Electronic Voltmeters

In all electronic voltmeter circuits the principle involved is that an indication on a permanent magnet moving coil instrument (normally abbreviated as PMMC or D’Arsonval movement) proportional to the input voltage is obtained by means of amplification in one or more stages with a high input impedance.

Although electronic instruments are usually more costlier than electrical instruments but are becoming more and more popular because of their numerous advantages over conventional ones, as discussed below :

A moving coil voltmeter draws a large current, hence a large power from the test circuit during measurement. Generally current drawn by the voltmeter does not make much difference in electrical circuits but when measurement is involved in electronic circuits, current drawn by the voltmeter load the test circuit, and therefore, voltmeter shows an error in the reading.

In electronic voltmeters power required for deflection of a permanent magnet moving coil (PMMC) movement is not drawn from the test circuit but supplied from the amplifier whose output is proportional to the test circuit voltage i.e. input to the amplifier, so power drawn from the test circuit becomes almost negligible. So elec­tronic voltmeters draw very low power from the test circuit and it can be said that it has very high input impedance. This feature of electronic voltmeters is indispensible for voltage measurements in many high impedance circuits such as encountered in communication of the order of micro-volts.

Voltages of the order of micro-volts are measured in electronic circuits which is not possible with the sensitive PMMC voltmeters but these voltages can be measured with   I electronic voltmeters by using its amplifying properties

Electronic voltmeters measure voltage both at audio as well as radio frequency power level, as the action of thermionic valves or transistors can be made independent of frequency up to 10 Hz — 100 MHz or even higher. Hence such an instrument has an extremely wide frequency range (from direct current to frequencies of the order of hundred of MHz), if the circuit is properly designed. The high frequency range may also be attributed to low input capacitance (of the order of a few pF) of most electronic devices. An electronic voltmeter calibrated at power frequency of 50 Hz can be used at radio frequency without any error.

Transistor voltmeters can be designed for measuring very high voltages such as hun­dreds or thousands of volts.

Owing to amplifying properties electronic instruments possess very high sensitivity and because of high sensitivity, their input impedance is increased. It results in re­duced loading effect while carrying out measurements.

Electronic voltmeters give faster response and flexibility.

Such instruments can monitor remote signals. Electronic voltmeters may use either vacuum tube or transistor. Later one is called transistorized voltmeter (TVM) and former is called the vacuum tube voltmeter (VTVM).

Transistorized voltmeters have numerous advantages over vacuum tube voltmeters and hence replacing them in almost every field of electronics.

The advantages of TVMs over VTVMs are given below:

• A TVM does not require any warm up time because of absence of heating element.
• Use of transistor makes the instrument portable because of its light weight.
• A transistorized instrument can be operated on low voltage i.e. on battery so TVM is well suited for the field work where power source is not available.
• VTVM cannot measure current directly because of its high resistance whereas TVM can do.
• TVM consumes very less power because of absence of heating element.
• The only disadvantage with TVM is low input impedance which it offers in comparison to VTVM. But this short-coming is overcome by using field effect transistor (FET) in input stage of voltmeter because FET offers input impedance almost equal to vacuum tubes.
• In electronic voltmeters current proportional to the voltage to be measured is gener­ated by use of amplifier, rectifier etc. and then this current is measured by PMMC.

Electronic voltmeters are of two types namely dc voltmeters and ac voltmeters.