General Purpose 20MHz Function Generator Amp

I often find we need more current than our lab function generators can output.

Does anyone here have a good circuit for a current amp that will sink at least 500mA?

The range of most interest is 1MHz to 10MHz, although the wider the better.

We don't require any voltage gain, since the usual 10-20V is usually enoough.

Because the application is general experimentation, the amp needs to be able to handle a wide variety of loads, i.e. coils, ss switches, etc.

Unfortunately, we cannot justify $3,000 for a manufactured lab amp. Just a "basic" circuti for tinkering will do.

Barry Latham

Reply to
blatham
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Complementary emitter followers on a heat sink, and a couple of bench supplies?

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John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

** Maybe try a complementary pair of these lateral FETs.

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Two diode drops between gates will do for forward bias.

No need to insulate the paks from the heatsink, as the source terminal is the tab, so the heatsink is at output voltage.

A+/- 15 volt supply limited to say 1A should do fine.

... Phil

Reply to
Phil Allison

Does it just need to sink current? Can you handle a bit of attenuation and voltage shift? If so, why not just a PNP or P-channel FET?

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com
Reply to
Tim Wescott

Diodes. I hadn't seen that used before. I will try it.

Barry Latham

Reply to
blatham

Just saw this in a mag....

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John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

You might want to take a look at the power op amps from Apex Microtechnology. They have several models which would easily meet your output current requirements and appear to be fast enough. Some of them are described as being suitable for driving motors or piezo drivers; they have built-in current limiting, inductive-kickback clamping diodes, and thermal shutoff.

The PA119 would be one possibility... 100 MHz gain-bandwidth product,

5 amp maximum output current ("within SOA"), 75-watt internal power dissipation limit. Supply voltage range from +/- 15 to +/- 40.

8-pin TO-3 case. Use a good heatsink :-)

Not cheap (about $300 in onesies) but it would be fairly straightforward solution. Some of their smaller, lower-power parts might suffice for your needs.

Reply to
David Platt

$1875 for the cheaper model that does 16Vpp into 50 ohms. Several time I wanted more out of my sig gen, but not at that price. Mikek

Reply to
amdx

OK, that is expensive. Maybe I should do one as a product.

We sometimes need to make 26VRMS for synchro simulation, and sometimes 120 RMS, from a low-voltage ARB, but that's a different product.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

Interesting! Do you have a guess as to what your cost would be if you did?

Reply to
John S

not 500mA but in the ball park.

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Reply to
mook johnson

I almost made that suggestion when I found the price. Mikek

Reply to
amdx

A similar device was previously discussed in this newsgroup. The best suggestion was a pair of LT1210 devices in the form of a bridge amplifier for 9 watts into 50 ohms to about 14 Mhz. Note that it's NOT DC coupled. Search Google Groups in this newsgroup for LT1210 .

Data sheet:

App Note. See Fig 70:

My tweaks:

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

** Did you see the price ??
Reply to
Phil Allison

It's only 0.625 of $3000!

This looks like it could be done with available opamps, maybe a few in parallel at the output. THS6022 or some such. Or something discrete for slower but a bit more voltage swing.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin

Den tirsdag den 1. oktober 2013 02.36.11 UTC+2 skrev John Larkin:

haven't looked too closely at it but maybe:

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-Lasse

Reply to
Lasse Langwadt Christensen

That circuti looks worth trying.

Can you possibly recommend a "workable" substitute for the CoilCraft transformer?

Something more commonly availalble or self-made on a generic core.

Thanks,

Barry Latham

Reply to
blatham

"John Larkin" "Phil Allison"

** And only 0.5 of the current he asked for too.

Two FETs as I suggested will fit his spec for under $20 - plus a PSU.

.... Phil

Reply to
Phil Allison

The most critical part of any design is the specification. And yours is beyond vague.

It's not unusual for customers to think vaguely about what they might need, then hedge their bets by asking for 10x that much. The difference between 10 and 20V might make a BIG difference in the cost of the devices. Ditto for 1 vs 10 MHz. And what does 10MHz. mean? Are you content with a 3dB bandwidth of 10MHz.? Or do you want competent square waves at 10MHz. rate? Transient response requirements? Linearity of the triangle? Flatness of a frequency sweep? Flatness of the top of a square wave? DC coupled? Voltage or current offset?

And how are you gonna get the signal to the load? Mismatched cables are not your friend.

It's been over 30 years since I designed a production function generator, but physics hasn't changed much since then. Meeting any few requirements is easy. Meeting all of them simultaneously in the same box is a bitch. "General purpose" only has meaning to the person who made the statement.

You need to be very clear about what you want.

Having said all that, I second the recommendation that you build complementary emitter followers. But there's art in even the simple approach. Thermal time constants can mess up the flatness of the tops of your square waves at low frequencies. Or induce variations in offset that are waveform dependent. The output impedance of an emitter follower has an inductive component that you have to compensate for. You're probably gonna want some current limit. Overcurrent shutdown is a lot simpler than a smooth transition to current limit in active mode. Carefully consider the load fault current when selecting devices that may have safe-operating-area issues. You can also get blind-sided by SOA issues when driving inductive loads.

Be aware that this isn't a current amplifier. It's a voltage amplifier with a gain less than one and lower output impedance.

If your objective is to supply current, there may be better ways.

Reply to
mike

"VPH5-0155-R" That seems to be a Coiltronics part number.

You could possibly substitute Coilcraft HPH5-0155L

Hit their website

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Search for the part. They'll mail you a free sample.

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Reply to
Jasen Betts

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