Transistor Voltmeters Using Field Effect Transistor

Transistor Voltmeters Using Field Effect Transistor Points : transistor voltmeters using field effect transistor Shows a TVM d.c voltmeter with a field effect transistor (FET) forming the input stage of a direct coupled amplifier. The circuit has the advantage that since a FET has high input impedance, the sensitivity of this meter is high and it does not load the circuit under measurement.

The d.c input voltage is applied to range selecting (attenuator) switch. The attenuator is necessary to provide input voltage levels which can be accommodated by the d.c. amplifier.

The two transistor Q₁ and Q₂ form a direct coupled d.c. amplifier driving the meter movement. Provided that the amplifier operates within the limits of its dynamic range, the deflection of the meter is directly proportional to the magnitude of the input applied voltage. The gain of d.c. amplifier allows the instrument to be used in the mV range. The amplifier has the added advantage that accidental over load voltages do not burn out the meter since the amplifier saturates and thus limits the maximum current through the meter to a safe value. The input impedance of this voltmeter is high enough to make it unnecessary to correct for the loading effect of the circuit under measurement.

Instruments used for microvolt ranges require a high gain d.c. amplifier in order to supply current for driving of movement . To avoid the drift problems usually associated with direct-coupled d.c. Amplifiers, the “chopper type d.c. Amplifiers” is normally employed in high sensitivity instruments. In a chopper type d.c amplifier, the d.c. input is converted into an a.c voltage, before it is applied to an a.c amplifier. The amplified a.c voltage is then converted back into a d.c voltage.