1. MALVINO
Electronic
PRINCIPLES
SIXTH EDITION

2. Transistor Biasing
Chapter 8

3. { Voltage divider bias +VCC RC R1 R1 and R2 form a voltage divider R2RE

4. 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.

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

6. 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.

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

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

9. 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

10. 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

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

12. 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

13. 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

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

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

16. 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

17. Two-supply emitter bias:
Very stable
Requires 2 supplies

18. 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

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

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

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