SAMPLE AND HOLD CIRCUIT

CIRCUIT CONSTRUCTION

The circuit samples the input and holds the last sample until the input sampled again.

The circuit has an operational amplifier and a N-channel E-MOSFET.

Here the MOSFET acts like a switch.

CIRCUIT OPERATION

The input signal is applied in the drain, and the sample and hold control voltage is applied to the gate of the MOSFET.

During the positive portion of the sample and hold control voltage, E-MOSFET conducts.

During this time, the capacitor c is charged by the input voltage.

The input voltage appears across C and in turn to the output.

When Vs is equal to zero, the E-MOSFET is off. It acts like as an open switch.

Hence the voltage across C is retained.

SAMPLING AND HOLD PERIODS

Ts -- Sampling period of Vs. (During this time the voltage across the capacitor is equal to the input voltage)

TH -- Hold period (Here the voltage across the capacitor is constant)

APPLICATIONS

The sample and Hold circuit is commonly used in

1. Digital Interfacing

2. Communications

INPUT AND OUTPUT WAVEFORMS

CLIPPERS

Clipping is an important wave shaping technique.

In op-amp clipper circuits a rectifier diode may be used to clip off a certain portion of the input signal.

POSITIVE CLIPPER

POSITIVE CLIPPER

A positive clipper, a circuit that removes positive parts of the input signal.

We can set the clipping level by using the reference voltage.

The value of the reference voltage should be less than the input voltage of the op-amp.

FORWARD BIAS

During the positive half cycle, the diode D1 conducts only Vin=Vref.

When the input voltage less than the reference voltage, the output becomes negative to drive D1 into conduction

When the input is greater than the reference voltage the output of the op-amp becomes sufficiently positive to drive D1 in to cut off.

For this condition we can drive the reference voltage from the positive supply voltage

INPUT AND OUTPUT WAVEFORMS

NEGETIVE CLIPPER

NEGATIVE CLIPPER

The positive clipper is converted into a negative clipper by simply reversing diode D1 and changing the polarity of reference voltage Vref.

The negative clipper clip off the negative parts of the input signal below the reference voltage.

CIRCUIT OPERATION

Diode D1 conducts, when input voltage is greater than the reference voltage. During this period, the output follows the input.

The Negative portion of the output voltage below the reference voltage is clipped off because D1 is off for Vin< -Vref

INPUT AND OUTPUT WAVEFORMS