1. ac Load Line Analysis Maximum Symmetrical Swing
BJT AMPLIFIER
ac Load Line Analysis
Maximum Symmetrical Swing

2. Common-emitter amplifier with emitter bypass capacitor

3. DC Load line: KVL on C-E loop

4. AC Load Line - KVL on C-E loop

6. Example
Determine the dc and ac load line. VBE=0.7V, β=150, VA=∞

7. Solution
To determine dc Q-point, KVL around B-E loop

9. dc & ac load line

10. Maximum Symmetrical Swing
Steps to design a BJT amplifier
Write DC load line equation (relates of ICQ & VCEQ)
Write AC load line equation (relates ic, vce; vce = - icReq, Req = effective ac resistance in C-E circuit)
Generally, ic = ICQ – IC(min), where IC(min) = 0 or some other specified min collector current
Generally, vce = VCEQ – VCE(min), where VCE(min) is some specified min C-E voltage
Combination of the above equations produce optimum ICQ & VCEQ values to obtain maximum symmetrical swing in output signal

11. Example
Determine the maximum output symmetrical swing for the ac load line in previous figure.
Solution
From the dc & ac load line, the maximum negative swing in the Ic is from mA to zero (ICQ). So, the maximum possible peak-to-peak ac collector current:
The max. symmetrical peak-to-peak output voltage:
Maximum instantaneous collector current:

12. Common-collector Amplifier Common-base Amplifier
BJT AMPLIFIER
Common-collector Amplifier
Common-base Amplifier

13. C-C Small Signal Voltage Gain
Common Collector Circuit
Small Signal Equivalent Circuit

15. C-C Input & Output Impedance

16. Example Calculate the small signal voltage gain,
the input and output resistance.
Assume the transistor & circuit parameter are;β=100, VCC=5V, VBE=0.7V, VA=80V and ro=100kΩ.

17. C-C Small Signal Current Gain
Can be determine by using the input resistance & the concept of current dividers.

18. Common Base Amplifier

19. C-B Small Signal Equivalent Circuit

20. C-B Small Signal Voltage Gain

21. C-B Small Signal Current Gain

22. Input Impedance

23. Output Impedance

24. Summary & Comparison Configuration Common Emitter Common Collector
Common Base
Voltage Gain
Av > 1
Av  1
Current Gain
Ai > 1
Ai  1
Input Resistance
Moderate
(kΩ)
High
(50-100kΩ)
Low
(Ω)
Output
Resistance
Moderate to High