Question

The amplifier A shown in the figure has open loop gain of 100, input impedance $Z_i$ of $1k\Omega$ and output impedance $Z_0$ of $100\Omega$. If a negative feedback factor $\beta =0.99$ is used, the new values of $Z_i$ and $Z_0$ respectively are

• A. $10\Omega and10\Omega$
• B. $100K\Omega and1\Omega$
• C. $10\Omega and1\Omega$
• D. $100\Omega and10\Omega$

Answer 1

The correct option is B $100K\Omega and1\Omega$

Open loop gain , A = 100

Feedback factor, $\beta =0.99$

Input impedance $Z_i=1k\Omega$

$Z_i^{\prime }=Z_i(1+A\beta )$

$={10}^3(1+100\times 0.99)=100k\Omega$

Output impedance after negative feedback

$=\frac{Z_0}{1+A\beta }=\frac{100}{1+100\times 0.99}=1\Omega$