Amplifier Output Voltage, Current, And Power

Canonical Definition

An amplifier applies a time-varying voltage across a load. Current follows the load's complex impedance. Instantaneous power is p(t) = v(t)i(t); average real power requires voltage, current, their phase relationship, waveform, bandwidth, and averaging interval.

Known Resistor

Irms = Vrms / R
Paverage = Vrms × Irms = Vrms² / R = Irms² × R

These equations are appropriate when the load is a verified resistor and the voltage and current measurements are valid for the waveform and bandwidth.

Real Loudspeaker

A loudspeaker's impedance has magnitude and phase. At one frequency the current may be nearly in phase with voltage; at another it can lead or lag. Using nominal impedance in V²/R gives only an estimate and can hide the current stress the amplifier actually experiences.

Preal = average[v(t) × i(t)]

For a real-power result, capture voltage across the load and current through the load on the same time base.

Crest Factor Done Correctly

Crest factor in decibels for voltage is:

CFdB = 20 log10(Vpeak / Vrms)

A 10 dB crest factor means Vpeak/Vrms ≈ 3.162. For the same resistance, peak power is ten times average power—not one hundred times.

Paverage / Ppeak = 10^(-CFdB/10)

What Each Instrument Proves

Instrument	Useful result	Does not prove by itself
Oscilloscope	Waveform, peak voltage, timing, clipping at its node	Real load power without current
True-RMS DMM	Steady waveform voltage inside its bandwidth	Waveform cleanliness or fast peaks
Current probe or shunt	Load or supply current	Power without synchronized voltage
Power analyzer	Synchronized voltage, current, phase, and power	Speaker thermal or mechanical safety

Class-D And Measurement Bandwidth

Switching amplifiers can place high-frequency energy on the output terminals. State whether the measurement includes, filters, or rejects that energy. A meter can report the wrong RMS value if its input stage or bandwidth is not suitable for the switching waveform.

Floating And Bridged Outputs

Both terminals may be active.

Measure across the load with a suitable differential or isolated method. Do not attach an earth-referenced scope ground to an unknown speaker terminal.

Minimum Reporting Set

State amplifier mode, supply voltage, load, frequency or program, duration, voltage detector, current method, bandwidth, averaging interval, waveform condition, and whether the result is instantaneous, RMS, average real, peak, or apparent power.