1. Principles & Applications Small-Signal Amplifiers
Electronics
Principles & Applications
Sixth Edition
Charles A. Schuler
Chapter 7
More About
Small-Signal Amplifiers
© Glencoe/McGraw-Hill

2. INTRODUCTION
Amplifier Coupling
Voltage Gain
FET Amplifier
Negative Feedback
Frequency Response

3. VCC Capacitive coupling is convenient in cascade ac amplifiers.
These two points are at different dc voltages.

4. Direct coupling is required for dc gain.
VCC

5. The darlington is a popular dc arrangement.
VCC

6. Transformer coupling offers the advantage of impedance matching.
VCC
10:1
ZRATIO = TRATIO2 P S
10 W
= 102 = 100
ZCOLLECTOR = 100 x 10 W = 1000 W

7. VCC fR Transformer coupling can be used in bandpass amplifiers
to achieve selectivity.
VCC
Gain fR

8. Amplifier Coupling Quiz
Capacitive coupling is not useful for
_________ amplifiers. dc
Dc frequency response requires ________ coupling.
direct
Transformer coupling offers the advantage of _________ matching.
impedance
Tuned transformer coupling provides frequency _____________.
selectivity
A darlington amplifier is an example of _________ coupling.
direct

9. More about solving the practical circuit for its ac conditions:
VCC
= 12 V
RB1
22 kW RL
= 2.2 kW C
Zin = ? B E
RB2
2.7 kW RE
= 220 W

10. Zin is a combination of RB1, RB2, and rin of the transistor.
VCC
= 12 V
Determine rin first:
rin = b (RE + rE)
RB1
22 kW RL
= 2.2 kW
rin = 150 (220 W W) C
rin = 34.4 kW B E
RB2
2.7 kW RE
= 220 W
Note: rin = brE
when RE is bypassed.

11. RB1, RB2, and rin act in parallel to load the input signal.
VCC
= 12 V
Zin = 1
RB2
rin +
RB1
RB1
22 kW RL
= 2.2 kW +
Zin = 1
2.7 kW
34.4 kW
22 kW C B E
RB2
2.7 kW RE
= 220 W
Zin = 2.25 kW

12. VCC RL RB1 RB2 RE What happens when an amplifier is loaded?
RL and the Load act in parallel.
VCC
= 12 V
RP = 1.1 kW RL
= 2.2 kW
RB1
22 kW
Load = 2.2 kW
RB2
2.7 kW RE
= 220 W

13. There are two saturation currents for a loaded amplifier.
VCC
RL + RE
ISAT(DC) =
= 4.96 mA
VCC
= 12 V
VCC
RP + RE
ISAT(AC) =
= 9.09 mA
RB1 RL
22 kW
= 2.2 kW
RP = 1.1 kW
Load = 2.2 kW
RB2
2.7 kW RE
= 220 W

14. There are two load lines for a loaded amplifier.
The DC load line connects VCC and ISAT(DC).
100 mA 14 12
80 mA 10
60 mA
IC in mA 8
TEMPORARY AC
40 mA 6 4
20 mA DC 2
0 mA 2 4 6 8 10 12 14 16 18
VCE in Volts
A temporary AC load line connects VCC and ISAT(AC).

15. The quiescent VCE is projected to the DC load line to
establish the Q-point. The AC load line is drawn through
the Q-point, parallel to the temporary AC load line.
100 mA 14 12
80 mA 10
60 mA
IC in mA 8
40 mA AC 6
TEMP. AC 4
20 mA DC 2
0 mA 2 4 6 8 10 12 14 16 18
VCE in Volts
5.3 V

16. The AC load line shows the limits for VCE
and if the Q-point is properly located.
100 mA 14 12
80 mA 10
60 mA
IC in mA 8
40 mA 6 4 AC
20 mA 2
0 mA 2 4 6 8 10 12 14 16 18
VCE in Volts
5.3 V
With loaded amplifiers, the Q-point is often closer to saturation.

17. What about voltage gain for a loaded amplifier?
AV = RP
RE + rE
VCC
= 12 V
AV =
1.1 kW
220 W W
= 4.8
RB1 RL
= 2.2 kW
22 kW
RP = 1.1 kW
Load = 2.2 kW
RB2
2.7 kW RE
= 220 W

18. VCC When analyzing cascade amplifiers, remember: 1st 2nd
Zin of the 2nd stage loads the 1st stage.

19. Amplifier ac Conditions Quiz
Emitter bypassing _________ an amplifier’s input impedance.
decreases
Loading at the output of an amplifier
________ its voltage gain.
decreases
A loaded amplifier has two load lines: dc and ___________. ac
The clipping points of a loaded amplifier are set by its _______ load line. ac
In a cascade amplifier, the Zin of a stage _______ the prior stage.
loads

20. Common-source JFET amplifier.
VDD = 20 V
20 V
ISAT =
= 4 mA
5 kW
RL = 5 kW
Drain
Gate
Input
signal CC RG
Source
Phase-inverted
output
VGS = 1.5 V
Fixed bias

21. N-channel JFET characteristic curves
The Q-point is set by the fixed bias.
Load line 4
1 VP-P
-0.5 3
-1.0
VGS in Volts
ID in mA 2
-1.5
-2.0 1
-2.5 5 10 15 20 25
VDS in Volts
8 VP-P
AV = 8

22. Determining forward transfer admittance:4
ID in mA
-0.5 3
-1.0
VGS in Volts
1.6 mA 2
-1.5
-2.0 1
-2.5 5 10 15 20 25
VDS in Volts
DID
Yfs =
VDS
= 1.6 mS
DVGS

23. When the forward transfer admittance is known,
the voltage gain can be determined using:
VDD = 20 V
RL = 5 kW
AV = Yfs x RL D G
= 1.6 mS x 5 kW
= 8 CC RG S
This agrees with the
graphic solution.
VGS = 1.5 V

24. VDD IS = ID RL VGS = ID x RS CC RG RS
Source bias eliminates the need for a separate VGS supply.
VDD
IS = ID RL D
VGS = ID x RS G CC S RG RS
This resistor also provides
ac negative feedback which
decreases the voltage gain.

25. A negative feedback model
Summing
junction
A negative feedback model
A(Vin - BVout)
Vin
Vout A
Vin - BVout
A = open loop gain B
Feedback
BVout
B = feedback ratio
AVin
Vout
1 =
- AB
AVin
Vout
AB +1 =
Vin
Vout
AB +1 A =
Vin
Vout
AB +1 A =
Vout = A(Vin - BVout)
Vout = AVin - ABVout
AB +1 A
Vin
Vout
A simplified model

26. VDD RL CC RG RS The feedback ratio (B) for this circuit
is easy to determine since the source and
drain currents are the same.
VDD RL
= 5 kW D
800 W G
B =
= 0.16
5 kW CC S RG RS
= 800 W

27. Use the simplified model:A
Vin
Vout
AB +1
Use the simplified model: 8
A(WITH NEG. FEEDBACK) =
= 3.51
(8)(0.16) + 1

28. VDD RL CC RG RS CS The source bypass capacitor will
eliminate the ac negative feedback
and restore the voltage gain.
VDD RL D G CC RG CS RS

29. JFET Amplifier Quiz
In a common-source amplifier, the input signal goes to the _______.
gate
In a common-source amplifier, the input
to output phase relationship is ____.
180o
The voltage gain of a C-S amplifier is equal to Yfs x _________.
load resistance
Source bias is produced by current flow through the _______ resistor.
source
An unbypassed source resistor _______ the voltage gain of a C-S amp.
decreases

30. Amplifier Negative Feedback
DC reduces sensitivity to device parameters
DC stabilizes operating point
DC reduces sensitivity to temperature change
AC reduces gain
AC increases bandwidth
AC reduces signal distortion and noise
AC may change input and output impedances

31. The frequency response curve of an ac amplifier
Midband
Amax
0.707 Amax
-3dB f
Bandwidth
The gain is maximum in the midband.
The bandwidth spans the -3 dB points
which are called the break frequencies.

32. The emitter bypass capacitor in this amplifier has
a significant effect on both gain and bandwidth.
50 W
10 mF
1 kW
100 W
1k W
6.8 kW

33. Gain and bandwidth with and without the emitter bypass50
BW1
Gain in dB
BW2
10 Hz
100 MHz
Frequency

34. Amplifier Frequency Response
The lower break frequency is partly determined by coupling capacitors.
It is also influenced by emitter bypass capacitors.
The upper break frequency is partly determined by transistor internal capacitance.
Both break frequencies can be influenced by negative feedback.

35. REVIEW Amplifier Coupling Voltage Gain FET Amplifier Negative Feedback
Frequency Response