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Old 1st February 2010, 02:19 AM   #1
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Hi, I am looking to build an amplifier which amplifies mainly highish frequencies (50Khz-13MHz) and I decided upon a class D amplifier because efficiency is very important. High voltage up to ~1000V is required and very low current is needed of about a microamp. After the amplifier i decided to use a cockroft walton stack to pump up the voltage.

All was looking fine until I had a browse in an electronics book and I can't really figure why a specific amplifier type (Class D or any other class) would be chosen over a simple common emitter amplifier. I suspect the reasons are obvious but they aren't jumping out at me at the moment!

Thanks for any help
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Old 1st February 2010, 07:48 AM   #2
Mooly is offline Mooly  United Kingdom
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You haven't really said what you are trying to do.
50khz to 13mhz is a wide range (LRF to RF) to amplify. And you want 1000 volts output !!!
What is the input to this amplifier ?

A CW multiplier gives a high voltage output from a steady ac input... so I don't really see the connection between an amplifier and this.

Forget amplifier classes at the moment... what are you trying to achieve ?
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Old 1st February 2010, 12:19 PM   #3
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The amplifier is for a university project and I need to transmit an oscillating signal (which ideally should be 13.56MHz but component limitations mean we are starting at around 100K and if that works we'll stick with it). The oscillating signal is not going to vary so I only need to amplify one frequency, and the high voltage is required because the amp must be capacitively coupled across a physical air gap between two copper plates.

So the input is 100K sine wave for now of about 10V
What do you think would be best for this? The device should ideally be battery powered which is why I wanted a class D amp. Despite the high voltage I thought batteries would be ok because the current drawn here is tiny. The CW stack should be ok I think because of the constant signal.
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Old 1st February 2010, 12:26 PM   #4
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Apart from everything else would the requirement to amplify those high frequencies exclude the use of a class D amp given that they seemingly need a low pass filter in the output to remove the carrier frequency?
From what I gather the typical carrier freq. is around 500kHz.
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Old 1st February 2010, 12:31 PM   #5
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Hmmm, I'm something familiar with broadband RF amplification and what you want here ain't gonna happen in one box without bandswitching. Even broadband military rigs will have at least three power amplification modules to cover this range.

There's some projects around qsl.net that can do class-D broadband nicely with a pair of IRF510's and you could use toroidal step-up transformers (use good teflon wire) to get your voltage.

Cheers!
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Old 1st February 2010, 12:52 PM   #6
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Class D amps not being fast enough was a bit of a worry but there are some pretty fast components out there and i've seen amps capable of 10MHz and I found mosfets with some good charactoristics so it doesnt seem too much of a problem acheiving the frequency needed. Besides, as I said I hope the device to work at lower frequencies.

Geek - I think you misunderstood the design in that I amp only amplifying 1 frequency. I probably wasn't clear but I want the design to work at around 100K but if I am struggling to transmit across an air gap I will increase the frequency so help matters. Using a high voltage was there to allow me to reduce frequency. High frequency is problematic in all parts of the design so I don't really want to use it if at all possible!

So yes for the time being its best off said that I'm designing an amplifier to significantly increase the voltage a 100KHz sine wave. I'll look into the torroidal transformers to see whats available. Do you think it is possible to run such a device off batteries. The idea was for a hand held device you see. It is possible to create ~30KV using a 9V battery and a flyback transformer so I thought all hope wasn't lost!
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Old 1st February 2010, 12:54 PM   #7
Mooly is offline Mooly  United Kingdom
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OK then.... ha what do I know

Still not 100% clear on this. If I'm talking rubbish please tell me... I just don't get what you are trying to do.

A CW stack generates a high DC voltage from an AC one... and you can't "transmit" DC... it doesn't propagate as a EM wave. The CW stack and it's diodes mean the input frequencies are "low", perhaps up to a few khz at most (I would guess... because you need high speed diodes for HF, to eliminate losses).

You can generate something that will "jump" from one electrode to another but that's not transmitting. The fundamental frequency you put into the CW is "lost" as it just becomes DC.
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Old 1st February 2010, 12:59 PM   #8
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Is it possible to generate 30kv from 9 volts... yes of course.

Just clarify something... are you trying to recover the fundamental (100khz/ 13mhz whatever across this air gap or are you just trying to generate a voltage to cause the air to breakdown and for a spark to jump across ?
Are you trying to transmit "information" across this gap by modulating the carrier in some way ?
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Old 1st February 2010, 01:03 PM   #9
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I've never used a CW stack before so you're points are no doubt very relevant. I see the problem you are saying - I was unaware that the stack produced a DC output and if that's the case then it is indeed about as useful as a chocolate teapot! Might be best off with a transformer or something else. Any ideas?
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Old 1st February 2010, 01:05 PM   #10
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I'm trying to transmit the signal across the air gap from one device to another and will be modulating the carrier in some way to send information. The 30Kv thing was irrelevant really!
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