Performance sample and hold

Description: When switch SW1 is in the positive position, the FET is activated, exhibiting a resistance of approximately 400 ohms. The input voltage charges the capacitor through the FET. Conversely, when SW1 is in the negative position, the FET is deactivated (pinched off). To achieve a prolonged storage time, the operational amplifier must exhibit a very low input bias current. For the CA3140, this bias current is about 10 picoamps.

The discharge rate of the capacitor due to this current can be described by the equation C (dv/dt) = i, where dv/dt represents the rate of voltage change across the capacitor.

The circuit described involves a switch (SW1) controlling a field-effect transistor (FET) that regulates the charging and discharging of a capacitor. When SW1 is in the positive state, the FET allows current to flow, charging the capacitor to a certain voltage level. The resistance of the FET in this state, approximately 400 ohms, indicates that it is in the 'on' state, allowing efficient charging of the capacitor.

Once the switch is flipped to the negative position, the FET transitions to an 'off' state, effectively isolating the capacitor from the input voltage source. This pinching off of the FET is crucial for maintaining the charge stored in the capacitor, as it prevents any further current from flowing into or out of the capacitor.

The operational amplifier (op-amp) used in this configuration is the CA3140, which is selected for its exceptionally low input bias current of around 10 picoamps. This low bias current is essential for applications requiring long storage times, as it minimizes the discharge of the capacitor over time. The relationship between the capacitor's charge and its discharge rate is governed by the equation C (dv/dt) = i, where C is the capacitance, dv/dt is the rate of voltage change, and i is the current flowing through the capacitor. This equation highlights the importance of maintaining low current levels to ensure that the voltage across the capacitor changes slowly, allowing for prolonged storage of the charge.

In summary, this circuit utilizes a FET to control the charging and discharging of a capacitor, with the operational amplifier ensuring minimal bias current to extend the storage duration of the charge. The careful selection of components and their configurations is critical for achieving the desired performance in applications that rely on stable voltage levels over extended periods.When switch SWl is positive, the FET is turned on, and has a resistance of about 400 ohm. The input voltage charges up the capacitor through the FET. When SWl is negative, the FET is turned off (pinched off). To get a long storage time, the op amp must have a very low input bias current. For the CA3140, this current is about 10 pico amps. The rate at which the capacitor will be discharged by this current is based on the equation, C (dv/dt) = i where dv/dt is the rate of change of voltage on the capacitor.