1. MALVINO
Electronic
PRINCIPLES
SIXTH EDITION

2. AC Models
Chapter 9

3. { The coupling capacitor XC R R OPEN R SHORT Good coupling:
1. For ac analysis, the capacitor is a short.
2. For dc analysis, the capacitor is open.

4. +30 V
5 kW
1 MW
bdc = 100
100 kW
100 mV
A base-biased amplifier with capacitive coupling
A dc analysis reveals IB = 30 mA, IC = 3 mA and VC = 15 V.

5. IB
30 mA t IC
3 mA t VC
15 V t

6. The base-biased amplifier with voltage waveforms
5 kW
1 MW
bdc = 100
100 kW
+0.7 V
100 mV
The base-biased amplifier with voltage waveforms

7. vin vout vout A = vin The voltage gain of an amplifier is
the ac output divided by the ac input.
A = 200
vin
vout
vout
A =
vin

8. The bypass capacitor
Ac ground R XC
Good bypassing:
XC < 0.1 R

9. A VDB amplifier with voltage waveforms
3.6 kW
10 kW
+1.8 V
100 kW
+1.1 V
2.2 kW
1 kW
100 mV

10. A TSEB amplifier with voltage waveforms
3.6 kW
+5.32 V
100 kW
-0.7 V
100 mV
2.7 kW
1 kW
-2 V

11. IE Q
VBE
Input signal
Large-signal operation produces distortion

12. Total emitter current: IE = IEQ + ie
IEQ = 10 mA
Less than
1 mA
VBE
Total emitter current: IE = IEQ + ie
Small-signal operation: ie(PP) < 0.1IEQ

13. Use capital letters for dc quantities and lowercase letters for ac.
The dc current gain is given as: IC
bdc = IB
The ac current gain is given as: ic ib
bac =
Use capital letters for dc quantities
and lowercase letters for ac.

14. IE
VBE
The size of the ac emitter current depends on the Q point.

15. Total emitter current: IE = IEQ + ie
Total base-emitter voltage: VBE = VBEQ + vbe
The ac resistance of the emitter diode is defined as:
vbe
re’ = ie

16. vbe re’ = ie IE Larger ie Smaller ie VBE
Note that re’ varies with the operating point.
This implies that re’ is a function of the dc emitter current.

17. A very useful formula:
25 mV
re’ = IE

18. The T model of a transistor:
zin(base) =
vbe ib ic
vbe = iere’
zin(base)
zin(base) =
iere’ ib ib
re’ ie
zin(base) = bre’

19. The p model of a transistor is based on zin(base) = bre’ :ib
zin(base)
bre’ ic ie

20. Amplifier analysis Perform a complete dc analysis
Short all coupling and bypass capacitors
Visualize all dc supply voltages as ac grounds
Replace the transistor by its p or T model
Draw the ac equivalent circuit

21. Data sheets The four h parameters are traditional
hfe is the ac current gain
hie is equivalent to input impedance
bac = hfe
re’ = hie/hfe
hre and hoe are not needed for basic design and troubleshooting