Trouble with amplifying white noise  - Page 1
Forum section: electronics / projects

author: spacescience
author_rank: Newbie
timestamp: June 30, 2023, 08:44:38 pm
content: Hello,A project I am working on for an internship is giving me more than a few problems. It's quite electronics heavy and my electronics knowledge is lacking; I have taken a single college level electronics course. My team is trying to amplify a white noise signal up to over 250V ptp. I know that's a lot, we are not trying to output an audio signal. Our setup so far is a white noise generator to a preamp, then to a high pass filter and then to a piezo amplifier. We added the high pass filter because our output from the preamp had a DC offset which we needed to get rid of. I've included a schematic of the setup. The issue we're having is that when inputting a signal to the piezo amplifier above 250V ptp it seems to overload the amplifier. The green light starts flashing and the output on the oscilloscope is cutting in and out. The higher the input, the quicker the cuts happen. We believe it could be a grounding issue and/or an issue with the high pass filter. We've tried a few different component values within the high pass filter, and the values in the schematic are what seemed to work the best. Any help would be appreciated.Thanks!Edit: My wording may be unclear, when I said 250V ptp, I meant 250V total, so about +/- 125V. The Piezo amplifier should be able to output +/- 150V or 300V ptp with the 24 V power supply.

author: srb1954
author_rank: Super Contributor
timestamp: June 30, 2023, 11:18:51 pm
content: Quote from: spacescience on June 30, 2023, 08:44:38 pmWhat is your source of white noise?If you have a true white noise source the waveform is extremely spiky and the peak voltage can occasionally be considerably higher than the RMS voltage indicated on a multi-meter. Theoretically, the peak voltage of a true white noise signal is unlimited. Pseudo white noise generated by something like a LFSR or S/W application has a slightly more predictable maximum amplitude but its peak to RMS ratio will still be quite high.In amplifying white noise you normally have to allow a minimum 5x overload factor (sometimes known as a "crest factor") from the RMS voltage, to ensure that your amplifier can handle the signal without excessive clipping. Passing the white noise through a high pass filter will tend to emphasise the spikiness of the waveform so you may need to have an even greater crest factor. You could reduce the required crest factor a little if you also put a suitable LPF in circuit to restrict the noise bandwidth to no more than is necessary for your application.Can you post some scope traces of the signal at various points in the circuit?

author: Infraviolet
author_rank: Super Contributor
timestamp: July 01, 2023, 02:08:17 pm
content: Don't know precisely what the equivalent would be for such high voltages, but whatever the equivalent of a zener diode would be might be an appropriate solution. A fast acting one of whatever a high voltage equivalent of a zener would be, perhaps one designed for ESD suppression so long as your currents are low, could cut off spikes going too far past your maximum tolerable amplitude. It would ofcourse make the noise no longer ideal white-noise when chopping off those peaks, but I guess you can tolerate that?Depending what your high voltage noise signal ends up feeding in to (is the piezo device what it is going in to in the end, or is that just further amplification?), that thing may have a maximum frequency it responds to anyway, so some low pass filtering (if you did it before the "zeners" then it would reduce the speed at which those "zeners" need to respond too) to an extent of a cut-off frequency somewhere close to the maximum frequency your end application can take, would probably cause no change to measurable output.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: July 01, 2023, 02:12:36 pm
content: If it is just the voltage amplification you are looking for, then you could use a step up transformer for this application. You could get some transformers which are working in the audio range and beyond. Note that the transformer will not amplify power.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 01, 2023, 02:32:49 pm
content: Don't know what you're doing, but if trying to drive a PZT device it's going to be difficult. The signal is going to have peaks and spikes and thus the 5X headroom advice. A PZT device is close to a pure capacitance, so the current requirements on those peaks and spikes will be very large. Just a guess, but you're probably way under powered. There's also the question of how much drive the PZT device can take without damage. If this is a bipolar situation it might also depole. Again, I could be way off base because I don't know exactly what the application is.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 02, 2023, 12:43:29 pm
content: You say you're inputting a 250v pk-pk to the amplifier input and its complaining about being overdriven.... and the query is?What are the specs of this scary amp.Help us out with some idea of your kit and a copy of the the schematic might help.My first guess is that you need to generate a drive signal in the volts range so from the generator and filter combo.

author: mawyatt
author_rank: Super Contributor
timestamp: July 02, 2023, 03:35:47 pm
content: If the OP can get an Apex PA443 (Duals) or AP441 (Single) HV Op Amp IC, then an HV amplifier can be built around such.See this thread for some ideas,https://www.eevblog.com/forum/testgear/hv-buffer-amp-for-awg/msg3623953/#msg3623953Best,

author: mawyatt
author_rank: Super Contributor
timestamp: July 02, 2023, 04:53:10 pm
content: We've updated the thread mentioned above, showing a version of one of our HV AWG amplifiers based upon a Apex PA443 producing 270VPP output waveforms. This can go higher by increasing the +- rail values (see pot in images), as the PA443 can withstand ~350V total DC bias.Best,

author: Terry Bites
author_rank: Super Contributor
timestamp: July 02, 2023, 08:07:01 pm
content: Its a non-trivial task to build a high voltage amp with its psu and fault protection.It is irresonsible to reccomend it to an inexpeienced constructor with limited ee know-how.Lethal voltages will be present.HV power opamps drifted through my mind as well.They tend to be slow- GBPs in the 10s of MHz.You need the BW to be at least twice the piezo self resonance freq.Even at a gain of 10V/V you'd be hard pushed to get a useful drive signal.A bridge tied load can be used to double the slew rate.Without knowing the load capacitance it will be tricky to get the compensation right.As op notes, he's not building an audio amp here.Lets wait for more of the details of this experiment.

author: mawyatt
author_rank: Super Contributor
timestamp: July 03, 2023, 04:47:07 pm
content: Irresonsible by who's judgment, yours??Dealing with a single integrated HV amp solution such as the mentioned PA441 or PA443, is certainly safer and more responsible than attempting something with a bunch of discrete devicesSure HV is present (OP mentioned 250VPP), and will be in any attempts including a transformer, anyone, including the OP, needs to be careful around such. This is no different than long ago when folks dealt with devices that had vacuum tubes (300~500VDC plate supplies), some TVs had ~25KV, even restoring an old Tektronix analog scopes involves high voltages ~2KV.Regarding piezo devices, we've done a fair amount of development work with them at high voltages, mostly to drive custom ultra-precision nanometer positioning systems and agree they are tricky, as such the OP will likely require some consultation with experienced folks whom can also provide some guidance, maybe helping the HV needsBTW the mentioned HV Amp based around the PA443 has overcurrent limiting, 5K output Resistance, 250V 20ma HV Output PTC Fuses, 2.5A 12 Power Input PTC fuse and each HV supply is limited to under 2 Joules stored energy, all for "safety" reasons. Of course YMMV!!Best,

author: spacescience
author_rank: Newbie
timestamp: July 05, 2023, 03:53:39 pm
content: Thank you all for the replies, and I apologize for not responding sooner. I sent out my question right before we went on break for the fourth and completely forgot about this. The piezo amplifier is not the final part of the circuit, we will be putting the output from the amplifier into a PZT actuator. It is a piece of hardware that expands and contracts when introduced to varying voltages. For our application, we want it to vibrate as much as possible, hence the extremely high white noise voltage requirement. I will attach data sheets for the piezo amplifier and an image of the PZT actuator.