Download From Kirchhoff`s voltage law

Chapter 4:
DC Biasing–BJTs
Biasing
Biasing refers to the DC voltages applied to a transistor in order to turn it
on so that it can amplify the AC signal.
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Operating Point
The DC input establishes an
operating or quiescent point
called the Q-point.
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Biasing and the Three States of Operation
• Active or Linear Region Operation
Base–Emitter junction is forward biased
Base–Collector junction is reverse biased
• Cutoff Region Operation
Base–Emitter junction is reverse biased
• Saturation Region Operation
Base–Emitter junction is forward biased
Base–Collector junction is forward biased
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DC Biasing Circuits
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Fixed-bias circuit
Emitter-stabilized bias circuit
Collector-emitter loop
Voltage divider bias circuit
DC bias with voltage feedback
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Fixed Bias Circuit
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Base-Emitter Loop
From Kirchhoff’s voltage law:
+VCC – IBRB – VBE = 0
Solving for the base current:
IB 
VCC  VBE
RB
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Collector-Emitter Loop
The collector current is given by:
I C  I B
From Kirchhoff’s voltage law:
VCE  VCC  I C R C
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Transistor Saturation Level
When the transistor is operating in the saturation region
it is conducting at maximum current flow through the transistor.
VCC
I Csat 
RC
VCE  0 V
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Load Line Analysis
The end points of the load line are:
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ICsat
o IC = VCC / RC
o VCE = 0 V
VCEcutoff
o VCE = VCC
o IC = 0 mA
The Q-point is the particular operating point:
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where the value of RB sets the value of IB
where IB and the load line intersect
that sets the values of VCE and IC
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Circuit Values Affect the Q-Point
more …
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Emitter-Stabilzed Bias Circuit
Adding a resistor (RE) to
the emitter circuit
stabilizes the bias circuit.
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Base-Emitter Loop
From Kirchhoff’s voltage law :
 VCC - I E R E - VBE - I E R E  0
Since IE = ( + 1)IB:
VCC - I B R B - (  1)I B R E  0
Solving for IB:
IB 
VCC - VBE
R B  (  1)R E
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Collector-Emitter Loop
From Kirchhoff’s voltage law :
 I E R E  VCE  I C R C  VCC  0
Since IE  IC:
VCE  VCC – I C (R C  R E )
Also:
VE  I E R E
VC  VCE  VE  VCC - I C R C
VB  VCC – I R R B  VBE  VE
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Improved Biased Stability
Adding RE to the emitter improves the stability of a transistor.
Stability refers to a bias circuit in which the currents
and voltages will remain fairly constant for a wide
range of temperatures and transistor Beta () values.
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Saturation Level
The endpoints can be determined from the load line.
VCEcutoff:
ICsat:
VCE  VCC
I C  0 mA
VCE  0 V
IC 
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VCC
RC  RE
Voltage Divider Bias
This is a very stable bias
circuit.
The currents and voltages
are almost independent of
variations in .
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Approximate Analysis
Where IB << I1 and I2 and I1  I2 :
VB 
R 2 VCC
R1  R 2
Where RE > 10R2:
IE 
VE
RE
VE  VB  VBE
From Kirchhoff’s voltage law:
VCE  VCC - I C R C - I E R E
IE  IC
VCE  V CC -I C (R C  R E )
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Voltage Divider Bias Analysis
Transistor Saturation Level
I Csat  I Cmax 
V CC
RC  RE
Load Line Analysis
Cutoff:
Saturation:
VCE  VCC
I C  0mA
IC 
VCC
RC  RE
VCE  0V
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DC Bias with Voltage Feedback
Another way to
improve the stability of
a bias circuit is to add a
feedback path from
collector to base.
In this bias circuit the
Q-point is only slightly
dependent on the
transistor beta, .
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Base-Emitter Loop
From Kirchhoff’s voltage law:
VCC – I C R C – I B R B – VBE – I E R E  0
Where IB << IC:
I C  I C  I B  I C
Knowing IC = IB and IE  IC, the
loop equation becomes:
VCC – I B R C  I B R B  VBE  I B R E  0
Solving for IB:
IB 
VCC  VBE
R B  (R C  R E )
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Collector-Emitter Loop
Applying Kirchoff’s voltage law:
IE + VCE + ICRC – VCC = 0
Since IC  IC and IC = IB:
IC(RC + RE) + VCE – VCC =0
Solving for VCE:
VCE = VCC – IC(RC + RE)
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Base-Emitter Bias Analysis
Transistor Saturation Level
I Csat  I Cmax 
V CC
RC  RE
Load Line Analysis
Cutoff
Saturation
VCE  VCC
I C  0mA
VCC
IC 
RC  RE
VCE  0V
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Transistor Switching Networks
Transistors with only the DC source applied can be used as electronic
switches.
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Switching Circuit Calculations
Saturation current:
I Csat 
VCC
RC
To ensure saturation:
I
I B  Csat
 dc
Emitter-collector
resistance at saturation
and cutoff:
R sat 
VCEsat
I Csat
R cutoff 
VCC
I CEO
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Switching Time
Transistor switching times:
t on  t r  t d
t off  t s  t f
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Troubleshooting Hints
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Approximate voltages
– VBE  .7 V for silicon transistors
– VCE  25% to 75% of VCC
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Test for opens and shorts with an ohmmeter.
Test the solder joints.
Test the transistor with a transistor tester or a curve tracer.
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Note that the load or the next stage affects the transistor operation.
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PNP Transistors
The analysis for pnp transistor biasing circuits is the same as that
for npn transistor circuits. The only difference is that the currents
are flowing in the opposite direction.
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