1. Recall Lecture 12 Voltage Transfer Characteristics Biasing of BJT
Vo versus Vi
Biasing of BJT
Three types of biasing
Fixed Bias Biasing Circuit
Biasing using Collector to Base Feedback Resistor

2. Bipolar Transistor Biasing

3. Voltage Divider Biasing Circuit
This is a very stable bias circuit.
The currents and voltages are almost independent of variations in .

4. Analysis
Redrawing the input side of the network by changing it into Thevenin Equivalent
RTH
RTh: the voltage source is replaced by a short-circuit equivalent

5. Analysis VTH VTh: open-circuit Thevenin voltage is determined. VTH
Inserting the Thevenin equivalent circuit
Use voltage divider

6. Analysis
The Thevenin equivalent circuit

7. Example Find VCE ,IE, IC and IB given
β=100, VCC=10V, R1 = 56 k, R2 = 12.2 k,
RC = 2 k and RE = 0.4 k
VTH= R2 /(R1 + R2 )VCC
VTH = 12.2k/(56k+12.2k).(10)
VTH = 1.79V
RTH = R1 // R2
= 10 k

8. BJT Biasing in Amplifier Circuits
VTH = RTH IB + VBE + RE IE
1.79 = 10k IB k ( +1)IB
IB = µA
IC = IB = 100(21.62)=2.16mA
IE = IC + IB = 2.18mA
VCC = RC IC + VCE + RE IE
10 = 2k(2.16m)+VCE +0.4(2.18m)
VCE = 4.8 V

9. VCE(V) IE (mA) IC (mA) IB (mA)
VCE(V)
IE (mA)
IC (mA)
IB (mA)
Measured
0.16 V 
5.441 mA 
5.382 mA 
0.059 mA

10. Example – A Design Question
VCC = 10 V
IE = 1 mA
=100
RE = k
RTH = 33 k
Determine the values of R1 and R2

11. R2 = 38.8 k BE Loop: IBRTH + 0.7 + IERE – VTH = 0
VTH = 1.5 V
We know:
1.5 (220 + R2) = 10R2
330 = 8.5 R2
R2 = 38.8 k