Discussion
To obtain 30% feedback factor for the circuit shown below :
Answer: C
The feedback factor of the network is the fraction of the output fed back to the input of the amplifier. The output is divided by R1 and R2 in series, and the feedback voltage is taken across R2, so
\[ \beta = \frac{R_2}{R_1 + R_2} = 0.3 \]
\[ R_2 = 0.3\,R_1 + 0.3\,R_2 \Rightarrow 0.7\,R_2 = 0.3\,R_1 \]
\[ 3R_1 = 7R_2 \]
Checking against the options: with 3R1 = 7R2 the ratio R2/(R1 + R2) = 3/(7 + 3) = 0.3, as required. The relation R1 = 3R2 would give a feedback factor of 0.25, 2R1 = 5R2 gives 2/7, which is about 0.29, and 5R1 = 9R2 gives 5/14, about 0.36, so none of them gives 30 per cent. The answer is 3R1 = 7R2.
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