How to Build a Simple Sample and Hold Circuit with TL071 Op-Amp

Here I want to show how one can build a Simple Sample and Hold Circuit with TL071 Op-Amp. This circuit allows you to capture a snapshot of a changing analog voltage/analog current and hold that value steady so it can be measured or used elsewhere. This circuit is a fundamental building block in analog-to-digital interfacing and signal processing. While this version uses manual switches for demonstration, the core principle is used across many advanced technologies. The circuit are used in ADC(Analog to Digital Converter), DAC (Digital to Analog Converter) for signal reconstruction, data distribution as demultiplexer, peak detectors and analog delay lines.

1. The Sampling Stage

When SW1 is closed (moved to the Sample position), the input voltage ($V_{in}$) is connected directly to the capacitor C1.

• Charging: The capacitor rapidly charges or discharges until the voltage across its terminals exactly matches the input voltage.

• Monitoring: Since the TL071 is configured as a unity-gain voltage follower (output tied to inverting input), the output at Pin 6 instantly mirrors whatever voltage is currently on the capacitor.

2. The Hold Stage

When SW1 is opened (moved to the Hold position), the capacitor is physically disconnected from the input source.

• Charge Trapping: Because the TL071 has a JFET input stage, it possesses an extremely high input impedance. This means almost no current flows into the Op-Amp's input.

• Stability: With nowhere for the electrons to go, the charge remains "trapped" in the capacitor. As a result, the voltage remains constant for a substantial period of time, allowing you to read the meter even if the original input signal disappears or changes.

3. The Reset Stage

To prepare for a new measurement, SW2 is momentarily closed to the Reset position.

• Discharging: This creates a path directly to Ground, allowing all the stored energy in C1 to drain away.

• Zeroing: The voltage across the capacitor drops to $0\text{V}$, and the ammeter returns to a zero reading.

Key Components & Roles

• Capacitor (C1): Acts as the "memory" of the circuit. High-quality units like Mylar are preferred because they have low internal leakage, meaning the "hold" lasts longer.

• Op-Amp (TL071): Provides the high-impedance "buffer". Without this, connecting a standard meter or load would immediately drain the capacitor.

• Potentiometer (RV1) & Ammeter: These form a calibrated voltmeter. By adjusting RV1, you can scale the current so that a specific input voltage (like $1.5\text{V}$) displays a specific current (like $0.15\text{mA}$) on the meter.