JNTU B.Tech III ANALOG AND DIGITAL IC APPLICATIONS Previous Question Paper (September - 2021 - R16)

A sample and hold circuit is used to sample an incoming analog signal and hold its value for a specific duration, typically to allow an Analog-to-Digital Converter (ADC) to process the signal. It consists of an input buffer, a sampling switch (usually a MOSFET), a holding capacitor, and an output buffer.

During the sampling mode, the switch is closed, allowing the capacitor to charge to the instantaneous voltage of the input signal through the input buffer. The output buffer provides a high input impedance to prevent the capacitor from discharging.

During the hold mode, the switch is opened. The capacitor maintains the sampled voltage level because the high-impedance output buffer prevents significant leakage current. The circuit effectively freezes the analog signal, providing a stable input to the ADC during the conversion process.

An AC amplifier is designed to amplify time-varying signals while blocking DC components. This is typically achieved by using coupling capacitors at the input and output stages. These capacitors act as high-pass filters, allowing AC signals to pass through while presenting an open circuit to DC bias voltages.

The operation relies on a transistor (BJT or FET) biased in the active region. The AC input signal is superimposed on the DC bias point (Q-point). The coupling capacitors ensure that the DC bias of one stage does not affect the next, preventing shifts in the Q-point. By setting the capacitor values appropriately, the lower cutoff frequency is determined, ensuring the signal frequency of interest is within the amplifier's bandwidth.

Bypass capacitors are also used across emitter or source resistors to provide a low-impedance path for AC signals, thereby maximizing voltage gain without disturbing the DC stability provided by the resistor.