Electronic Multimeters

Posted on Feb 6, 2014

It is one of the most versatile general purpose instruments capable of measuring dc and ac voltages as well as current and resistances. The solid-state electronic multimeter (or VOM) generally consists of the following elements. In addition, the instrument is usually provided with a built-in power supply for operation on ac mains and, in most case

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s, one or more batteries for operation as a portable test instrument. The schematic diagram of a balanced-bridge dc amplifier using two field effect transistors (FETs) is given in figure. It is to be noted that two FETs used in circuit should be reasonably well matched for current gain to ensure circuit thermal stability. The two FETs and the source resistors Rx and R2, together with zero adjust control resistor R3, constitute a bridge circuit. The PMMC meter is connected between the source terminals of the FETs, representing two opposite corners of the bridge. In the absence of input signal, the gate terminals of the FETs are at ground potential and the transistors operate under identical quiescent conditions. Ideally no current should flow through the PMMC movement but in practice, on account of some mismatch between the two FETs and slight tolerance differences in the values of various resistors a current doesflow and causes the meter movement to deflect from zero position. This current is reduced zero by the adjust control resistor R3. Now the bridge is balanced. With a positive input signal applied to the gate of input transistor T1 its drain current creases causing the voltage at the source terminal to rise. The resulting unbalance between the two transistors T1 and T2 source voltages is shown by the meter movement, whose scale is calibrated in terms of the magnitude of the applied input voltage. The maximum voltage...

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