Question

An amplifier has a gain of 20 without feedback, if 10 % of it is feed back by means of negative feedback,

determine the actual gain?

(A) 100

(B) 3.67

(C) 6.67

(D) 12​

Answer 1

Answer:

(i) Gain with feedback

`A_(f)=(A)/(1+ betaA)=(100)/(1+(1//25)xx100) =20`

(ii) `beta = (1)/(25)`

(iii) Out put voltage

`V_(0)' =A_(f)V_(i) = 20 xx 50 mV = 1` volt

(iv) Feedback voltage `beta V_(0)'= (1)/(25) xx 1 = 0.04` volt

(v) New increased input voltage `V_(i)^(1) =V_(i)(1+beta A)`

=`50 (1+ (1)/(25) xx 100) = 250 mV`.

Answer 2

Given :

Av = 20

mv = 10% = 0.1

To find :

Actual gain

Solution :

$A_v_f = \frac{A_v}{1+A_v*m_v }$

$A_v_f = \frac{20}{1+20*0.1}$

$A_v_f = \frac{20}{1+2}$

$A_v_f = \frac{20}{3}$

Avf = 6.67

So, the actual gain is 6.67

Extra information :

Negative feedback

V₀ / Vs = A / (1 + Aβ)

Af = V₀ / Vs

Af = A / (1 + Aβ)

Positive feedback

V₀ / Vs = A / (1 - Aβ)

Af = V₀ / Vs

Af = A / (1 - Aβ)