FET Voltmeters

A good voltmeter disturbs the circuit under test as little as possible, and this requires that the meter have high internal resistance. Besides the electrostatic-type voltmeter, there is another way to get high internal resistance. This is to sample a tiny current, far too small for any meter to directly indicate, and then amplify this current so a conventional milliammeter or microammeter can display it. When a minuscule current is drawn from a circuit, the equivalent resistance is always extremely high.

The most effective way to accomplish voltage amplification, while making sure that the current drawn is exceedingly small, is to use a field-effect transistor, or FET. A voltmeter that uses a FET voltage amplifier to minimize the current drain is known as a FET voltmeter (FETVM). It has extremely
high input resistance, along with good sensitivity and amplification.