Why cant we measure the impedances of amplifier circuit or any other circuits directly with multimeter?

Question

need a precise answer..

Answer 1

In a few cases you can. It really depends on the circuit, and whether you want AC impedance, and at what frequency or DC resistance.

Measuring "impedance" with a multimeter is actually a measure of DC resistance, and must be done with all power removed from the circuit. This means that any diodes, transistors, and IC's will have no bias on them, or the incorrect bias, so their contribution to the resistance measurement will be missing or incorrect.

If you want to measure output resistance of an opamp inverter, for example, that resistance is a function of the opamp gain and it's output stage, so therefore the resistance is not a real resistance that could be measured by a multimeter. In this case, it can only be measured by making voltage and current tests on the circuit with power applied.

But if you want to measure the DC resistance of a resistor in the emitter of a transistor amplifier, you may be able to do it with a multimeter, as long as the power is off.

A little more detail on the opamp circuit above. You need to know the maximum current the opamp can supply, typically 5ma. You setup the input so the opamp output is at 10.000 volts, unloaded. You then put a 2kohm resistor from the output to ground, and measure the output voltage again. If it changed to 9.999 volts, then

Rout = (10-9.999)/5ma = 0.2 ohm

(real measurements are usually lower than this)

If you tried to do that measurement with a multimeter and no power on the circuit, you would measure the junction resistances of the output stages, in series with some internal resistors or other transistors, probably in the hundreds or thousands of ohms.

That's at DC. AC you would apply a sine wave at the frequency of interest and set it so the output is between ± 10 volts (20 v P-P). Apply the load and again measure the change in voltage.

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