How would I calculate, from datasheet information or elsewhere, the input and output impedances of a Class D power amplifier, in general or for say a chip like the LM4670 or the TPA2000D1?

{Note that in TPA2000D1's datasheet, \$ Z_{in}\$ is given (about \$ 15k\Omega\$), but not \$ Z_{out}\$.}

What differences would there be to measuring the input and output impedances of a Class-D amplifier like this, from techniques used for other amplifier classes, like class A, AB, B, C?

Would the input and output impedances vary with frequency of the input signal?

  • \$\begingroup\$ If the output doesn't have feedback after the filter (like, say, UcD), the impedance will vary with the load (and frequency). The input may vary according to input capacitances, but given the relatively large resistance, its frequency response may be fairly fixed (but not necessarily flat). \$\endgroup\$ Jan 11, 2017 at 7:23
  • \$\begingroup\$ Please explain why you feel it is necessary to measure the output impedance of a class D audio amplifier? \$\endgroup\$
    – Andy aka
    Jan 11, 2017 at 9:52
  • \$\begingroup\$ @Andyaka, I'm still reading and learning up myself on class D amplifiers. 1. While I have done Z_out measurements on class A & B amps in class in the now distant past, I want to know if the concept of Z_out is applicable to class D amps, for completeness' sake. 2. I'm doing a "simple" design myself which will use one of the chips I mention, or similar, as my understanding grows. I feel if I'm designing an amp, I should know about its basic system parameters, or their absence, as the case may be. \$\endgroup\$ Jan 11, 2017 at 11:12
  • 1
    \$\begingroup\$ @Andyaka When trying to evaluate the Thiele-Small parameters (TSP) for a speaker, it requires an amplifier with a low output impedance, preferably below 0.1Ω. This is something I will be doing myself shortly - and wondering if a class D amp is suitable. \$\endgroup\$
    – brewmanz
    Jul 13, 2020 at 6:26
  • \$\begingroup\$ @brewmanz, if you want to be sure of a low Z_out, try putting in buffer stage after the amp output. \$\endgroup\$ Jul 14, 2020 at 7:53

1 Answer 1


Class D means that the output stage of these amplifiers are switched. The output stage consists of fast switches which connect the outputs to the positive and negative supply rails and so generating a PWM signal.

This PWM signal is then filtered to recover the wanted analog signal for the speaker.

As such a class D stage has a strongly non-linear behavior making it less trivial to determine the output impedance of such a stage.

As you should know, determining the output impedance of the classic (linear,non-switching) class A, AB, B and C stages involves choosing an operating point around which the amplifier stage is assumed to be linear. Then a small signal model can be made and the output impedance determined. There might be a feedback loop which can lower the output impedance as well.

For class D it is possible to determine a certain linearized behavior model but it is more cumbersome. Some clues might be found in this Thesis.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.