Calculation and measurement of input and output impedances of a Class-D amplifier

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?

• 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). Jan 11, 2017 at 7:23
• Please explain why you feel it is necessary to measure the output impedance of a class D audio amplifier? Jan 11, 2017 at 9:52
• @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. Jan 11, 2017 at 11:12
• @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. Jul 13, 2020 at 6:26
• @brewmanz, if you want to be sure of a low Z_out, try putting in buffer stage after the amp output. Jul 14, 2020 at 7:53