Download Voltage Amplifiers

MALVINO & BATES
Electronic
PRINCIPLES
SEVENTH EDITION
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Chapter
Voltage Amplifiers
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10
Topics Covered in Chapter 10
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Voltage gain
The loading effect of input impedance
Multistage amplifiers
Swamped amplifier
Two-stage feedback
Troubleshooting
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Voltage gain
• Ac output voltage divided by ac input
voltage
• Can be derived by using the p model of a
transistor
• Ac collector resistance divided by the ac
resistance of the emitter diode
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A VDB common-emitter amplifier
+VCC
R1
RC
RL vout
vin
R2
RE
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p model of the common-emitter amplifier
This model best illustrates that zin(stage) = R1 R2 bre’
ib
vin
R1
R2
bre’
ic
RC
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RL vout
T model of the common-emitter amplifier
rc
This model best illustrates that AV = r ’
e
ic
vin
R1
R2
re’
RC
RL vout
ic @ ie
ie
rc = RC RL
vout
AV = v
in
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Calculating voltage gain
• Solve the dc circuit to find the
emitter current.
• Use the emitter current to find re’.
• Combine RC and RL to find rc.
• Divide rc by re’.
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Loading effect of input impedance
• An ideal ac voltage source has zero
source resistance
• The input impedance of a stage includes
biasing resistors and base input
impedance
• When the ac source is not stiff, the input
voltage is less than the source voltage
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Real signal sources are not ideal.
Rg
zin(stage) vin
vg
When a source is not stiff, use:
zin(stage)
vin =
Rg + zin(stage)
vg
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Multistage amplifier
• Two or more amplifiers cascaded
• Provides increased gain
• Two CE stages produce an amplified inphase signal
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To get more gain, a cascade amplifier can be used.
Stage 1
AV1
vin
Stage 2
AV2
zin(stage 2)
The overall voltage gain: AV = AV1AV2
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RL vout
Ac equivalent circuit for the two-stage amplifier
Rg
zin(stage 2)
zin(stage 1)
ic
RC
RC
ic
RL
vg
The 2nd stage loads the 1st stage:
Rc1 = Rc1 ║ Zin (stage 2)
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Swamped amplifier
• Some of the emitter resistance is
unbypassed to get negative feedback
• Voltage gain is stabilized
• Input impedance is increased
• Large-signal distortion is decreased
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Swamped
amplifier
circuit
+VCC
R1
RC
RL vout
vin
R2
rE
ac feedback resistor
RE
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T model of the swamped common-emitter amplifier
Emitter feedback
decreases the gain
and increases the
impedance.
vin
R1
R2
ic
re’
re
RC
RL vout
rc
AV = r ’ + r
e
e
zin(base) = b(re’ + re)
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Large signals produce distortion since re’ is non-linear.
re’ =
vbe
IE
ie
Q
VBE
Input signal
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ic
vin
R1
R2
RC
RL vout
re’
re
This resistor is linear
and can swamp re’.
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vb versus ie in a swamped amplifier
ie
Q
re’ + re
vb
Input signal
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Emitter ac feedback
• A swamped amplifier is an example of
single-stage feedback
• Decreases voltage gain (but the gain is
more stable)
• Increases the base input impedance
• Decreases large-signal distortion
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Two-stage feedback
Stage 1
vin
Stage 2
RL vout
rf
The feedback signal can be connected to
the emitter end of resistor re in stage 1.
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+VCC
Stage 1
R1
RC
To stage 2
Feedback
from stage 2
vin
R2
rE
RE
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Voltage gain with two-stage feedback:
Stage 1
Stage 2
RL vout
vin
rf
AV =
rf
re(stage 1)
+1
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Troubleshooting
• Open coupling or bypass capacitors will
not cause dc error.
• Shorted coupling or bypass capacitors
will cause dc error.
• An open coupling capacitor will cause a
loss of signal.
• An open emitter bypass capacitor will
cause the gain to decrease.
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More troubleshooting
• The supply line is normally an ac ground.
• The supply line will not be an ac ground
if a bypass or filter capacitor opens.
• An open bypass may allow other signals
to affect the amplifier.
• An open filter capacitor will allow power
supply ripple to get into the amplifier.
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