1. Types of BJT Biasing CircuitsBy
G.Kavitha
ECE Dept
Paavai Engineering College

2. Biasing Circuits of the BJT
Normally, three types of circuits are used to establish dc biasing in a BJT, these are;
(a) Fixed bias circuit
(b) Collector-to-Base Bias Circuit
(c) Voltage-Divider with Self-Bias Circuit (or) Self-Bias (or) Emitter Bias (or) Emitter-Stabilized Bias Circuit
While designing the all four types of biasing circuits of BJT it is aimed to obtain Q-point parameters (IBQ, ICQ, VCEQ)

3. Fixed Bias

4. The Base-Emitter Loop From Kirchhoff’s voltage law:
+VCC – IBRB – VBE = 0
Solving for base current:

5. Collector-Emitter Loop
Collector current:
From Kirchhoff’s voltage law:

6. Saturation
When the transistor is operating in saturation, current through the transistor is at its maximum possible value.

7. The end points of the load line are: ICsat
Load Line Analysis
The end points of the load line are:
ICsat
IC = VCC / RC
VCE = 0 V
VCEcutoff
VCE = VCC
IC = 0 mA
The Q-point is the operating point:
where the value of RB sets the value of IB
that sets the values of VCE and IC

8. Collector to Base Bias

9. Base-Emitter Loop From Kirchhoff’s voltage law: Where IB << IC:
Knowing IC = IB and IE  IC, the loop equation becomes:
Solving for IB:

10. Collector-Emitter Loop
From Kirchhoff’s voltage law:
Since IC  IC and IC = IB:
Solving for VCE:

11. Voltage Divider Bias (or) Self Bias

12. Approximate Analysis Where IB << I1 and I1  I2 :
Where bRE > 10R2:
From Kirchhoff’s voltage law:

13. Voltage Divider Bias Analysis
Transistor Saturation Level
Load Line Analysis
Cutoff: Saturation:

14. Emitter-Stabilized Bias Circuit

15. Base-Emitter Loop From Kirchhoff’s voltage law: Since IE = (b + 1)IB:
Solving for IB:

16. Collector-Emitter Loop
From Kirchhoff’s voltage law:
Since IE  IC:
Also:

17. Improved Biased Stability
Stability refers to a circuit condition in which the currents and voltages will remain fairly constant over a wide range of temperatures and transistor Beta () values.
Adding RE to the emitter improves the stability of a transistor.

18. Saturation Level The endpoints can be determined from the load line.
VCEcutoff:
ICsat:

19. Emitter-Stabilized Bias Circuit

20. Base-Emitter Loop From Kirchhoff’s voltage law: Where IB << IC:
Knowing IC = IB and IE  IC, the loop equation becomes:
Solving for IB:

21. Collector-Emitter Loop
From Kirchhoff’s voltage law:
Since IC  IC and IC = IB:
Solving for VCE:

22. Base-Emitter Bias Analysis
Transistor Saturation Level
Load Line Analysis
Cutoff: Saturation:

23. THANK YOU