Negative voltage feedback
( 1 ) Gain without
feedback
Av
= Vo / Vin
Where
Av = Gain
Vo
= Output voltage
Vin
= Input voltage
( 2 ) Feedback fraction (
mv )
= Output voltage of feedback / Output voltage
of amplifier
< 1
( 3 ) Power gain
Ap
= Av × Ai
Where
Ap
= Power gain
Av
= Voltage gain
Ai
= Current gain
( 4 ) Gain of negative
voltage feedback amplifier
Avf
= Av / ( 1 + Avmv )
Where
Avf
= Gain of amplifier with feedback
Av
= Gain of amplifier without feedback
mv
= Feedback fraction
( 5 ) Non linear distortion
Dvf
= D / ( 1 + Avmv )
Where
Dvf
= Distortion of amplifier with feedback
D
= Distortion of amplifier without feedback
Av
= Gain of amplifier without feedback
mv
= Feedback fraction
( 6 ) Input impedance
Zin’
= Zin ( 1 + Avmv )
Where
Zin’
= Input impedance without feedback
Zin
= Input impedance with feedback
( 7 ) Output impedance
Zout’
= Zout / ( 1 + Avmv
)
Where
Zout’
= Input impedance without feedback
Zout
= Input impedance with feedback
( 8 ) Gain stability
Avf
= Av / ( 1 + Avmv )
Avf
≈ Av / Avmv
Avf
≈ 1 / mv
|
Parameters |
Negative voltage feedback |
|
Gain stability |
Avf ≈ 1 / mv |
|
Input impedance |
Zin’ = Zin ( 1 + Avmv
) |
|
Output impedance |
Zout’ = Zout / ( 1 + Avmv ) |
|
Non linear distortion |
Dvf = D / ( 1 + Avmv
) |
|
Voltage Gain |
Avf = Av / ( 1 + Avmv
) |
Negative current feedback
( 1 ) Feedback fraction
of negative current feedback
Feedback
Fraction = Feedback current / Output current
mi
= If / Iout
Where
If
= Feedback current
Iout
= Output current
( 2 ) Current gain with
negative feedback
Aif
= Ai / ( 1 + Ai mi )
Where
Aif
= Current gain with feedback
Ai
= Current gain without feedback
mi
= Current feedback fraction
( 3 ) Input impedance
with negative current feedback
Zin’
= Zin / ( 1 + Ai mi )
Where
Zin’
= Input impedance with negative current feedback
Zin
= Input impedance without negative current feedback
( 4 ) Output impedance
with negative current feedback
Zout’
= Zout ( 1 + Ai mi
)
Where
Zout’
= Output impedance with negative current feedback
Zout
= Output impedance without negative current feedback
( 5 ) Bandwidth with
negative current feedback
BW’
= BW ( ( 1 + Ai mi )
BW’
= Bandwidth with negative current feedback
BW
= Bandwidth without negative current feedback
|
Parameters |
Negative current feedback |
|
Feedback Fraction |
mi = If / Iout |
|
Current gain |
Aif = Ai / ( 1 + Ai mi
) |
|
Input impedance |
Zin’ = Zin / ( 1 + Ai mi
) |
|
Output impedance |
Zout’ = Zout ( 1 + Ai mi ) |
|
Bandwidth |
BW’ = BW ( ( 1 + Ai mi ) |
Emitter Follower
( 1 ) Voltage gain of
emitter follower
Av
= RE / re’ + RE
Where
Av
= Voltage gain of emitter follower
re'
= 25mV / IE
RE
= Emitter resistance
( 2 ) Input impedance of
Emitter follower
Zin
= R1 ‖ R2 ‖ Zin ( base )
Where
Zin
( base ) = β ( re' + RE’ )
re'
= 25mV / IE
RE’
= RE ‖ RL
Generally, the impedance
of base is very large as compared to R1 ‖ R2 so Zin ( base ) is
neglected.
Therefore Zin ≈
R1 ‖ R2
( 3 ) Output impedance of
Emitter follower
Zout
= RE ‖ ( re' + R’in / β )
The value of RE is
very large in the practical circuit
Zout
= ( re' + R’in / β )
Where
re' = 25mV / IE
R’in
= R1 ‖ R2 ‖ Rs
Where
Rs
= Source resistance
β
= Current amplification factor
Darlington Transistor
( 1 ) Current gain of
Darlington transistor
β
= β1 β2
Where
β1
= Current gain of transistor 1
β2
= Current gain of transistor 2
( 2 ) Input impedance of
Darlington transistor
Zin
= ( β1 )( β2 )( RE )
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