I built this circuit:

enter image description here

It is based on this design for a single-supply non inverting amplifier:

enter image description here

When I tested my output waveforms with earphone A I got normal results:

enter image description here

When I used earphone B, I got abnormal results:

enter image description here

Blue - Vout Red -Vin

I suspect this has something to do with earphone B pulling more current than earphone A, and the op-amp upper current restriction, though I am not too sure yet.

Why do you think the waveforms of voltage behaves this way?

  • 2
    \$\begingroup\$ First thing to realize is that you don't want to put a DC voltage on a speaker; it's wasteful and bad for the speaker, and yes, somewhat increases the likelihood of exceeding ratings and going into compression. A single supply amplifier will almost always use capacitive coupling at the output to pass only AC to the coil; a better design would drive each side with its own complementary amplifier. Today for a practical use you might want to look at Class-D solutions, too... \$\endgroup\$ Dec 6, 2020 at 4:50
  • \$\begingroup\$ You are clipping positive peaks, and as you suggest, earphone B having a lower impedance makes it harder to drive. Chris has nailed the first thing to fix : the next is basically reduce the output level or find an opamp with higher output crrent capability. \$\endgroup\$
    – user16324
    Dec 6, 2020 at 12:03
  • \$\begingroup\$ When the current source which drives the top half of the output stage runs out of current because the load on the output is too large (too low a load resistance), it means that that the compensation capacitor is current starved and so it must charge in a linear manner. \$\endgroup\$
    – user173271
    Dec 6, 2020 at 15:15

1 Answer 1


This is mostly the 'Impedance Match' issue. Every amplifier circuit is designed for a specific load impedance and can deliver the maximum power to that load. So by changing the load, you may not be able to deliver max power to the load, and also the output may contain harmonics (the distortion that you are seeing in the waveform). Take a look at this link:

Impedance Matching

It may help you to solve the issue.


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