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Old 30th August 2012, 09:00 AM   #1
KLR650 is offline KLR650  United Kingdom
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Default High Voltage Hexfet Amp?

Hello, a newbie here!

I have an application that requires as much voltage output as possible. I want to drive a piezo stack actuator up to 100V or more. Is there anything stopping me using the simple ‘maplin’ type design but up-grading components for the higher supply voltages? I’m only interested in +ve output too so could I run the amp off say +130 and -20V ? The piezo BTW behaves like a 100n capacitor at the frequency’s I’m interested in i.e up to 4KHz.

Thanks.
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Old 30th August 2012, 02:33 PM   #2
jcx is online now jcx  United States
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do you need DC response? - if you can get by with 50 Hz low frequency limit then you can use a transformer to step up a audio amp's output V, could C couple, add DC bias supply if you really need the offset

could use (cheap) standard toroidial mains transformers for 50 Hz-4 kHz range
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Old 30th August 2012, 03:17 PM   #3
KLR650 is offline KLR650  United Kingdom
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Yes I think DC is required - but I didnt think of a transformer to step up. I guess these tranformers are optimised for 50/60Hz - would be interesting to know how low you could go before response falls off too much.
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Old 30th August 2012, 04:55 PM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by KLR650 View Post
Hello, a newbie here!

I have an application that requires as much voltage output as possible. I want to drive a piezo stack actuator up to 100V or more. Is there anything stopping me using the simple ‘maplin’ type design but up-grading components for the higher supply voltages? I’m only interested in +ve output too so could I run the amp off say +130 and -20V ? The piezo BTW behaves like a 100n capacitor at the frequency’s I’m interested in i.e up to 4KHz.

Thanks.
You can find some ideas in this thread:
Self-cascoding, a tool for creating ultra high voltage building blocks:
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Old 30th August 2012, 05:14 PM   #5
jcx is online now jcx  United States
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cascaded Q not necessary, many multi-hundred V Q are available today, 400-600 V Mosfet are popular for offline switching supplies


true piezo will give a fixed displacement for DC

if you are generating sound, mechanical waves or other "AC", relative motion then the DC isn't required

transformer lower frequency limit is V*T product - use a 240 AC rated mains xfmr @ 50 Vac (= 140 V pk-pk ) and you could go as low as 50/4.8 ~= 10 Hz


electrostriction requires a DC bias - preferably a lot larger than the AC amplitude for good linearity

Last edited by jcx; 30th August 2012 at 05:22 PM.
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Old 30th August 2012, 05:20 PM   #6
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I have a pohotcopied page from a book showing how to drive high voltage swings at low current using an op-amp and some discrete components. Will find it out and scan in. Should be just what you are after.
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Old 31st August 2012, 08:08 AM   #7
KLR650 is offline KLR650  United Kingdom
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Thanks for that.

Well I only need 100V or so peak to peak and that only positive. I'm wondering if I can use something like:

http://www.ebay.co.uk/itm/EBB-250-Mosfet-Amplifier-Module-250W-RMS-Audio-Disco-PA-/360484078775?pt=UK_AudioTVElectronics_HomeAudioHiF i_Amplifiers&hash=item53ee8684b7

and run it off say +120 and -20V. The output devices are 200V so they should be OK. I would need to change capacitors and maybe some of the other transistors to work on the higher voltage but this would be a cheap solution if it works.
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