Transcript Voltage Divider Bias

Voltage Divider Bias
• Voltage-divider bias is the most widely used type of bias circuit. Only
one power supply is needed and voltage-divider bias is more stable
(independent) than other bias types.
• Two current path to ground, I2 and IE. R1 and R2 are used to provide
the needed voltage to point A (base).
• A voltage divider in which the base current is small compared to the
current in R2 is said to be a stiff voltage divider because the base
voltage is relatively independent of the different transistor and
temperature effects.
Stiff
RINBASE =>10R2
VB = (R2/(R1 + R2))VCC
VE = VB – VBE
IC = IE = VE / RE
VC = VCC - ICRC
• In some cases, you need to analyze the circuit with more accuracy.
• If the circuit designer wanted to raise the input resistance, the divider
string may not be stiff.
• If IB is not small enough, resistance to ground from the base, RINbase is
significant enough to consider in most cases.
• In the case where base to ground resistance (input resistance) RINbase
is low enough to consider, so RIN(base) = βDCRE
• We can view the voltage at point A of the circuit in two ways, with or
without the input resistance (point A to ground) considered.
Not stiff
• RINbase = VIN/IIN = ßDCIBRE/IB = ßDCRE
• If RINbase < 10R2
VB = ((R2 || ßDCRE)/((R1 + R2 )|| ßDCRE) VCC
VE = VB – VBE
IE = VE/RE and assume, IC = IE
VC = VCC – ICRC
VCE = VC – VE
VCC – ICRC – IERE – VCE = 0
Since IC = IE
VCE = VCC – IC(RC + RE)
Exercise : Calculate VCE and IC if ß = 100
Base Bias
VCC – VRB – VBE = 0
VCC – IBRB – VBE = 0
IB = (VCC – VBE)/RB
VCC – ICRC – VCE = 0
VCE = VCC – ICRC
Substituting the expression for IB into the formula IC = ßDC yields
IC = ßDC ((VCC – VBE)/RB)
This type of circuit is very unstable since it’s ßDC changes with temperature
and IC also from one transistor to another due to manufacturing variations.
Exercise : How much Q-point (IC, VCE) will change when ß
increase from 85 to 100
Emitter Bias
The circuit is independent of ßDC making it as stable as the voltage-divider
type, but it requires two power supplies (VCC and VEE).
IC =IE= (-VEE-VBE) /(RE + RB/ ßDC)
VE = VEE + IERE
VB = VE + VBE
VC = VCC – ICRC
IE depend to VBE and βDC, both change
with temperature and IB
This can improve by,
RE >> RB/ β, IE = -(VEE – VBE)/RE
VEE >> VBE, IE = VEE/RE
Collector-Feedback Bias
It kept stable with negative feedback, but not as stable as voltage-divider or
emitter. If IC, try to increase, VRC increase, VC decrease, VRB decrease, IB
decrease.
IB = (VC – VBE)/RB
VC = VCC - ICRC
IC = (VCC – VBE)/(RC + RB/ βDC)
Since emitter is ground, VCE = VC
VCE = VCC – ICRC
IC depend on βDC and VBE, and can
minimize by,
RC >> RB and VCC >> VBE