Saturday, May 18, 2013

Simple and hold circuit using op-amp Circuits Diagram

As the name indicates , a sample and hold circuit is a circuit which samples an input signal and holds onto its last sampled value until the input is sampled again. Sample and hold circuits are commonly used in analogue to digital converts, communication circuits, PWM circuits etc. The circuit shown below is of a sample and hold circuit based on uA 741 opamp , n-channel E MOSFET BS170 and few passive components.

Description

As the name indicates , a sample and hold circuit is a circuit which samples an input signal and holds onto its last sampled value until the input is sampled again. Sample and hold circuits are commonly used in analogue to digital converts, communication circuits, PWM circuits etc. The circuit shown below is of a sample and hold circuit based on uA 741 opamp , n-channel E MOSFET BS170 and few passive components.
In the circuit MOSFET BS170 (Q1) works as a switch while opamp uA741 is wired as a voltage follower. The signal to be sampled (Vin) is applied to the drain of MOSFET while the sample and hold control voltage (Vs) is applied to the source of the MOSFET. The source pin of the MOSFET is connected to the non inverting input of the opamp through the resistor R3. C1 which is a polyester capacitor serves as the charge storing device. Resistor R2 serves as the load resistor while preset R1 is used for adjusting the offset voltage.
During the positive half cycle of the Vs, the MOSFET is ON which acts like a closed switch and the capacitor C1 is charged by the Vin and the same voltage (Vin) appears at the output of the opamp. When Vs is zero MOSFET is switched off and the only discharge path for C1 is through the inverting input of the opamp. Since the input impedance of the opamp is too high the voltage Vin is retained and it appears at the output of the opamp.
The time periods of the Vs during which the voltage across the capacitor (Vc) is equal to Vin are called sample periods (Ts) and the time periods of Vs during which the voltage across the capacitor C1 (Vc) is held constant are called hold periods (Th). Taking a close look at the input and output wave forms of the circuit will make it easier to understand the working of the circuit.

Circuit diagram

 Sample and Hold circuit using uA741 opamp

Input and output waveforms.

sample and hold waveforms
Input and output waveforms - Sample and hold circuit

Notes

  • The circuit can be assembled on a vero board.
  • Use +15V/-15V DC dual supply for powering the opamp.
  • Capacitor C1 must have minimum leakage current possible and thats why a polyester capacitor is used here.
  • Mount the IC uA741 on a holder.
  • The type number of the MOSFET Q1 is not very significant here and so substitution is possible if BS170 is not available.
  • BS170 is a 60V, 500mA n-channel enhancement mode MOSFET available in TO-92 package.
  • Preset resistor R1 can be used for offset adjustments.

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