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Old 28th February 2010, 08:47 AM   #1
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Default OPA547F Waveform Generator

Hi All,

I'm new here and thought I'd share what I'm doing.

I'm designing a 73.5v Pk-Pk 1kHz 50mA waveform generator (sine only).

This is the output stage (it still has the OPA452, but I've decided to use the OPA547 as its more easily available).
Click the image to open in full size.

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Old 28th February 2010, 09:18 AM   #2
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Here's the PSU.

Click the image to open in full size.
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Old 28th February 2010, 09:30 AM   #3
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The only thing missing seems to be the waveform generator

You can obviously also use any of the "chip amps" in your bridge amp.
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Old 28th February 2010, 09:43 AM   #4
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here it is, it's a slightly hacked Miniosc Miniature Audio Oscillator

Click the image to open in full size.

I've been nutting out why I get 2.5% THD.

I've placed C? across R10 (10nF MKT).
All other caps are polystyrene (except bypass).

D1 ~ D4 were 1n4148 but the temperature stability was woeful so I changed them to some 11DQ03's that somehow magically arrived this also greatly reduced the unwanted Harmonics.

I've managed to bring it down to 0.6%

Amplitude stability is still pretty bad though and I'd like to get the THD down a bit.

Last edited by _-_-_Adam; 28th February 2010 at 09:49 AM.
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Old 28th February 2010, 10:46 AM   #5
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Dare we ask why you need 73.5 volts output and from a bridge output too as that must limit it's connectivity.

Just wondered
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Old 28th February 2010, 11:22 AM   #6
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Quote:
I've been nutting out why I get 2.5% THD.
There are several reasons why your plan shows poor performance. The most important ones:

* You need a better plan for the level stabilisation. These clamp diodes necessarily introduce frightening amounts of distortion. As a simple fix you could probably replace them with a diode bridge which drives a zener (there's something in one of Jung's audio opamp books, details on request).
* I'm not particularly familiar with the oscillator topology you use (I'd rather start with a state-variable filter) but probably you need some sort of trim to pre-trim the Q of the filter. Otherwise you might get excess drive to the diode clamp.
* TL064 likely show rather high levels of common-mode distortion, particularly at low supply voltages. Use +/-15 V and a decent opamp type (I suggest you try TLE2704).

Quote:
All other caps are polystyrene.
The choice of capacitors becomes important (assuming a *reasonable* cap to start with and not something like ceramic X7R) once you approach 0.001%.

Samuel

Last edited by Samuel Groner; 28th February 2010 at 11:30 AM.
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Old 28th February 2010, 11:49 AM   #7
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Another one: How does an opamp with +/-24 V rails provide an output of 36.8 Vp..?

Samuel
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Old 28th February 2010, 08:39 PM   #8
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Opamps are gained for 18v Peak with 180 degree phase inversion.

So the differential voltage is 36v Pk (but still only 18v pk to GND).

When Opamp 1 is at +18v, Opamp 2 is at -18V. (My bad labelling the 26Vrms, as it's actually the differential voltage, not the pin-gnd voltage)

Last edited by _-_-_Adam; 28th February 2010 at 08:45 PM.
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Old 28th February 2010, 09:06 PM   #9
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Quote:
Originally Posted by Samuel Groner View Post
a diode bridge which drives a zener (there's something in one of Jung's audio opamp books, details on request).
I would have thought that would just raise the clamping voltage, or are Zeners less noisy? Any info appreciated.

Quote:
Originally Posted by Samuel Groner View Post
TL064 likely show rather high levels of common-mode distortion, particularly at low supply voltages. Use +/-15 V
I'll bump up the rails and have another look at the TL064 before I rip it out of the circuit

Thanks for the feedback.
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Old 28th February 2010, 10:27 PM   #10
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Quote:
Originally Posted by Mooly View Post
Dare we ask why you need 73.5 volts output and from a bridge output too as that must limit it's connectivity.

Just wondered
Sure I'm working with these things
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