1. ELEC 121
January 2004
BJT Fixed Bias
ELEC 121
Fixed Bias

2. BJT Biasing 1 For Fixed Bias Configuration:
Draw Equivalent Input circuit
Draw Equivalent Output circuit
Write necessary KVL and KCL Equations
Determine the Quiescent Operating Point
Graphical Solution using Loadlines
Computational Analysis
Design and test design using a computer simulation
January 2004
ELEC 121

3. Complete CE Amplifier with Fixed Bias
January 2004
ELEC 121

4. Fixed Bias and Equivalent DC Circuit
January 2004
ELEC 121

5. Fixed-Bias Circuit
January 2004
ELEC 121

6. DC Equivalent Circuit
January 2004
ELEC 121

7. Base-Emitter (Input) Loop
Using Kirchoff’s voltage law: – VCC + IBRB + VBE = 0
Solving for IB:
January 2004
ELEC 121

8. Collector-Emitter (Output) Loop
Since: IC =  IB
Using Kirchoff’s voltage law: – VCC + IC RC + VCE = 0
Because: VCE = VC – VE
Since VE = 0V, then: VC = VCE
And VCE = VCC - IC RC
Also: VBE = VB - VE
with VE = 0V, then: VB = VBE
January 2004
ELEC 121

9. BJT Saturation Regions
When the transistor is operating in the Saturation Region, the transistor is conducting at maximum collector current (based on the resistances in the output circuit, not the spec sheet value) such that:
January 2004
ELEC 121

10. Determining Icsat ELEC 121 January 2004 Fixed Bias January 2004

11. Determining ICSAT for the fixed-bias configuration
January 2004
ELEC 121

12. Load Line Analysis
January 2004
ELEC 121

13. Load Line Analysis
The end points of the line are : ICsat and VCEcutoff
For load line analysis, use VCE = 0 for ICSAT, and IC = 0 for VCEcutoff
ICsat:
VCEcutoff:
Where IB intersects with the load line we have the Q point
Q-point is the particular operating point:
Value of RB
Sets the value of IB
Where IB and Load Line intersect
Sets the values of VCE and IC.
January 2004
ELEC 121

14. Circuit values effect Q-point
January 2004
ELEC 121

15. Circuit values effect Q-point (continued)
January 2004
ELEC 121

16. Circuit values effect Q-point (continued)
January 2004
ELEC 121

17. Load-line analysis
January 2004
ELEC 121

18. Fixed-bias load line
January 2004
ELEC 121

19. Example
January 2004
ELEC 121

20. DC Fixed Bias Circuit Example
January 2004
ELEC 121

21. Loadline Example Family of Curves
ELEC 121
January 2004
Loadline Example Family of Curves
January 2004
ELEC 121
Fixed Bias