EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: max_torque on August 26, 2014, 03:26:34 pm
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I have an input filtering circuit that takes in a variable frequency & variable amplitude sine wave. I want to test that circuit, on the bench, so i would like to generate a suitable sine wave. My signal gen obviously doesn't go up that high, so what options do i have?
One immediate thought was get a cheap old audio amplifier and use that? (no idea what the rail to rail volts on a typical seperates amplifier might be, but i suspect one with a decent wattage much be able to get close to +-50v?)
Ideally, i'd like to be able to sweep from say 5hz @ 1Vpkp to around 100Hz @ 100Vpkp
Any other suggestions? (there seem to be some "high voltage" op amps availible)
(of yeah, i'm not too worried about distortion or non linearity of the sinewave, but the closer to what my signal generator is putting out in terms of waveform shape, the better really)
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You could use a mosfet(s) and external PSU to boost the output. I do this quite often when I need a higher voltage/current signal from my generator.
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Any audio amplifier or receiver rated for ~125W or so into 8ohms will have rails over +/-50V.
The 100Hz will be no problem, but you might run into issues at 5Hz because of the coupling caps in the signal path.
Wouldn't be difficult to build one. 1K isn't much of a load.
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These bipolar supplies (https://www.eevblog.com/forum/reviews/review-lambdaemi-boss-bipolar-operational-ps-or-'40-pound-op-amp'/msg161338/#msg161338)are like a huge op amp. I have a extra 50V model I will sell shown here (https://www.eevblog.com/forum/chat/show-your-favorite-and-mostly-used-benchtop-psu/msg404318/#msg404318).
Edit: added link to 50V 2A units
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You can make your own around Linear Tech LT1210 high current op amp, either alone or with some power transistors, in case you desire even higher voltage/current. The links from a product page point to couple of appnotes (one by JW) with examples of various amplifiers -> http://www.linear.com/product/LT1210 (http://www.linear.com/product/LT1210)
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You'll need a fairly high powered amp to hit those numbers. Maybe an old Crown DC300A would be the ticket. Actually there won't be much supply sag, so maybe a bit lesser amp would do it. Since you don't need much power, how about some lower powered amp, driving a step-up transformer? You don't need much bandwidth, so even a 50/60 Hz transformer might surprise you. I'd start with a 24V filament or control transformer.
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With a 1k load, all you need is a booster circuit for an opamp for it to work. An audio power amp will do as well but that's likely too complicated and too expensive.
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I have a cheap Yamaha AVR and it has +/-65V rails. Under load they might sag somewhat, but I can't imagine 1k giving issues.
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I was thinking that i could mess around bread boarding something up for a few hrs, but only a low cost, or just buy a s/h audio amp on ebay. Cheap ones seem to go, er, cheaply, so i wouldn't worry too much about damaging etc etc!
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They still make the LM12?
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What about an audio amplifier plus a transformer? The transformer could be a common 24V HVAC control transformer used backwards.
BTW, most consumer audio amps run at 20-50V supply rails depending on wattage. Some of the newest ones vary the supply rails depending on the volume setting.
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You don't need a lot of power------remember both peaks aren't present simultaneously,& amplifiers are normally rated at average power,which is Vrms^2/R,or in this case,1.25 watts.
What you really need is voltage swing,which makes the transformer idea seem the easiest.
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I was thinking that i could mess around bread boarding something up for a few hrs, but only a low cost, or just buy a s/h audio amp on ebay. Cheap ones seem to go, er, cheaply, so i wouldn't worry too much about damaging etc etc!
You should be able to cook up something in couple of hours. Biggest broblem is the power supply, but since the power requirements are very low you can use voltage doublers or triplers and so on to get the desired voltage. Or use transformer coupled scheme as already suggested.
All-discrete amplifier for your specs wont need more than few components. Or you can add extra voltage gain stage for opamp of your choice. Audio amplifier circuits would be a good starting point, you can even run your output stage on class-a since you dont need much power.
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They still make the LM12?
The LM12 is no longer produced but the Linear Technology LTC6090 can operate to +/- 70 volts. It would require an output current buffer or several in parallel to drive 1k but that is not difficult. Unfortunately it is only available in surface mount:
http://www.linear.com/product/LTC6090_LTC6090-5 (http://www.linear.com/product/LTC6090_LTC6090-5)
I would either bootstrap the operational amplifier more more likely use some type of simple discrete output stage since the current output needs to be increased anyway.
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Hi,
Take a small audio amplifier, connect it to the low side off a toroid transformer connect a 1 to 4K7 as a preload resistor on the 120/240V primairy, done!
It wil have a Frequentie range from 40 tot at least 10Khz.
Kind regarts
Blackdog
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For +/- 50 V, an audio power amp is probably the simplest solution. If you want a high voltage opamp, the apex microtechnology ones like the PA85 are pretty well known and easy to use. They can do +/- 200 V with no problem. You pay quite a premium for the convenience, but that may or may not matter to you for a one-off application.
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Burr-Brown and National made all kinds of integrated and hybrid high voltage amplifiers but I think those products are gone since TI absorbed them.
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I was also thinking at Apex. Long time ago, I used PA88A on boards, but the price was high (and not the only one on this board) !
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I have a couple of Apex samples and while they are really neat, they are expensive enough that I have always gone the discrete design route.
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An audio amp should do it. Even a modest 100W @ 8 Ohm is only <25 VRMS, or +/- 40 V peaks. At a 1 kOhm load, you will usually get higher voltage output than loaded to 8 Ohms. At 5 Hz, you will very likely get significant rolloff, so you will need to measure the output with something that will properly respond to just 5 Hz (not all True RMS multimeters will do that).
A couple of cheap audio power amp ICs like LM3875 or LM3886 wired in a full bridge output will also give +/- 50 V but not referenced to ground.