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. Fig 5-10a & b

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. Fig 5-19 Base bias circuit

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 Fig 5-21a npn emitter bias

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