Mumbai University > Electronics Engineering > Sem 4 > Discrete Electronic Circuits

Marks: 10M

Year: Dec 2015

Feedback is a network which take samples of output which may voltage or current and feeds back to the input. If the feedback signal is out of phase with the input signal then it is called as Negative Feedback.

Advantages of Negative Feedback :

• Bandwidth of amplifier increases.

• Noise gets reduced.

• Stability of amplifier increases.

• Decreases frequency distortion.

Disadvantages of Negative Feedback :

• Gain of amplifier decreases due to negative feedback.

• Input Resistance decreases incase of current shunt and voltage shunt.

• Output Resistance increases incase of current shunt and current series.

The different types of negative feedback are as follows :

1) Voltage series negative feedback.( Shunt Series)

2) Voltage shunt negative feedback.( Shunt Shunt)

3) Current series negative feedback.( Series Series)

4) Current shunt negative feedback.( Series Shunt)

Current Shunt Negative Feedback :

Figure 1 shows the block diagram of current shunt negative feedback. Since current shunt is mentioned, output is connected in series configuration because of current sampling and input is connected in parallel mixing. It decreases input impedance and increases output impedance.

Figure 1: Block diagram of current shunt negative feedback

1. The input signal $I_S$ is applied as a input, $I_f$ is the feedback current and $I_O$ is the output current.
2. $A_f$ indicate relationship between $I_O$ and input current $I_S$. The feedback current is given by $I_f = β I_O$.
3. Current amplifier is called so because output is current and input is current, hence ratio gives us current gain.
4. The sample of output current is applied as a input to feedback network which feeds back the output signal to the input.
5. The difference of input signal and feedback signal gets amplified by the current amplifier.

Derivation for Rif, Rof and Af :

1. Rif (Input Resistance with feedback):

$Rinf = \frac{Vin}{I_S} = \frac{Vin}{Iin + I_f} = \frac{Vin}{Iin + \beta I_O}$

But, $A = \frac{I_O}{Iin}$

$I_O = A Iin$

$Rinf = \frac{Vin}{Iin + \beta A Iin} = \frac{Vin}{Iin(1+A\beta)}$

$Rinf = \frac{Rin}{1+A\beta}$

Input impedance decreases by a factor 1+Aβ.

2. Rof (Output Impedance with feedback):

Figure 2 shows the equaivalent circuit for Rof.

Figure 2: Equivalent circuit for Rof

For Rof :

i. Open the input current source.

ii. Remove output load resistance.

iii. Connect an imaginary current source that delivers current $I_O$.

Hence $Zof = \frac{V_O}{I_O}$

Applying KCL at output,

$I_O = I_1 + A Iin$

But $I_1 = \frac{V_O}{R_O}$

$I_O = \frac{V_O}{R_O} + A Iin$.......(1)

But for finding Rof, $I_S = 0$

$Iin = I_S - I_f$

$Iin = - I_f$

But, $I_f = \beta I_O$.......(2)

Substitute (2) in (1),

$I_O = \frac{V_O}{R_O} + A \times (-\beta I_O)$

$I_O (1+A\beta) = \frac{V_O}{R_O}$

$Rof = R_O(1+A\beta)$

Output impedance increases by a factor 1+Aβ.

3. Af (Current gain with feedback ):

$Aif = \frac{I_O}{I_S} $

Apply KCL at input side,

$Iin = I_S - I_f$

$I_S = Iin + I_f = Iin + \beta I_O$

$A_f = \frac{I_O}{Iin + \beta I_O}$

Divide numerator and denominator by $Iin$,

$A_f = \frac{A}{1+\beta A}$

Current gain decreases by a factor 1+Aβ.