reverse bias, + on N, - on P
(Expand W, Vbi)
Allow holes to jump over barrier
into N region as minority carriers
Remove holes and electrons away
from depletion region
I
I
V
V
Transistor
• Trans—relation b/w input and output
• Istor—is taken from the RESISTOR
• Output resistance by input
resistance
Origin of the names
• the Emitter 'emits' the electrons which pass
through the device
• the Collector 'collects' them again once
they've passed through the Base
• ...and the Base?...
Currents
Conventional View
types
•
•
•
•
Bc 107---driver stage
2n2222--switching
Bc547
Bc548
CONFIGURATIONS
•
•
•
COMMON BASE
COMMON COLLECTOR
COMMON EMITTER
Common Base NPN
Common Collector NPN
How does IC vary with VCE for various IB?
Note that both dc sources are variable
Set VBB to establish a certain IB
Common Emitter NPN
Common Emitter Characteristics
• We can therefore draw an input characteristic
(plotting base current IB against base-emitter
voltage VBE) and
• an output characteristic (plotting collector
current Ic against collector-emitter voltage VCE)
IDEAL CE INPUT (Base) Characteristics
IDEAL CE OUTPUT (Collector)
Characteristics
ACTUAL CE OUTPUT Characteristics
IB =
Various Regions (Modes) of Operation of BJT
Active:
• Most important mode of operation
• Central to amplifier operation
• The region where current curves are practically flat
Saturation:
• Barrier potential of the junctions cancel each other out causing a
virtual short (behaves as on state Switch)
Cutoff:
• Current reduced to zero
• Ideal transistor behaves like an open switch
* Note: There is also a mode of operation called inverse
active mode, but it is rarely used.
DC  and DC 
 = Common-emitter current gain
 = Common-base current gain
 = IC
 = IC
IB
IE
The relationships between the two parameters are:
= 
=

+1
1-
Note:  and  are sometimes referred to as dc and dc
because the relationships being dealt with in the BJT are DC.
The DC Operating Point
For a transistor circuit to amplify it must be properly biased with dc voltages. The dc
operating point between saturation and cutoff is called the Q-point. The goal is to
set the Q-point such that that it does not go into saturation or cutoff when an a ac
signal is applied.
NEED FOR STABLIZING Q-POINT
• Ambience temperature
• Human Error
• Cut-in voltage [ -2.5mv/c]
STABILITY FACTOR: Measure of stability of
operating point
biasing
•
•
•
•
Fixed bias
Collector to base bias
Fixed bias with emitter resistor
Self bias or voltage divider bias
Fixed Bias
Collector to
base bias
Emitter Bias
Self bias
Transistors as Switches