Question:medium

In the circuit shown, \(V(t) = 2\sin(2000\pi t)\) Volts, where \(t\) is in seconds. The source \(V(t)\) drives the non-inverting (+) input of an opamp directly, the inverting (-) input is tied to ground (0 V), and the opamp is powered from +15 V and -15 V rails with no feedback network between its output and either input. Take the opamp to be ideal. Which of the following options is correct?

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With no feedback, an ideal opamp comparator output only ever sits at one of its two supply rails, comparing \(V(t)\) against the grounded inverting input tells you when it flips.
Updated On: Jul 28, 2026
  • \(V_{out}(t)\) is square wave with peak-to-peak voltage = 30 V and time period is 1 ms.
  • \(V_{out}(t)\) is a sine wave with peak-to-peak voltage = 4 V and time period is 1 ms.
  • \(V_{out}(t)\) is sine wave with peak-to-peak voltage = 30 V and time period of 1 ms.
  • \(V_{out}(t)\) is square wave with peak-to-peak voltage = 4 V and time period is 1 ms.
Show Solution

The Correct Option is A

Solution and Explanation

Step 1: Identify the circuit function.
There is no resistor feeding the output back into either input, so this is not a negative-feedback amplifier stage. With an ideal (infinite gain) opamp and no feedback, output can only rail to $+15$ V or $-15$ V, whichever input is higher wins completely.

Step 2: Track the sign of $V(t)$ over one cycle.
The inverting input sits at 0 V (grounded), so the comparison reduces to the sign of $V(t)=2\sin(2000\pi t)$ at the non-inverting input. Checking a few instants: at $t=0$, $V=0$ and about to go positive; for small $t>0$, $\sin(2000\pi t)>0$ so $V_{out}=+15$ V; once $2000\pi t$ passes $\pi$, $\sin(\cdot)<0$ so $V_{out}$ flips to $-15$ V. This toggling repeats every time $\sin(2000\pi t)$ changes sign.

Step 3: Get the frequency, then the period.
Comparing $2000\pi t$ with the standard form $2\pi f t$ gives $f = \dfrac{2000\pi}{2\pi} = 1000$ Hz. So the period is
\[ T = \frac{1}{f} = \frac{1}{1000\ \text{Hz}} = 1\ \text{ms} \]
Since the output only ever sits at one of the two rails, its peak-to-peak swing is just the rail separation:
\[ V_{pp} = 15\ \text{V} - (-15\ \text{V}) = 30\ \text{V} \]

Step 4: Eliminate the wrong shapes and amplitudes.
A sine-shaped output (options B, C) would need the opamp working in its linear region with feedback, which is not present here, so both are ruled out on shape alone. A 4 V swing (option D) is far too small for a rail-to-rail comparator running off $\pm15$ V supplies.

Final Answer:
The comparator toggles between $+15$ V and $-15$ V at the input's zero crossings, giving a square wave of 30 V peak-to-peak and 1 ms period. \[ \boxed{V_{pp} = 30\ \text{V},\ T = 1\ \text{ms}} \]
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