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Electronics
Principles & Applications
Fifth Edition
Charles A. Schuler
Chapter 7
More About
Small-Signal Amplifiers
©1999
Glencoe/McGraw-Hill
INTRODUCTION
• Amplifier Coupling
• Voltage Gain
• FET Amplifier
• Negative Feedback
• Frequency Response
Capacitive coupling is convenient in cascade ac amplifiers.
VCC
These two points are at different dc voltages.
Direct coupling is required for dc gain.
VCC
The darlington is a popular dc arrangement.
VCC
VCC
Transformer coupling offers the
advantage of impedance matching.
10:1
P
S
10 W
ZRATIO = TRATIO2
= 102 = 100
ZCOLLECTOR = 100 x 10 W = 1000 W
VCC
Transformer coupling can be
used in bandpass amplifiers
to achieve selectivity.
Gain
fR
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
More about solving the practical
circuit for its ac conditions:
VCC = 12 V
RB1 22 kW
RL= 2.2 kW
C
Zin = ?
B
RB2 2.7 kW
E
RE = 220 W
Zin is a combination of RB1, RB2,
and rin of the transistor.
VCC = 12 V
RB1 22 kW
Determine rin first:
RL= 2.2 kW rin = b (RE + rE)
C
B
RB2 2.7 kW
E
rin = 150 (220 W + 9.03 W)
rin = 34.4 kW
RE = 220 W
Note: rin = b rE
when RE is bypassed.
RB1, RB2, and rin act in parallel
to load the input signal.
VCC = 12 V
1
Zin =
1
1
1
+
+ r
RB2
in
RB1 22 kW
RL= 2.2 kW RB1
C
B
RB2 2.7 kW
Zin =
E
RE = 220 W
1
1
1
+
+
22 kW 2.7 kW 34.4 kW
1
Zin = 2.25 kW
What happens when an amplifier is loaded?
VCC = 12 V
RL and the Load act in parallel.
RP = 1.1 kW
RB1 22 kW
RL= 2.2 kW
Load = 2.2 kW
RB2 2.7 kW
RE = 220 W
There are two saturation currents for a loaded amplifier.
VCC
= 4.96 mA
ISAT(DC) =
RL + RE
V = 12 V
CC
RB1 22 kW
VCC
ISAT(AC) =
= 9.09 mA
RP + RE
RL= 2.2 kW
RP = 1.1 kW
Load = 2.2 kW
RB2 2.7 kW
RE = 220 W
There are three load lines for a loaded amplifier.
The DC load line connects VCC and ISAT(DC).
14
12
10
IC in mA 8
TEMPORARY AC
6
4 DC
2
0 2 4 6
8 10 12 14 16 18
VCE in Volts
100 mA
80 mA
60 mA
40 mA
20 mA
0 mA
The temporary AC load line connects VCC and ISAT(AC).
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
10
IC in mA 8
AC
6
4 DC
2
80 mA
60 mA
40 mA
TEMP. AC
0 2 4 6
5.3 V
8 10 12 14 16 18
VCE in Volts
20 mA
0 mA
The AC load line shows the limits for VCE
and if the Q-point is properly located.
14
12
10
IC in mA 8
6
4
2
100 mA
80 mA
60 mA
40 mA
20 mA
AC
0 2 4 6
8 10 12 14 16 18
VCE in Volts
0 mA
5.3 V
With loaded amplifiers, the Q-point is often closer to saturation.
What about voltage gain for a loaded amplifier?
RP
AV =
R
+
r
E
E
VCC = 12 V
1.1 kW
= 4.8
AV =
220 W + 9.03 W
RL= 2.2 kW
RB1 22 kW
RP = 1.1 kW
Load = 2.2 kW
RB2 2.7 kW
RE = 220 W
When analyzing cascade amplifiers, remember:
VCC
1st
2nd
Zin of the 2nd stage loads the 1st stage.
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
Common-source JFET amplifier.
VDD = 20 V
20 V
= 4 mA
ISAT =
5 kW
RL = 5 kW
Drain
Gate
Input
signal
CC
VGS = 1.5 V
RG
Source
Phase-inverted
output
Fixed bias
ID in mA 2
-1.5
1
-2.0
0
8 VP-P
5
10
20
15
VDS in Volts
-2.5
25
AV = 8
VGS in Volts
N-channel JFET characteristic curves
The q-point is set by the fixed bias.
0
Load line 4
1 VP-P
-0.5
3
-1.0
1.6 mA
2
-1.5
1
-2.0
0
Yfs =
DID
DVGS
5
10
VDS = 1.6 mS
15
-2.5
20
25
VDS in Volts
VGS in Volts
Determining forward transfer admittance:
0
4
ID in mA
-0.5
3
-1.0
When the forward transfer admittance is known,
the voltage gain can be determined using:
VDD = 20 V
RL = 5 kW
D
= 1.6 mS x 5 kW
G
CC
VGS = 1.5 V
RG
AV = Yfs x RL
S
=8
This agrees with the
graphic solution.
Source bias eliminates the need for a separate VGS supply.
VDD
IS = I D
RL
D
G
CC
RG
VGS = ID x RS
S
RS
This resistor also provides
ac negative feedback which
decreases the voltage gain.
Summing
junction
A negative feedback model
A(Vin - BVout)
Vin
Vin - BVout
A
Vout
A = open loop gain
BVout
B
Feedback
B = feedback ratio
Vin
V
Vin
AV
A AV
in
out
in)
VVout
=
A(V
BV
AB
+1
==AV
ABV
in - =
out
AB
out1AB
in
out
+1 =A
Vout VV
Vout
A +1
out
AB
in
AB +1
A simplified model
Vout
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
G
CC
RG
B=
S
RS = 800 W
800 W
5 kW
= 0.16
Vin
A
AB +1
Use the simplified model:
8
A(WITH NEG. FEEDBACK) =
= 3.51
(8)(0.16) + 1
Vout
The source bypass capacitor will
eliminate the ac negative feedback
and restore the voltage gain.
VDD
RL
D
G
CC
RG
RS
CS
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
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
The frequency response curve of an ac amplifier
A
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.
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.
REVIEW
• Amplifier Coupling
• Voltage Gain
• FET Amplifier
• Negative Feedback
• Frequency Response