these slides discuss the basic properties of negative feedback, with example of basic feedback amplifiers.

1. UNIT-2
Mohammad Asif Iqbal
Assistant Professor,
Deptt of ECE,
JETGI, Barabanki

2. General Feedback Structure
Source
(ME)
∑ A
(YOU)
Load
(Result)
B
(Feed back)
xs + xi xo
xf
xo =Axi
xf=Bxo
xi=xs -xf

3. Continue…
The gain of the feedback amplifier can be obtained by combining all three
equations
Loop gain
Close loop
gain
open loop
gain
•From the above equation we can observe that the gain with feedback will be smaller than the open
loop gain by the quantity 1+ AB, which is called the amount of feedback.
•If , as in the case in many circuits the loop gain AB is large, AB 1, then from above equation it
follows that
This is something very interesting: the gain is almost
entirely determined by the feedback network. Since the
feedback network usually consist passive components,
which can be chosen to be as accurate as one wishes

4. Some properties of negative feedback
•Gain de-sensitivity
•Bandwidth extension
•Noise reduction
•Reduction in nonlinear distortion

5. 1#Gain de-sensitivity
We have
Assume that β is constant. Taking differentials of both sides of above Eq. results in
Dividing Eq. 2 by Eq. 1 yields
Eq -1
Eq -2
which says that the percentage change in 𝐴 𝑓 (due to variations in some circuit parameter)
is smaller than the percentage change in A by a factor equal to the amount of feedback. For
this reason, the amount of feedback, 1 + Aβ, is also known as the desensitivity factor.

6. Upper 3-dB
frequency
2#Bandwidth extension
Consider an amplifier whose high-frequency response is characterized by
a single pole. Its gain at mid and high frequencies can be expressed as
Mid-band
gain
Upper 3-dB
frequency
What are
these?
Now suppose if we apply a negative feedback, with a frequency
independent factor B, around this amplifier, we will get a closed-loop gain
Put the value of A(s) from above equation

7. Continue..
After a little manipulation, we will get.
Recall the gain equation without feedback
Comparing both, you may observe
Midband gain has changed
from
And upper 3-dB frequency has
changed

8. More about bandwidth extension

9. 3#Noise reduction
Negative feedback can be employed to reduce the noise or interference in
an amplifier or, more precisely, to increase the ratio of signal to noise. Lets
consider following amplifier with gain 𝐴1, an input signal 𝑉𝑠, and noise, or
interference, 𝑉𝑛.
So the signal to noise ratio for this amplifier is
S/N = 𝑉𝑠/𝑉𝑛

10. Continued…
Now consider this
We have assume
that it is possible
to build another
amplifier stage
with gain A2 that
does not suffer
from the noise
problem.
This feedback is
connected across
the overall
cascade of such
an amount to
keep the overall
gainconstant.
Now lets find the output voltage of this
circuit byusingsuperposition:
WhatwillbenewS/N?

11. But this is not the case when
nonlinearity comesinthepicture
4#Reduction in nonlinear distortion
Because of negative feedback, system becomes more liner, it can handle
larger signals at output without distortion occurring.
For understanding these thing, consider the output of following system
A
(YOU)
xi xo
xo =Axi
xo =xoffset + Axi + K2xi
2 + K3xi
3….
A function which is
independentof xi
Nonlinearities
Any amplifier which is working in active reason,
then the term Axi is dominating and other
factors are forced to zero by choosing an
appropriate operating point. But this is
possible for certain signal levels only, as the
signal levels increases these other factors will
play there part

12. Now lets connect the feedbcak
∑ A
(Nonlinear)
B
(Linear)
xs + xo =Axi
xf=Bxo
xi=xs -xf
-
xo =xoffset + Axi + K2xi
2 + K3xi
3….

13. NOW, LETS TALK ABOUT FOUR BASIC TYPES OF
FEEDBACK AMPLIFIER…
•The series-shunt or voltage series or series voltage feedback amplifier
•The series-series or current series or series current feedback amplifier
•The shunt-shunt or voltage shunt or shunt voltage feedback amplifier
•The shunt-series or current shunt or shunt current feedback amplifier
Shunt:-voltage
Series:-current
These represents the
type of connection
between output-
feedback
These represents the
type of connection
between input-
feedback
In input we will decide the parameter by
consideringthefollowingfact:
We will take that parameter which get divided in
the givenconnection type.Meaninseries:-voltage
& in shunt-current
In these cases the given connection type is
for input, and the parameter type tells you
abouttheoutputconnection

14. Continue..
xo =xoffset + Axi + K2xi
2 + K3xi
3….
xi=xs -xf xf=Bxo
xi=xs –Bxo = Є
xo =xoffset+A(xs –Bxo)+K2Є2 +K3Є3
xo+ABxo=xoffset+Axs +K2Є2 +K3Є3
xo=xoffset /(1+AB)+Axs/(1+AB)+(K2Є2 +K3Є3)/(1+AB)
Upper 3-dB
frequency
Replacing
xi by these
values
Replacing
xi by these
values
As these values are
large enough, we can
ignore these fractions
xo= Axs/(1+AB)
Finally we
get an
expression
free from
nonlinearities

15. 1#The series-shunt feedback amplifier
Vs
+
Vo
-
+
Vo
-
+
Vf =BVo
-
+
Vi
-
Ri
+
AVi
-
RoIi
An amplifier, with
gain A, Input
resistance Ri output
resistance Ro
- Vf +
This is a mixer,
Feedback
network, with
gain B.

16. 1#The series-shunt feedback amplifier
•The output signal voltage is sampled and is applied as a
voltage into the mixer
•Both sampled and feedback signals are voltage
•It is also known as voltage amplifier whose gain is
•An ideal voltage amplifier has infinite input resistance, and
zero output resistance.
•The best practical examples are emitter follower, source
follower, voltage follower.

17. To find input and output resistances, Rif with
feedback
so, Rif = Vs / Ii =(Vi +Vf)/ Ii
but, Vf = BVo = BAVi
Then, Rif = (Vi + BAVi ) / Ii
Rif = (1+AB) Vi/ Ii
Rif = DRi
Ri = Vf / Ii
We know, the input resistance of the amplifier without feedback is Ri And from
the figure it is,

18. To find Rof
The standard procedure to find the output resistance of amplifier is to
reduce the Vs to zero, and apply a test voltage Vt at the output, as shown in
fig.
Rof = Vt / I
From figure we can write
I =
And since Vs =0 we can conclude that
Vi = −Vf = −BVo = −BVt
Thus
Leading to
+
AVi
-
Ro
Vt
I
0

19. Important Numerical
Q:1
Q:2