IC capable of driving 30MHz 10 Vp sin into 300pF

And you need to check the slew rate, 10Vp at 30MHz is 1885V/us.

Or three or four. Generally the slew rate capability will be degraded when an amp is used near its maximum output-current capability. Furthermore, when driving a capacitive load the current maximum occurs as the output voltage goes through zero, a situation that maximizes each amplifier's power dissipation.

Also, be sure to add a ballast resistor for each paralleled output, say 0.5 to 1.0 ohm.

Even so, the 2kV/us slew rate is typical, not a minimum spec, and furthermore is uncomfortably-close to your requirement.

In bridge form each amp only needs 942V/us, but that's still a bit past the AD9815's specs. You can solve this with some turns step-up in the transformer, and give yourself some good slew-rate safety margin. Again, if you use two AD815s (four amps) there'll be a better safety margin. Use separate feedback resistors for each amp's gain setting, and also ballast the paralleled amps. This will also help to prevent the AD815 IC from oscillating.

--
 Thanks,
    - Win
Reply to
Winfield Hill
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Yes, and he can lower both the voltage and current requirements by connecting his added inductor to ground, and driving a tap. If he can employ resonance, it's a whole new ball game.

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 Thanks,
    - Win
Reply to
Winfield Hill

That's a lot of current. Is it a fixed frequency? If so, resonate the capacitive load with an inductor and that will reduce your drive current requirement by maybe 10:1.

John

Reply to
John Larkin

An ic ?

That's a substantial current. Have you checked those numbers ?

Graham

Reply to
Pooh Bear

Hi group Can someone recoment a IC witch has the capability to drive 30MHz 10V p sin into 300pF cap. load.

Thanks in advance

Frank

Reply to
Frank Mikkelsen

How about a CRT driver amp? They can drive high currents at high frequencies. Greg

Reply to
Greg

"Pooh Bear" skrev i en meddelelse news: snipped-for-privacy@hotmail.com...

Xc for 300pF at 30MHz is app 17r give a peak current at 570mA.

My first ides was to use the EL2008 buffer from élantec. (BW 55MHz, 1A) but it is obsolete, and I can't find any other types driving > 570mA.

Had found two other types that can be taken into consideration:

-BUF634T, but it is only capable of driving 250mA maybe two in parallel.

-AD815 but then I must use both amps in one case and a transformer to make the output single ended.

Frank

Reply to
Frank Mikkelsen

Yipes, that's quite a load. You're talking several watts of power to be sure.

How's about a middling op amp with a couple RF power transistors tacked on?

Reply to
grg

Hmm, is the 300p really 300p?

At 30MHz the OP can tune the reactive part out with about 94nH serial inductance and adjust the voltage gain (Q) with a series resistor.

Say we want +/-10V excursion on the amplifier, that translates to Q=1, R=17.7R and Io=563mA. He can lower the voltage excursion requirements by setting a higher Q. Say Q about 3 / R=5.6R then this translates to Vo=3.2V and Io unchanged.

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Thanks,
Fred.
Reply to
Fred Bartoli

OK, how about a bunch of medium-power fast opamps in parallel? Use one to get the basic voltage swing, then a bunch of paralleled followers, each connected to the load rail through smallish resistors, so they don't fight one another too much.

TI has about the only fast, high voltage linear process around...

THS3061

THS3112

Both are available in the SO-8 PowerPad package, so can be heat sunk pretty well. They *will* get hot.

John

Reply to
John Larkin

"John Larkin" skrev i en meddelelse news: snipped-for-privacy@4ax.com...

Unfortunately I had to scan the frequency from 1 to 30MHz so there no way out of driving the large current into the cap. load.

Frank

Reply to
Frank Mikkelsen

Difficult spec:

THS6182 has enough output current (1A), small signal bandwidth of 80 MHz (not too much for my taste when operating at 30MHz), and slew rate is too low, i.e. 450V/us. Maybe bridged?

OPA2677 offers 500mA output current and has a slew rate of 2000 V/us, but has a maximum supply voltage of +/- 6.5 volt, allowing for an output swing of a bit below +/- 5 volts, so 10 volts peak peak are probably not possible.

Anyway, power dissipation is anyway a serious issue with these specs, so you probably have anyway to come up with a bridged/parallelled solution.

HTH Klaus

Reply to
Klaus Bahner

What are you working on? Is it a sinusoidal output whose frequency you scan, or some other kind of waveform? How precise must the output be?

BTW, you could still take advantage of output resonance, peaking-coil style, by placing a parallel inductor with a series R, making a resonance at the highest frequency, that's low-Q, but still helpful in reducing the current and shifting its phase. The Q-spoiling resistor becomes a simple resistive load at low frequencies. A series cap can be used to remove or reduce its effect at DC and the lowest 1MHz frequency region.

--
 Thanks,
    - Win
Reply to
Winfield Hill

Another candidate would be the THS3001, with 2 paralleled pairs of

2N4401/2N4403 type (or better) emitter followers helping the output current. Connect a resistor from the opamp output to the load, connect the transistor bases to the opamp output (possibly add stopper resistors in series), put small resistors from each emitter to the load to equalise currents (and help with stability). You'll need a better package than a TO92, at least an SO-89 or similar. A quick search throws up PXT4401, I don't know who makes those.

Regards Ian

Reply to
Ian

Look into rigid coax - I don't know what they call it these days, but you should be able to find some - military equipment uses it all over the place. If you have the six outputs, you could run rigid coax to each of your electrodes, and, much like the way car buffs do exhaust headers, see to it that they're all the same overall length.

Good Luck! Rich

Reply to
Rich Grise

141 semirigid line, terminated with SMA connectors, readily available (eBay is a good source), but matching line lengths is a small part of this prolblem.

The burr-brown buf634 will do the 2000v/us slew rate and 30 MHz bw to about 250 mA. It's a pretty bullet-proof part.

The old LH0002H from natsemi is (only) 200 v/us, but can do 250-400 mA and up to 30 MHz.

You might also check out the power op amps from Apex

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Apex have lots of app notes and sample circuits. As an example, they have a nice app note on driving capcitive loads (app note 25), going beyond slew rate and max current calculations to looking at stability, power dissipation, and the like.

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Namaste--
Reply to
artie

I am working on a sweep generator to exercise an ion trap, used on a atom physics experiments on cern

formatting link
The ion trap consists of 6 electrodes each having a capacity to ground of

300 pf. The design of the ion trap can't be changed and it is placed in a vacuum chamber and the temperature must be close to 0 Kelvin -> no power dissipation allowed in the chamber. The signal paths to each electrode are precisely matched. The generator consist of six outputs producing the same frequencies to each electrodes with a constant phase offset during the frequency sweep, I need therefore a close phase match f(frq) for each output stages, and therefore I attempt have as smoothly phase change as possible up to 30 MHz.

The frequencies with constant phase offset during the sweep are generated with six synchronize AD9954.

The requirement to phase match between the electrodes over a 1MHz - 30 MHz sweep are not defined but the scientist wished around 1 deg which I am not sure I can meet.

Frank

Reply to
Frank Mikkelsen

"Ian" skrev i en meddelelse news: snipped-for-privacy@newsreg.cos.agilent.com...

I had missed these TI amps, they are fast! I take a a closer look - Thanks

with 2 paralleled pairs of

Is it possible to make the feedback loop stable when adding emitter followers in the loop (when using op-amps with bandwidth that high) ? Or is the output emitter follower uotside the loop ?

Frank

Reply to
Frank Mikkelsen

I wouldn't waste time attempting it inside the loop, certainly not with those parts. Outside the loop is a non-starter, unless you're considering a class-A circuit.

I'd start by taking bench measurements on the current capability of BUF634T buffer ICs at their full slew rate, to evaluate paralleled opamps. BTW, Burr Brown (see AB-101) says series resistors aren't needed because of the amplifier's internal 10-ohm output resistance, but you may find some added external resistance may help it to deal with a capacitive load. You may also find an R+C network to ground helpful in reducing ringing.

I previously suggested evaluating inductor peaking of the capacitive load near 30MHz to shift the phase of the current, or to reduce the voltage swing at 30MHz, either of which can reduce the stress on the amplifiers. Something along those lines may be part of your solution, but a quick calculation reveals you may also have serious problems involving inductance. For example, if you have a high-capacitance electrode at the end of some coax, what the electrode sees at 30MHz won't be what you're sending at the start of the coax. Also, simple wiring may be a problem. Consider, even a modest 250nH of wiring inductance resonates with 300pF at only 18MHz. Sheesh! One wonders how your 300pF is constituted, and what inductances are involved. Your first task should be to evaluate these issues. Often a spice simulation can be helpful in evaluating a circuit with many complex elements.

--
 Thanks,
    - Win
Reply to
Winfield Hill

I think a similar aplication apeared here some time ago I sugested crt driver, but voltage was much higher . they are designed to drive lower capacitances but at much higher frequencies and voltages so the current should be within its capabilities, also you get 3 in a high power package wich you can parallel or use one per electrode etc...

Colin =^.^=

Reply to
colin

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