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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