MALVINO Electronic PRINCIPLES SIXTH EDITION

Transistor Biasing Chapter 8

{ Voltage divider bias +VCC RC R1 R1 and R2 form a voltage divider R2 RE

current is normally much smaller than the divider current. +VCC Divider analysis: R1 R2 VBB = VCC +VBB R1 + R2 R2 ASSUMPTION: The base current is normally much smaller than the divider current.

Now the circuit can be viewed this way: +VCC RC RE VBB To complete the analysis: IE = VBB - VBE RE IC @ IE VC = VCC - ICRC VCE = VC - VE

The six-step process 1. Calculate the base voltage using the voltage divider equation. 2. Subtract 0.7 V to get the emitter voltage. 3. Divide by emitter resistance to get the emitter current. 4. Determine the drop across the collector resistor. 5. Calculate the collector voltage by subtracting. 6. Calculate the collector-emitter voltage by subtracting.

{ Is the divider a stiff source? +VCC RC R1 Find the Thevenin resistance. RTH = R1 R2 R2 RE

A Thevenin model of the bias circuit: +VCC RC RTH VBB RE

The 100:1 rule applied to the bias circuit: +VCC RC RTH < 0.01 RIN VBB RTH RIN When the rule is met, the divider is stiff. RE

Sometimes a firm divider is chosen. +VCC R1 R2 < 0.1 bdcRE RC R1 A closer approximation: IE = VBB - VBE RE + bdc R1 R2 R2 RE

Two-supply stiff emitter bias: 3.6 kW 10 V IE = VEE - 0.7 V RE Assume 0 V 2.7 kW 1 kW IE = 2 V - 0.7 V 1 kW = 1.3 mA 2 V

Find the voltages: 3.6 kW 10 V 2.7 kW 1 kW 2 V VC = 10 V - (1.3 mA)(3.6 kW) = 5.32 V VCE = 5.32 V - (-0.7 V) = 6.02 V 2.7 kW 1 kW 2 V

Base bias: The least predictable Q point moves with replacement +VCC Base bias: RC RB The least predictable Q point moves with replacement Q point moves with temperature Not practical

Emitter-feedback bias: +VCC Emitter-feedback bias: RC RB Better than base bias Q point still moves Not popular RE

Collector-feedback bias: +VCC Collector-feedback bias: RC RB Better than emitter-feedback bias Q point still moves Limited application

Collector- and emitter +VCC Collector- and emitter -feedback bias: RC RB Better than emitter-feedback bias Not as good as voltage-divider bias Limited application RE

Two-supply emitter bias: Very stable Requires 2 supplies

Voltage divider bias: Very stable Requires 1 supply The most popular +VCC Voltage divider bias: RC R1 Very stable Requires 1 supply The most popular R2 RE

PNP Transistors IC IB IE IC IB IE Electron flow Conventional flow

PNP Biasing with a negative supply -VCC PNP Biasing with a negative supply RC R1 R2 RE

PNP Biasing with a positive supply +VEE PNP Biasing with a positive supply RE R2 R1 RC