Question

The op-amp based circuit of a half wave rectifier electronic voltmeter shown below uses a PMMC ammeter with a full scale deflection (FSD) current of 1 mA and a coil resistance of $1k\Omega .$ The value of R that gives FSD for a sinusoidal input voltage of 100 mV (RMS) is

• A. $45\Omega$
• B. $67.5\Omega$
• C. $100\Omega$
• D. $144.4\Omega$

Answer 1

The correct option is A $45\Omega$

$V_B=V_{avg}=\frac{100\sqrt{2}}{\pi }=45mV$

$V_A=V_B$ (By the concept of virtual ground)

$\therefore V_A=45mV$

also, it can be observed that $V_C=V_B$

$\therefore V_C=45mV$

at point 'c',

$V_C=IR$

$45\times {10}^{-3}=1\times {10}^{-3}\times R$

$\therefore R=\frac{45\times {10}^{-3}}{1\times {10}^{-3}}=45\Omega$