Q11Analog Electronics

Question

Write the short note on - (i) Summing amplifier (ii) Zero crossing detector (iii) Square wave detector (iv) Peak detector

Answer 1

A summing amplifier uses an Op-Amp's inverting input with multiple input resistors to produce an output that is the weighted sum of all inputs. For inputs $V_{1}$ , $V_{2}$ , $V_{3}$ with input resistors $R_{1}$ , $R_{2}$ , $R_{3}$ and feedback resistor $R_{f}$ :

V_{o u t} = - R_{f} (\frac{V _{1}}{R _{1}} + \frac{V _{2}}{R _{2}} + \frac{V _{3}}{R _{3}})

If all resistors are equal ( $R_{1} = R_{2} = R_{3} = R_{f} = R$ ): $V_{o u t} = - (V_{1} + V_{2} + V_{3})$ . Applications include audio mixers, digital-to-analog converters (DAC), and averaging circuits.

A zero crossing detector (ZCD) is an Op-Amp comparator with the non-inverting (+) input grounded. When the input signal $V_{in}$ crosses zero (goes positive), the output swings to $+ V_{s a t}$ ; when it goes negative, the output swings to $- V_{s a t}$ . The circuit converts any sinusoidal or triangular wave into a square wave synchronized with zero crossings of the input.

V_{o u t} = {+ V_{s a t} - V_{s a t} if V_{in} > 0 if V_{in} < 0

Applications: frequency measurement, phase-locked loops, synchronization circuits. A Schmitt trigger variant adds hysteresis to prevent false triggering due to noise.

A square wave generator using an Op-Amp consists of a comparator with positive feedback (Schmitt trigger) and an RC timing circuit. The capacitor charges and discharges through a resistor, and when the capacitor voltage reaches the threshold set by the voltage divider, the output flips. The frequency of oscillation is:

f = \frac{1}{2 R C ln ( \frac{1 + β}{1 - β} )}, β = \frac{R _{1}}{R _{1} + R _{2}}

For $R_{1} = R_{2}$ : $β = 0.5$ and $f = \frac{1}{2 R C l n 3} \approx \frac{1}{2.2 R C}$ . Applications: clock signals, PWM generation, timing circuits.

A peak detector is an Op-Amp circuit with a diode and capacitor that captures and holds the peak (maximum) value of the input signal. During the positive half cycle, the diode is forward biased, the capacitor charges to the peak value $V_{m}$ , and the Op-Amp output holds it. When input drops below $V_{m}$ , the diode becomes reverse biased, the capacitor holds the peak voltage, and the output remains at $V_{m}$ .

The Op-Amp in the feedback loop compensates for the diode's forward voltage drop, giving a more accurate peak detection. The capacitor discharges slowly through the high input impedance. Applications include amplitude detection in AM demodulators, envelope detectors, and signal level monitoring.

Q11Analog Electronics

Question

Answer