elct604 fb amp

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Spring 2010 1

ELCT 604: Electronic CircuitsChapter 3:

Feedback Amplifiers

Associate Prof. Dr. Soliman Mahmoud

Faculty of Information Engineering and Technology

Electrical and Electronic Department

[email protected]

Assistant Prof. Dr. Ahmed Madian

Faculty of Information Engineering and Technology

Electrical and Electronic Department

[email protected]

Associate Prof. Dr. Soliman Mahmoud & Dr. Ahmed MadianElectronics and Electrical Engineering Department

ELCT 604, Electronic CircuitsSpring 2010 2

Feedback Amplifiers

Objectives and outline:

1. Introduction and definitions

2. Classification of Amplifiers

3. Feedback Concept

4. The effect of Feedback on Gain, frequency response,input resistance and output resistance.

5. Approximate Analysis of the Feedback Amplifiers

6. Examples

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1. IntroductionDefinitions:

1. Feedback: Is the process where by a portion of the output is returned to theinput to form a part of the system excitation.

Open-Loop Amplifier

-ve FB Amplifier (Closed Loop Amplifier)



2. Why Feedback ?

i. The feedback improve the frequency response and make the amplifier more stable

ii. The feedback improve the sensitivity of the amplifier.

Definition:

Sensitivity of the amplifier : SAA

dASA =

iii. The feedback decrease the nonlinear distortion.

iv. The feedback decrease the noise effect on the amplifier.

HH ff FB > & LL ff FB &H)

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4.(c) The effect of the FB on LowerCutoff Frequency

A

AA

FB

FB+

=

1And

)1(S

AA

L

MF

+

=

S

A

S

A

A

A

AFBL

FBMF

MFFB

L

MFFB

MF

FB

,

,

1)1(

1

)1(

+

=

++

+=

)1(, MFFB

L

FBL A

+= (&L,FB < &L)



4.(d) The effect of the FB on Input

Resistance

1. Series Shunt amplifier

Mixing - Sampling

Consider

VFB Vout= FB

Vs Vout

Volt.

Amp

From the I/P loop : in

outL

LvFBinoutFBins v

RR

RAvvvv

+

+=+=

)1()1( VFBininVFBins ARiAvv +=+=

)1(inf VFBinin

s ARi

vR +== ( Rinf > Rin ) (Series Mixing)

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4.(d) The effect of the FB on InputResistance

2. Series Series amplifier

Mixing - Sampling

Consider

vFB iout= FB

vs iout

GMAmp

)1(inf MFBin

in

s GRi

vR +==

( Rinf > Rin )(Series Mixing)

Similarly



4.(d) The effect of the FB on Input

Resistance

3. Shunt Series amplifier

Mixing - Sampling

Consider

iFB iout= FB

is iout

Current

Amp

)1(inf

IFB

in

S

in

A

R

i

vR

+== ( Rinf < Rin )

(Shunt Mixing)

Similarly

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4.(d) The effect of the FB on InputResistance

4. Shunt Shunt amplifier

Mixing - Sampling

Consider

iFB vout= FB

is vout

RMAmp

)1(inf MFB

in

S

in

R

R

i

vR

+==

( Rinf < Rin )(Shunt Mixing)

Similarly



4.(e) The effect of the FB on output

Resistance

1. Series Shunt amplifier

Mixing - Sampling

Consider

VFB Vout= FB

Vs Vout

Volt.

Amp

)1( vFB

outoutf

A

RR

+

=

( R\outf < R\

out )(Shunt sampling)

From the shown Figure:)1(

//

\\

VFB

outLoutfoutf

A

RRRR

+==

WhereLoutout RRR //

\=

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4.(e) The effect of the FB on outputResistance

2. Shunt Shunt amplifier

Mixing - Sampling

Consider

iFB vout= FB

is vout

RMAmp

)1( mFB

outoutf

R

RR

+=

( R\outf < R\

out )(Shunt sampling)

From the shown Figure:)1(

//

\\

MFB

outLoutfoutf

R

RRRR

+==

WhereLoutout RRR //

\=



4.(e) The effect of the FB on output

Resistance

3. Shunt Series amplifier

Mixing - Sampling

Consider

iFB iout= FB

is iout

Current

Amp

)1( iFBoutoutf ARR +=

( R\outf > R\

out )

(series sampling)

From the shown Figure:

)1(

)1(

//

\\

IFB

iFBoutLoutfoutf

A

AR

RRR

+

+

==

Where

LoutoutRRR //

\= and

0==

LRIi AA

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4.(e) The effect of the FB on outputResistance

4. Series Series amplifier

Mixing - Sampling

Consider

vFB iout= FB

vs iout

GMAmp

)1( mFBoutoutf GRR +=

( R\outf > R\

out )

(series sampling)

From the shown Figure:

)1(

)1(//

\\

MFB

mFBoutLoutfoutf

G

GRRRR

+

+==

Where

Loutout RRR //\= and

0==

LRMm GG



5. Approximate Analysis of the FBAmplifiers

1. Identify the Topology of the FB : Mixing Sampling andfind the FB gain (FB = XFB/ Xout ).

2. Draw the amplifier circuit without FB and find the gain ,input resistance and the output resistance.

3. Find the gain, input resistance and output resistance ofthe feedback amplifier using the previous equations.

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Drawing the amplifier without FB

Open Circuit Load

If vout = 0 XFB = 0

Short circuit load

If iout = 0 XFB = 0

+

-

vS

voutRL

VCC

RFB

O/PLOOP

I/P

LOOP +

-

vf

+vi -

iout

+

-

iout

Open Circuit I/P Loop

iS

vout

RL

VCC

RFBO/PLOOP

I/P

LOOP

iin

if

iout

-

+

O/pnode

vin

Short circuit I/P loop (vin=0)

iin

Iin = 0

vin = 0



6. Examples

Example 1:

For the shown FB Amplifier,

1. Identify the topology of the FB.

2. Find the feedback parameter FB .

3. Draw the amplifier without FB.

4. Find gain, input resistance andoutput resistance without FB.

5. Find the voltage gain, inputresistance and output resistance ofthe shown FB Amplifier.

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Solution

Series Shunt amplifier

Mixing - Sampling

1. Topology:

VFB Vout= FB

Vs VoutVolt.Amp

2. FB gain FB

From the shown figure

vFB = 1 * vout FB = 1



3. Amplifier without FB

I/P loop Sampling : shunt vout

=0 (Short circuit load)

O/P loop Mixing : series iin =0 (Open circuit I/P loop)

"$

'

$

"#

(

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Therefore, the amplifier circuit

without FB as shown

4. Gain, input resistance andoutput resistance without FB

Rin R\out

Rin =

dsSLout rRRR ////\=

+

-vgs

gmvgs

)////)((dsSLgsmoutrRRvgv +=

sgs vv =

)////( dsSLms

outV rRRg

v

vA ==



5. Gain, input resistance and output resistance with FB

)//(1

)//(

)////(1

)////(

1,

SLm

SLm

dsSLm

dsSLm

VFB

VFBV

RRg

RRg

rRRg

rRRg

A

AA

+

+

=

+

=

Rinf =Rin (1+FB AV) = Series Mixing

m

dsSL

dsSLm

dsSL

VFB

outoutf

grRR

rRRg

rRR

A

RR

1//////

)////(1

////

1

\\

=

+

=

+

=

m

SLoutfg

RRR1

////\

Shunt Sampling

elct604 fb amp

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