Electronic Measuring Instruments ECE 151

Introduction Volt-Ohm-Milliammeter (VOM) - a rugged, accurate instrument but lacks both sensitivity and high input resistance Ex: A sensitivity of 20,000Ω/V with a 0- to 0.5-V range has an input impedance of only 10kΩ.

Electronic Voltmeter (EVM) - have input resistance ranging from 10 to 100MΩ which remains constant over all ranges. - represents less loading to circuits under test than VOM.

Different Types of EVM Vacuum Tube Voltmeters (VTMS) Transistor Voltmeters (TVM) or Field-Effect Transistor Voltmeters (FETVM) 2 categories: 1. analog 2. digital - depending on the type of readout used.

When a meter movement is used as a readout, the instrument is of analog type. If the readout used is one of the various numerical readouts available, the instrument is digital.

The Differential Amplifier Also called the difference amplifier. - block diagram (It has two input signals and one output signals which is an amplified version of the algebraic difference of the two input signals.) - FET version

The difference amplifier can be represented as a bridge. - each FET is represented by a variable dc resistance that is controlled by the input signals. - when both input signals are equal, each FET resistance is equal, and we have a balanced bridge. - thus when input signals are unequal, the bridge becomes unbalanced, therefore an output is produced.

The Differential-Amplifier Type of EVM FETs can be used to increase the input resistance of a dc voltmeter. This isolates the relatively low resistance from the circuit under test. - also applies to ordinary BJTs. - FETs should be reasonably matched for current gain to ensure thermal stability of the circuit.

Zero Set Potentiometer - equalize both halves of the difference amplifier under zero-signal conditions to obtain bridge balance or zero meter current. Full-scale adjustment of calibration is affected by the potentiometer marked calibration, in series with meter movement internal resistance which is necessary because gm and rd are different from FET to FET.

The Source-Follower Type of EVM The two FETs form the upper arms of a bridge circuit and source resistors together form the lower arms. When the input is zero , the bridge is balanced and there is no ammeter current. When there is an input voltage, the current flows through the ammeter.

Since the gate of the right FET is grounded, no signal is applied to its gate, and therefore no signal at its terminal. The left FET is however, a source follower. This is a negative-feedback amplifier with the feedback factor β= -1.