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Old 9th October 2014, 12:12 AM   #1
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Default Extreme prominence of odd harmonics with increasing frequency

I'm working on a little piece of silliness in the simulator I've dubbed the 'Excessive Force' amplifier. This is basically a 'Wolverine' IPS bolted to a giant EF buffered BIGBT OPS. It is stripped down and DC coupled just for ease of simulation. It is really the OPS I'm interested in at the moment.

This thing shows close to zero distortion right up to full output voltage swing into rather low load resistances, but only at lower frequencies. Obviously as frequency rises we have less OLG and therefore less NFB to help keep distortion low, so we expect distortion to rise with frequency. I wouldn't say the high power / high frequency THD is horrible, but the harmonic distribution in the FFT spectrum is. Odd harmonics, especially 3rd and 5th, take off like a rocket! The conditions are well away from clipping.

I have even tried just the input / VAS section on its own and see a similar result, with lower overall THD but the same poor spectrum having a very prominent 3rd harmonic. The issue seems to be somewhere in the front-end. This is also evident at lower levels, though to an even lesser degree. Again it is higher frequency performance that is poor.

Where did I go wrong? I was super stoked when I started simulations using 1kHz but the enthusiasm waned when I jacked up the frequency. Did I make a fundamental error? Maybe it is the nature of the beast, but it would be nice to get the odd harmonics down, even if it means bringing the even harmonics up a little. Any ideas? Perhaps just can it is a bad idea...

Forgot to label the last image - 20kHz THD 0.000817% at 160Vp-p IPS only at EF buffer, no BIGBT connected.
Attached Images
File Type: png Excessive Force Schematic.png (80.4 KB, 102 views)
File Type: png Excessive Force 1kHz-160Vpp-8R.png (42.7 KB, 99 views)
File Type: png Excessive Force 20kHz-160Vpp-8R.png (47.6 KB, 93 views)
File Type: png Excessive Force 20kHz-160Vpp-IPS.png (32.4 KB, 92 views)
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Last edited by jkuetemann; 9th October 2014 at 12:15 AM. Reason: Added comment.
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Old 9th October 2014, 01:42 AM   #2
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My inclination would be to see if smaller die MOSFETs and/or smaller gate stoppers would cleanup the high frequencies. This comes from experiments with one of the Pass amps. A 230 or 220 has much less input capacitance.
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Old 9th October 2014, 04:43 PM   #3
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I would look at Vin-Vfb over f. Maybe you are asking too much even from a degenerated LTP.

Otherwise it looks like output stage biasing could also use some optimization.
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