Op amp power consumption ?

Discussion in 'General Electronics Chat' started by d91up, Feb 2, 2013.

  1. d91up

    Thread Starter New Member

    Feb 2, 2013
    This is a general question: What is the power consumption of an Op Amp with supply voltage Vcc, output voltage Vrms and a resistive load R?

    Ignoring the amp's quiescent current I would say:

    Power = Vcc * Vrms / R

    But this sounds strange: If the load is 16 Ohm, Vrms=0.1V and Vcc = 1.8V
    then the amp draws 11mW to supply only 0.6mW of power to the load, that's 5% efficiency. Aren't op amps nearly ideal devices?

    Any thoughts are welcome.
  2. crutschow


    Mar 14, 2008
    The power to the load is (Vrms^2) / R where Vrms is the rms voltage across the load. The power drawn from the supply is Vcc * Iave where Iave is the average (not RMS) load current. I leave you to do the math.

    Op amps are nearly ideal for many characteristics but their output is a Class B type stage and the efficiency of such a stage is always much less than 100%.
  3. d91up

    Thread Starter New Member

    Feb 2, 2013
    Thanks for the feedback.
  4. Audioguru


    Dec 20, 2007
    An opamp cannot drive a 16 ohm load. The minimum load for most opamps is 2k ohms but a few can drive 600 ohms.

    The output of an amplifier is a variable resistor in series with the supply voltage and the load. So of course the amplifier dissipates power making heat.