1. 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

2. 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.
Fig 5-10a & b

3. Not stiff
RINbase = VIN/IIN = ßDCIBRE/IB = ßDCRE
If RINbase < 10R2
VB = ((R2 || ßDCRE)/((R1 + R2 )|| ßDCRE) VCC

4. 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)

5. Exercise : Calculate VCE and IC if ß = 100

6. 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.
Fig 5-19 Base bias circuit

7. Exercise : How much Q-point (IC, VCE) will change when ß increase from 85 to 100

8. 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
Fig 5-21a npn emitter bias

9. 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
VB = VBE