Section 5.5
Biasing of BJT Amplifier circuit
Biasing of BJT Amplifier circuit
• Biasing to establish constant DC Collector
current Ic & should be
•
•
•
•
Calculatable
Predictable
Insensitive to temp. variations
Insensitive to large variations in β
– To allow max. output signal swing with no
distortion
Figure 5.43 Two obvious schemes for biasing the BJT:
(a) by fixing VBE; (b) by fixing IB.
Figure 5.44 Classical biasing for BJTs using a single
power supply:
• Typical Biasing
– Single power supply
– Voltage Divider Network
– RE in Emitter Circuit
Typical Biasing
 R2 

VBB  VCC 
 R1  R2 
 R1 R2 

RB  
 R1  R2 
Figure 5.44 Classical biasing for BJTs using a single
power supply:
Classical Discrete-circuit Bias arrangement
(a) I C  I S e
v BE
VT
Any variation in v BE , IC changes
(b) iC  i
i changes iC
Base Emitter Loop
VBB  I B RB  VBE  I E RE
IE
IB 
 1
RB 

I E  RE 
  VBB  VBE
  1

VBB  VBE
IE 
RB
RE 
 1
Classical Discrete-circuit Bias arrangement
• For stable Ic, IE must be
stable as IC =αIE
• To make IE insensitive to
VBE (temp.) & β variations
VBB  VBE
IE 
R
RE  B
 1
VBB >> VBE
RE>> RB/(β+1)
Classical Discrete-circuit Bias arrangement
VBB >> VBE
For higher VBB at given V CC
VRB2  VRB1
VRB1  VRC  VCB
For smallerVRB1
VRC smaller
But for higher gain VRC should be more Larger signal Swing (before cutoff)
Av=-VRC / VT
VCB be large VCE is large for larger signed swing (before saturation)
Compromise
Role of thumb
1
VBB  VCC
3
1
VCE or VCB  VCC
3
1
I C RC  VCC
3
Classical Discrete-circuit Bias arrangement
RE>> RB/(β+1)
For RE 
RB
 1
- RB be small, thuslower value
of R 1 & R 2 resultsin large current
drain from thepower supply
- Results in lower input impedanceof
Amplifier- T hus LoadingEffect
T radeoff
Current through R c & R 2  I E - 0.1IE
For Stable IE - Negative Feed Back through RE
If IE increases somehow, VRE increases,
hence VE increases correspondingly,
VBB = VBE + VE ; VBE decreases for maintaining
constant VBB
Reduces collector (Emitter) current. Stable IE
Figure 5.45 Biasing the BJT using two power supplies.
Two Power Supplies Version
For  independent biasing
R B can be eleminated, if signal
is not applied to theBase
& Base connectedto ground
Two Power Supplies Version
Base Emitter Loop
I B RB  VBE  I E RE  VEE 
IB 
IE
 1
I B RB  VBE  I E RE  VEE
IE 
VEE  VBE
R
RE  B
 1

VEE  VBE
RB
RE 
 1
Figure 5.46 (a) A common-emitter transistor amplifier
biased by a feedback resistor RB.
A common-emitter transistor amplifier biased by
a feedback resistor RB.
Biasing using collector- Base Feed Back Resistor
CommonEmitterconfiguration only
RB provide negative Feedback
A common-emitter transistor amplifier biased by
a feedback resistor RB.
I E  IC  I B
VCC  I E RC  I B RB  VBE
IB 
IE
 1
VCC  VBE
IE 
RB
RC 
 1
VCC  VBE
RB
RC 
 1
A common-emitter transistor amplifier biased by
a feedback resistor RB.
RB
RC 
 1
VCB
I E RB
 I B RB 
 1
 RB Det erminessignal swing at t hecollect or.
 RB small


Signal swing will be t hesmall.
Input resist ancewill be small Loading
A BJT biased using a constant-current source I.
Biasing using a constant current
source
• Current in Emitter means
– Constant IC
IC =α IE
– Independent of RB & β value thus RB can
be made large to
• Increase Input resistance
• Large signal swing at collector
•Q1 acts as Diode CBJ is short circuits
Biasing using a constant current source
Q1 acts as Diode CBJ is short circuits
VCC-IREFR-VBE+VEE=0
I = IREF=(VCC-VBE+VEE)/R
Since Q1 & Q2 have VBE is same
I constant till Q2 in Active Mode (Region) & can be
guaranteed by
–Voltage at collector
V > (-VEE+VBE)
Current Mirror
Biasing using a constant current source
• IE is independent of β & RB
• RB can be made large thus increasing
input resistance
• Simple Design
• Q1 & Q2 are matched pair
• Q1 is Diode collector- Base connected
• β high IB can be neglected
α = 1 IC = IE
I = IREF=(VCC-VBE+VEE)/R