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Chapter 5
Transistor bias circuits
1
Objectives
Discuss the concept of dc biasing of a transistor for
linear operation
Analyze voltage-divider bias, base bias, emitter bias
and collector-feedback bias circuits.and collector-feedback bias circuits.
2
Biasing
Biasing: The DC voltages applied to a transistor in
order to turn it on so that it can amplify the AC
signal.
3
Operating Point
The DC input establishes
an operating or
quiescent point called the
Q- point.
4
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• Base–Emitter junction is reverse biased
• Saturation Region Operation
• Base–Emitter junction is forward biased Base–
Collector junction is forward biased
5
DC Biasing Circuits
• Fixed-bias circuit
• Emitter-stabilized bias circuit
• Collector-emitter loop
• Voltage divider bias circuit• Voltage divider bias circuit
• DC bias with voltage feedback
6
Fixed Bias
7
The Base - Emitter Loop
From Kirchhoff’s voltage law:
+VCC – IBRB – VBE = 0
Solving for base current:Solving for base current:
B
− VBEVCC
RI B ====
8
Collector-Emitter Loop
Collector current:
I ==== ββββIBC
From Kirchhoff’s voltage law:From Kirchhoff’s voltage law:
VCE ==== VCC −−−− ICRC
9
Saturation
When the transistor is operating in saturation, current
through the transistor is at its maximum possible value.
VCCI Csat ====
R CCsat
R C
VCE ≅≅≅≅ 0 V
10
Load Line Analysis
The end points of the load line
are: ICsat
IC = VCC / RC
VCE = 0 V
VCEcutoffVCEcutoff
VCE = VCC
IC = 0 mA
The Q-point is the operating point:
• where the value of RB sets the value
of IB
• that sets the values of VCE and IC
11
Circuit Values Affect the Q-Point
12
Circuit Values Affect the Q-Point
13
Circuit Values Affect the Q-Point
14
Emitter-Stabilized Bias Circuit
Adding a resistor (RE)
to the emitter circuit
stabilizes the biasstabilizes the bias
circuit.
15
Base-Emitter Loop
From Kirchhoff’s voltage law:
+ VCC - IERE - VBE - IERE ====0
Since IE = (ββββ + 1)IB:
VCC - IBRB - (ββββ ++++1)IBRE ==== 0VCC - IBRB - (ββββ ++++1)IBRE ==== 0
Solving for IB:
EB
VCC - VBE
++++ (ββββ ++++1)RIB ==== R
16
Collector-Emitter Loop
From Kirchhoff’s voltage law:
CE C C CC+ I R − V = 0I
ER
E + V
Since IE ≅≅≅≅ IC:
VCE ==== VCC – IC(RC ++++ RE )VCE ==== VCC – IC(RC ++++ RE )
Also:
VE ==== IERE
VC ==== VCE ++++ VE ==== VCC - ICRC
VB ==== VCC – IRRB ==== VBE ++++ VE
17
Improved Biased Stability
Stability refers to a circuit condition in which the currents
and voltages will remain fairly constant over a wide range of
temperatures and transistor Beta (ββββ) values.
Adding RE to the emitter improves the stability of aAdding RE to the emitter improves the stability of a
transistor.
18
Saturation Level
The endpoints can be determined from the load line.
VCEcutoff: ICsat:
VCE ==== VCC
IC ==== 0mA VCC
RC ++++ RIC ====
VCE ==== 0 V
19
Voltage Divider Bias
This is a very stable
bias circuit.
The currents and The currents and
voltages are nearly
independent of any
variations in ββββ.
20
Approximate Analysis
Where IB << I1 and I1 ≅≅≅≅ I2:
21B
R ++++ R
R2VCCV ====
Where ββββRE > 10R2:
I ====VE
EE
R
VE ==== VB −−−− VBE
From Kirchhoff’s voltage law:
VCE = VCC − ICRC −IERE
IE ≅ IC
VCE =VCC−IC(RC + RE )
21
Voltage Divider Bias Analysis
Transistor Saturation Level
VCCIEC
==== ICmax ====CsatR ++++ R
Load LineAnalysis Load LineAnalysis
Cutoff: Saturation:
VCE ==== VCC
IC ==== 0mA
CE
VCC
RC ++++ RE
V ==== 0V
IC ====
22
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.collector to base.
In this bias circuit the Q-
point is only slightly
dependent on the
transistor beta, ββββ.
23
Base-Emitter Loop
From Kirchhoff’s voltage law:
VCC – I′′′′CRC – IBRB – VBE – IERE ==== 0
Where IB << IC:
CI' = I
C + I
B ≅ I
C
Knowing I = ββββI and I ≅≅≅≅ I , the loop Knowing IC = ββββIB and IE ≅≅≅≅ IC, the loop
equation becomes:
VCC – ββββIBRC −−−− IBRB −−−− VBE −−−− ββββIBRE ==== 0
Solving for IB:
RB ++++ ββββ(RC ++++ RE )
VCC −−−− VBEIB ====
24
Collector-Emitter Loop
Applying Kirchoff’s voltage law:
IE + VCE + I’CRC – VCC = 0
Since I′′′′′′′′C ≅≅≅≅ IC and IC = ββββIB:
IC(RC + RE) + VCE – VCC=0
Solving for VCE:
VCE = VCC – IC(RC + RE)
25
Base-Emitter Bias Analysis
Transistor Saturation Level
ECCsat
VCCIR ++++ R
==== ICmax ====
Load LineAnalysis
Cutoff: Saturation:
V= VCCVCE
IC = 0mA ECC
I
VCE = 0 V
R + R= CC
26
PNP Transistors
The analysis for pnp transistor biasing circuits is the same as that for
npn transistor circuits. The only difference isthat the currents are
flowing in the opposite direction.
27
Base voltage
Emitter voltage
By Ohm’s Law,
Analysis of Voltage Bias for PNP Transistor
EE
EDC
B VRRR
RV
+=
ββββ21
1
BEBE VVV +=
VRIRIVV +++= By Ohm’s Law,
And,
DC
B
E
BEBEE
E RR
VVVI
β+
−−=
EECCCCEC
CCC
RIRIVV
RIV
−−=
=
BEEEBBBEEVRIRIVV +++=
28
Evaluate IC and VEC for pnp transistor circuit in Figure below.
Given VEE = +15V, R1 = 63kΩ, R2 = 27kΩ, RC = 1.8kΩ, RE =
2.6kΩ, βDC =120.
Example 1
29
Figure below shown the schematic with a negative supply
voltage, determine IC and VCE for a pnp transistor circuit with
given values: R1 = 25kΩ, R2 = 60kΩ, RC = 6kΩ, RE = 9kΩ,
VCC = -12V, and βDC = 90
Example 2
30
Construct a complete circuit required to replace the transistor in
Figure below with a pnp transistor. Given VCC = 10V, R1 =
78kΩ, R2 = 100kΩ, RC = 18kΩ, RE = 8kΩ.
Example 3
31