1. Chapter 1 – Revision Part 2
EKT 104
Analog Electronic Circuits 1
DC Biasing - BJTs
Chapter 1 – Revision Part 2

2. Biasing
Biasing: Applying DC voltages to a transistor in order to turn it on so that it can amplify AC signals.

3. Operating Point
The DC input establishes an operating or quiescent point
called the Q-point.

4. The Three Operating Regions
Active or Linear Region Operation
Base–Emitter junction is forward biased
Base–Collector junction is reverse biased
Cutoff Region Operation
Base–Emitter junction is reverse biased
Saturation Region Operation
Base–Collector junction is forward biased

5. DC Biasing Circuits Fixed-bias circuit Emitter-stabilized bias circuit
Collector-emitter loop
Voltage divider bias circuit
DC bias with voltage feedback

6. Fixed Bias

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

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

9. Example: Fixed-Bias Configuration

10. Example: Fixed-Bias Configuration
* Remember VBE = 0.7 V
when the transistor is in
ACTIVE state (ON).
a 𝐼 𝐵 𝑄 = 𝑉 𝐶𝐶 − 𝑉 𝐵𝐸 𝑅 𝐵 = 𝑉 − 0.7 𝑉 240 𝑘Ω =47.08 𝜇𝐴
𝐼 𝐶 𝑄 = 𝛽𝐼 𝐵 𝑄 =(50) 𝜇𝐴 =2.35 𝑚𝐴
(b) 𝑉 𝐶𝐸 𝑄 = 𝑉 𝐶𝐶 − 𝐼 𝐶 𝑅 𝐶 =12.0 𝑉−(2.35 𝑚𝐴)(2.2𝑘Ω)
= 6.83 V
c 𝑉 𝐵𝐸 = 𝑉 𝐵 − 𝑉 𝐸 = 𝑉 𝐵 =0.7 V
𝑉 𝐶𝐸 = 𝑉 𝐶 − 𝑉 𝐸 = 𝑉 𝐶 =6.83 V
d 𝑉 𝐵𝐶 = 𝑉 𝐵 − 𝑉 𝐶 =0.7 V −6.83 V=−6.13 V

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

12. Load Line Analysis ICsat VCEcutoff The load line end points are:
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 controls the values of VCE and IC .

13. The Effect of VCC on the Q-Point

14. The Effect of RC on the Q-Point

15. The Effect of IB on the Q-Point

16. Emitter-Stabilized Bias Circuit
Adding a resistor (RE) to the emitter circuit stabilizes the bias circuit.

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

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

19. Example: Emitter-Bias Configuration

20. Example: Emitter-Bias Configuration
= V
d 𝑉 𝐶 = 𝑉 𝐶𝐶 − 𝐼 𝐶 𝑅 𝐶 =20.0 𝑉− 2.01 𝑚𝐴 2 𝑘Ω
=15.98 𝑉
(e) 𝑉 𝐸 = 𝑉 𝐶 − 𝑉 𝐶𝐸 =15.98 V−13.97 V=2.01 V
(f) 𝑉 𝐵 = 𝑉 𝐵𝐸 + 𝑉 𝐸 =0.7 V+2.01 V=2.71 V
g 𝑉 𝐵𝐶 = 𝑉 𝐵 − 𝑉 𝐶 =2.71 V −15.98 V
=−13.27 V

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

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

24. Voltage Divider Bias This is a very stable bias circuit.
The currents and voltages are nearly independent of any variations in .

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

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

27. DC Bias With Voltage Feedback
Another way to improve the stability of a bias circuit is to add a feedback path from collector to base.
In this bias circuit the Q-point is only slightly dependent on the transistor beta, .

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

29. Collector-Emitter Loop
Applying Kirchoff’s voltage law:
IE + VCE + I’CRC – VCC = 0
Since IC  IC and IC = IB:
IC(RC + RE) + VCE – VCC =0
Solving for VCE:
VCE = VCC – IC(RC + RE)

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

31. Transistor Switching Networks
Transistors with only the DC source applied can be used as electronic switches.

32. Switching Circuit Calculations
Saturation current:
To ensure saturation:
Emitter-collector resistance at saturation and cutoff:

33. Switching Time
Transistor switching times:

34. Troubleshooting Hints
Approximate voltages
Test for opens and shorts with an ohmmeter.
Test the solder joints.
Test the transistor with a transistor tester or a curve tracer.
Note that the load or the next stage affects the transistor operation.
VBE  .7 V for silicon transistors
VCE  25% to 75% of VCC

35. PNP Transistors
The analysis for pnp transistor biasing circuits is the same as that for npn transistor circuits. The only difference is that the currents are flowing in the opposite direction.