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sine wave current booster

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Turkey9

Tue Jul 29 2008, 01:29AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Hi everyone

I have been searching google for a while now tryin to find a simple way to boost the current output from my function generator to no avail. I'm thinkin that something along the lines of the an audio amp that instead of amplifing the voltage will keep the voltage level the same and just allow for more current draw. It would need to go up to 5 mHz. Any help would be very much appreciated!

HV Enthusiast

Tue Jul 29 2008, 02:25AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

how much output power (current)?

And yes, its going to be an amplifier circuit, but certaintly not simple unless your only talking very low power output.

Turkey9

Tue Jul 29 2008, 03:26AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

For my perposes i think that about 5 amps at 20v should be good enough (100w?). i know that with audio amps distortion is an issue but will that be the case with what i'm looking for?

Hazmatt_(The Underdog)

Tue Jul 29 2008, 04:38AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Wow 0 to 5 milli-Hertz, that's a pretty narrow and low frequency response... hehehe. Nah, I know what you mean... heheh.

I'm working on practically the same thing right now. This is a driver that I think is up to ~5 MHz but is only 9 Watts out, so it will need to have some 250W transistors replacing the transistors I have in the sim. I'm thinking of using the NTE 87 and 88 for this application because I already have the T03 heatsink material available.

I'm also working on an HF amp to cover 100kHz to 2MHz, but if you really want to go from DC to 5MHz its going to need to be direct coupled for optimal response. Ferrites just don't go that low. Specialty iron cores can go pretty low, I think 2KHz to 3MHz, but its still not enough of a range to cover what you're looking for.

To amplify at nearly 0 Hz you will need a lot of power because the driver is going to be burning up a lot of current. I wouldn't be surprised if you need 400W of power for the 100W driver.

I threw together an amp real quick based off the same design, not much output but it goes to 1 MHz. It will need some more front-end driving before I can get the full capabilities of the transistors going, but this is a start and fairly clean too.
So you have there 130mA RMS at 1V rms at 96KHz, not great but it is a start.

Steve Conner

Tue Jul 29 2008, 09:53AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Terry Fritz made a function generator booster for Tesla coil tuning with the LM12CLK. It's a power op-amp in a 4-pin TO-3.

I just use an old Maplin MOSFET audio amp that puts out about 100W. I already had one, and when I saw the price of the LM12, I decided I could do without unity gain and DC coupling.

No audio amp will give full output at 5MHz: they are limited by the turn-off time of the big, slow power transistors. The LM12 craps out at 60kHz which is kind of typical. My Alexander stereo amp went up to 2MHz, but that was just small-signal bandwidth: the power bandwidth was 100kHz or so. If you want into the MHz, you probably need to use RF power transistors, or small MOSFETs.

Hazmatt's thingy is just the output stage of a Class-B audio amp. (hint: try capacitors across the diodes)

GeordieBoy

Tue Jul 29 2008, 02:59PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Tegam and Amplifier Research make broadband amplifiers that might do what you want. I have a Tegam 2350 on my bench here which boosts the output from a function generator up to +/- 400V. Whilst it's bandwidth goes from DC to a few MHz it won't put out 100W though.

-Richie,

Turkey9

Wed Jul 30 2008, 01:36AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I built the amp that Hazmatt gave and it worked well up to about 200kHz then dropped out. Probly the transistors i used, the data sheet said they had a cutoff frequency of 20MHz but i couldn't find the bandwidth. Could i just substitute mosfets that could take the current into the amp circuit? And also it doesn't need to go all the way to 0Hz, i can't see using it below about 500Hz.

Steve Conner

Wed Jul 30 2008, 10:18AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

No, it won't work with MOSFETs. (A cookie for anyone who can explain why)

Hazmatt_(The Underdog)

Wed Jul 30 2008, 07:33PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

if you want to go with mosfets they'll need their gates biased first and you'll need some capacitive coupling so the bias doesn't wander too far out of the linear range. Also they will need seperate bias - for P and + for N for linear amplification. That of course would forward the diodes and just short the gate and give you a mess.

Then on top of that you still need a matched pair N and P mos for the same setup and those are kinda hard to find in pairs. It's easier to find the N-mos because with the P-mos they have to put a P well in the N substrate before they can even start making the P-mos... extra pain in the #$$.
Then of course its kinda hard to push both past 1MHz because of capacitence, but I did have an almost success with a linear driver up to about 4MHz with the MOS.

At the moment I'm going to use 2 N-MOS for a push pull driver and I have some op amps coming that go to 160MHz with differential outputs. That will allow for the two mos to be 180* out of phase all the time and it will keep everything clean and happy. I was re-working a phase splitting transformer but I haven't finished deciding what transistor I want to use to drive the primary. That guy needs to be class A for the whole freq range and I don't have many power bjt's that are fast enough for 5MHz.

I just looked up the 2SC2078, its a real winner! gotta get some of them in stock! heheh.

Turkey9

Fri Aug 01 2008, 04:06AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I have a question about the circuit you gave me, Hazmatt. On the version that i built r3 and r4 get very hot. i looked at the picture you had and it looked like you were only using 1/4w resistors. I'm using 1 w ones now and they still get really hot afoter a couple seconds of use. They're 100 ohm right? Also, what are the function of these resistors?

And some success news as well. I tested it and it puts out 1.8A rms at 100kHz and 4V rms!

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