DC Isolation from Op Amp?  - Page 1
Forum section: electronics / projects

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 04, 2014, 03:28:58 am
content: Hello, all.I'm building a wideband RF non-inverting amplifier with an LT1222 op amp (voltage gain = 40) for use from about 500 kHz to 6 MHz.This is going to feed a germanium diode rectified DC meter circuit.What I need to do is provide DC isolation up to about 600V for the amplifier input. In the past, for instruments or discrete component amplifiers I have simply used a .02 to .05uF capacitor, but when I add the capacitor in series with the +In pin of the op amp, the waveform severely distorts. It looks to me like third harmonics on the scope. (I assume this has to do with phase shift?) This will not do, as I need to be able to use the amplifier/meter circuit to determine RMS of a relatively pure RF sine wave riding on vacuum tube B+.Do I need a transformer input instead, or can I use a 1M resistor to isolate the DC blocking capacitor or swamp out any significant phase shift?Thank ya.

author: ConKbot
author_rank: Super Contributor
timestamp: February 04, 2014, 04:41:52 am
content: Do you have biasing resistors to bias the input to a range the amp can handle and to allow bias currents to go somewhere?

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 04, 2014, 10:31:41 am
content: Here's what I have (+ and - power) not shown:(The 47 ohm resistors are there for stability--it works better with them, for me.)

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 04, 2014, 10:37:11 am
content: I was thinking about replacing the Vin 47 ohm resistor with a 1M ohm, and feeding the 1M ohm from the left side with a .02uF DC blocking cap in series.

author: R_G_B_
author_rank: Frequent Contributor
timestamp: February 04, 2014, 01:53:02 pm
content: I find using bode plotting helpful to look at magnitude phase and frequency response. Makes it easy to see what frequencies are effecting the shape of the waveform.Using a square wave to test you opamp response to D.C isolation also helps.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 04, 2014, 02:15:58 pm
content: Thanks you two.I looked a little further into what ConKbot posted, and I see the common error I made. I need an input resistor to ground between the blocking cap and the +in pin to keep the DC bias leakage from charging the cap and driving the op amp out of linearity. I will try that next.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 04, 2014, 02:24:48 pm
content: If the source impedance is modest, an off-the-shelf pulse transformer would do very nicely without the need for an op-amp (or risking 600V across its inputs).Tim

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 04, 2014, 02:30:06 pm
content: Thank, Tim, I do need the amplification for this application though. I am trying to drive an RF probe into a DMM, or maybe later, a 50uA D'Arsonval from about 5mV P-P at 500kHz to 2.5 MHz. I will probably add a stage if using a mechanical movement. The source impedance will vary, but usually 100K ohm to 5M ohm.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 05, 2014, 02:20:04 pm
content: I experimented a bit and found the highest input resistor (+in to ground) after the capacitor that gives a reasonably undistorted waveform at the voltage and frequency range needed is 220K. This is probably usable, but I would like to have more input Z, (at least 1M) if possible.Open to any hints...thanks guys.

author: ConKbot
author_rank: Super Contributor
timestamp: February 05, 2014, 02:55:40 pm
content: the LT1222 has an input bias current of ~100nA typ, 300nA max. which is 66mV of offset, multiplied by your gain of 40, makes an offset of 2.64V (!)Look for a different op-amp with a lower bias current if you want a high input impedance. Linear has precision op-amps that have input bias currents in the 200fA region. Not saying those ones are suited for your application, but certainly there is room for improvement. You might even have to use a low bias current amp to buffer your signal, before you send it to your gain stage. Some of the 20MHz GBW amplifiers have a typical 2, max 20 nA bias current, (which should let you get over 1M hopefully) so you could use something like that as a buffer.

author: dannyf
author_rank: Super Contributor
timestamp: February 05, 2014, 02:58:11 pm
content: QuoteUse two opamps: buffer + an amplifier.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 05, 2014, 06:52:23 pm
content: Thanks fellas, so if I use another of the same as I have now (LT1222), but with a unity gain for a buffer, it will have less of a problem with bias current? I'm afraid I'm finding out there is more to know about op amps than I thought. I didn't realize that the bias current leakage would be so high as to be a problem with an AV of 40, but it sure has been.It make sense though. Maybe I'll try an in-amp.

author: ConKbot
author_rank: Super Contributor
timestamp: February 05, 2014, 07:53:32 pm
content: Quote from: Mr. Coffee on February 05, 2014, 06:52:23 pmIf you reduce the gain, then your voltage offset by caused by the input bias currents is reduced also. So if 220k give you satisfactory results now, a 5x reduction in gain for marginal, and 10x for a bit of a buffer/safety margin for variation, would leave you with a AV of 4, but you should be able to get your 1Mohm input impedance. Follow that with another gain stage of AV=10, to get your gain of 40.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 06, 2014, 03:45:37 am
content: I tried a 10 x 10 tonight, but all I got was the default "no amplification." I must have roached the chips soldering them in.Oh well. I do appreciate the time you all have taken, but it's time to give up on this amplifier project. This is beyond my capabilities. I'm going to try to buy one ready made, or make one up out of discrete components sometime when I have time.

author: Richard Head
author_rank: Frequent Contributor
timestamp: February 06, 2014, 05:47:02 am
content: That's the attitude. Try once and if you fail admit defeat and never try it again.

author: fcb
author_rank: Super Contributor
timestamp: February 06, 2014, 08:28:49 am
content: Quote from: Mr. Coffee on February 06, 2014, 03:45:37 amMr Coffee with the Automatic Drop...

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 06, 2014, 01:14:02 pm
content: You last two assholes don't know the facts of the circumstances, the hours and money spent on this, my electronics and other accomplishments by persistance and determination, nor my present physical limitations in executing these tiny component projects.So STFU.

author: fcb
author_rank: Super Contributor
timestamp: February 06, 2014, 06:10:06 pm
content: Quote from: Mr. Coffee on February 06, 2014, 01:14:02 pmMy comment was meant in good-humour (review my other postings), we are not mind-readers here - just EE's.Now pick-up that soldering iron and solve the problem - we will all try and help.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 06, 2014, 07:35:30 pm
content: Fair enough. My apologies.I have ordered some op amps from Mouser with a very low bias current and 410MHz gain bandwdth product. I will see what I can do with those and post back. Thanks.(I just wish I could work my damn hands and not burn things up or drop them constantly.)

author: dannyf
author_rank: Super Contributor
timestamp: February 06, 2014, 10:05:38 pm
content: QuoteLet me be the first to say that high speed opamps are incredibly difficult to use, especially if you don't have a good layout engineer to help you.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 07, 2014, 11:21:10 am
content: Quote from: dannyf on February 06, 2014, 10:05:38 pmI'm finding that out the hard way. Fortunately, the bandwidth I need is less than 6 MHz, so at least I don't have a lot of self oscillation issues to deal with. Keeping my leads short and being carful not to run inputs and outputs close together or parallel has been working so far...This is why I was thinking about drawing up a simple disctrete component amplifier sometime and forgetting the op amp idea.Here's what I got:For now, I put the LT1222 back to a 220K ohm input resistor to drain off input bias current, and the single stage gain of 40. It works, but when the other op amps arrive--I will try one with a 1M ohm input just for giggles.

author: fcb
author_rank: Super Contributor
timestamp: February 07, 2014, 12:41:11 pm
content: What are your supply rails?

author: dannyf
author_rank: Super Contributor
timestamp: February 07, 2014, 02:25:38 pm
content: It is quite unique to require that much input impedance for an RF application. What are you trying to do here?The feedback resistors used seem to suggest that the opamp is a cfb type. Is it?

author: dannyf
author_rank: Super Contributor
timestamp: February 07, 2014, 02:26:47 pm
content: Another option you may consider, if you can put up with low input impedance, are MMICs. Their gain flatness isn't as great but their responses goes to Ghz range.

author: fcb
author_rank: Super Contributor
timestamp: February 07, 2014, 04:45:10 pm
content: Quote from: Mr. Coffee on February 04, 2014, 02:30:06 pm^^Dannyf^^

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 07, 2014, 04:47:17 pm
content: Quote from: fcb on February 07, 2014, 12:41:11 pm+/- 9V So 18VDC between V+ and V-.Quote from: dannyf on February 07, 2014, 02:25:38 pmIt is for an RF voltage level meter for vaccum tube radio work. It needs to pick up as low as a 5mV RMS sine wave at up to about 4-5MHz, and be able to drive a crystal RF probe into a DMM or VTVM. By amplifying the RF signal, I hope to be able to get all the needed readable RF levels up and out of the square law region of the detector and also maximize sensitivity.In short, if I connect this onto an vacuum tube RF or IF amplifier running at a few hundred kilohertz to a few megahertz, I would like to be able to read a DC voltage on a DMM or VTVM that is closely proportional to either the peak or RMS voltage of th RF being measured.No it is a VFB if I am correct, but these LT1222's do not work with high value resistors, in my experience. I started out with 100K over 2.56k, anf got no output at all. Working down to a few K ohms got gain, but also problems with waveforn distortion at above 1MHz and it kept breaking into oscillation. The absolute resistor values were determined empirically.Here is the datasheet:http://cds.linear.com/docs/en/datasheet/1222fc.pdfI can't find anything that would indicate a CFB type unless I missed something(?)

author: dannyf
author_rank: Super Contributor
timestamp: February 07, 2014, 08:52:10 pm
content: QuoteThen you need at least a few kohms of input impedance so you don't upset the rf/if amplifier.Have you thought about the possibility to use an follower (like a jfet or even a bjt) upfront and then couple it to your opamp?Also, a regular RF probe should work, assuming that a sufficiently strong signal is there. Many of them are nothing other than a jfet follower + diodes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 08, 2014, 03:20:23 am
content: Ok, so it's an RF AC voltage probe for high impedance circuits?Question: do you expect to measure simply if it's working, or do you expect that it be accurate without disturbing the circuit?If the latter, for those ~Mohm sources, you need an input impedance in the Gohms!The basic approach is going to be a small coupling capacitor (frequency dependent, but if you actually achieve ~Gohms, ~fF won't be a big deal), a JFET follower (the smallest, lowest capacitance device you can find), and subsequent amplification (probably with more bootstrapping and low capacitance and stuff, because that JFET won't provide much drive itself).This all takes place in the very tip of the probe, with as little stray capacitance as possible. Unfortunately, you still need input protection (diodes and resistors), which will add a *lot* of capacitance (fractional pF at best). It won't be easy.Personally, I would much rather either leave this sort of thing to the professionals (i.e., get a proper FET probe, and pay through the teeth for it), or use an alternative method, e.g., measuring after the IF strip, or with an inductive loop probe for qualitative response. The latter of which literally costs nothing (assuming you have a probe and scope already).Tim

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 08, 2014, 09:31:46 pm
content: I am thinking about an emitter follower buffer up front. I was not sure if a JFET would have too much input capacitance to ground so probably bjt. Ideally, I would like 10 megohms up front, shunted by a few pF, but that's probably unrealistic without an FET of some sort, or else a vacuum tube.I do use a plain detector type RF probe, but I want to get the lowest level signals up out of the square law region, and also add a buffer at the output, if necessary, to drive an analog meter movement (later on).Yes I do have a scope and an HP AC millivolt meter, signal generators, etc. to work with. This project is just for the challenge as much or more than the usefulness. But it needs to be able to have a flat gain and work well, or else I won't learn anything.

author: dannyf
author_rank: Super Contributor
timestamp: February 08, 2014, 10:23:32 pm
content: At RF frequencies, bjts have limited input impedance, and jfet is the way to go.You may also look into dual gate mosfets if you have access to them. NXP makes tons of them. There are also some 3SK devices out there, lower noise figures typically.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 09, 2014, 03:18:01 am
content: what is the advantage of a dual gate mosfet?

author: Richard Head
author_rank: Frequent Contributor
timestamp: February 10, 2014, 08:15:39 am
content: The dual gate MOSFET is a single package cascode circuit configuration.The advantage of this config is the feedback capacitance is almost non-existant. This feature is really great when building amplifiers at VHF/UHF etc.If you used a junction FET at UHF you would probably have to use it in common gate mode to avoid self oscillation due to the feedback capacitance.Common gate operation is not always the first choice since the power gain is not all that high.Also, a dual gate MOSFET allows you to control the power gain by changing the Gate 2 voltage.It's also a very useful device for making active mixers.They are available with noise figures well below 1dB.Try BF991. BF998, all SOT-23.Rich

author: dannyf
author_rank: Super Contributor
timestamp: February 10, 2014, 12:12:54 pm
content: Richard is absolute right. A dual gate mosfet is essentially one mosfet sitting on top of another. They offer a lot of configuration options.you can -simulate- that with two jfets, one jfet and one bjt or two bjt.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 10, 2014, 03:13:49 pm
content: Thanks for the info, Rich. and especially the numbers to try. I get overwhelmed looking through "stacks" of datasheets online.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 10, 2014, 03:18:52 pm
content: The RF amplifer, for the frequencies I need (and even to some extent, the impedance) is the easy part. Getting the combination of high input Z and a flat gain so I don't have to make and use a gain vs frequency chart, or make a separate RF amp with adjusted gain for each part of the bandwidth I need...thatis the challenge for me here.I look at the old HP400 series, and have more and more respect for the engineers everyday.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 16, 2014, 12:48:02 am
content: OK, a couple new things for anyone still following this thread:I received my new op amps; they are ADA4817-ARDZ and here is the datasheet:http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&ved=0CCgQFjAA&url=http%3A%2F%2Fwww.analog.com%2Fstatic%2Fimported-files%2Fdata_sheets%2FADA4817-1_ADA4817-2.pdf&ei=IAgAU6u1Le2yygHcp4DQDQ&usg=AFQjCNGt3iF0XI6TA0fSO51b_83tk8PGFQI have re-evaluated my requirements, and decided that a voltage gain of 20, with the current capacity to drive a 50uA, 2K ohm D'Arsonval movement through a diode/capacitor peak voltage rectifier. Will I still probably want a mosfet input buffer?Also, while I was waiting for these to arrive, I decided to build a BJT amp. It is working well, with a flat (Av=20) response from 500kHz - 3.5MHz and will drive my analog meter movement. The input Z seems generally sufficient, but I am thinking about bootstrapping the front end for higher input Z. It is still on the breadboard so I can easily experiment with that.This is the schematic for that:I still want to do the op amp idea for more compact size and less battery drain though.cheersRob

author: fcb
author_rank: Super Contributor
timestamp: February 16, 2014, 01:21:52 am
content: That buffer/gain/buffer/gain config is nice and old school.I would worry less about the gain/back end of the project and get the interface to your source worked out. Follow up on that JFET buffer idea.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 16, 2014, 03:59:03 am
content: Quote from: fcb on February 16, 2014, 01:21:52 amWell I'm old school, and not an EE. (I'm an engineer, but far from an EE) I do what I can...LOL.OK. Looks like I'm ordering some JFETs. What would be a good generic part number to get?Thanks!

author: fcb
author_rank: Super Contributor
timestamp: February 16, 2014, 09:24:07 pm
content: J309/J3102N4416 probably better but more expensive.I wouldn't bother with dual gate (BF991) or cascoding design yet, but if your placing an order - i'd get a few for the parts bin.

author: Marco
author_rank: Super Contributor
timestamp: February 16, 2014, 11:48:57 pm
content: Lots of different implementations of the J310 from different manufacturers BTW (with slightly different names). Also there is the ever popular BF862 (very high transconductance and the audio guys love it, well when they aren't matching them any way).

author: dannyf
author_rank: Super Contributor
timestamp: February 17, 2014, 01:22:14 am
content: While most jfets can be used for RF applications, there are exceptions: watch out for the input capacitance spec in the datasheet. That will quickly rule out some of the jfets mentioned earlier in this thread.while jefts have very high (input) dc resistance, their ac impedance is much lower at RF frequencies.what you are looking for is very difficult to do, and likely unnecessary.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 17, 2014, 03:51:31 am
content: Quote from: fcb on February 16, 2014, 09:24:07 pmExcellent, thank you!Quote from: Marco on February 16, 2014, 11:48:57 pmGood to know, thanks.Quote from: dannyf on February 17, 2014, 01:22:14 amThe input capacitance is largely why I stayed away in favor of Op amps or BJTs. The BJTs I am the most familiar with. The OP amp option seemed easier but was harder than I thought when I wanted to block DC and maintain a wide bandwidth.Do you mean that I am seeking too high of input Z to be practical?

author: dannyf
author_rank: Super Contributor
timestamp: February 17, 2014, 12:12:34 pm
content: QuoteYes, as I said earlier. You rarely need that high of a Z, especially when your circuit is unlikely to breach 10Kohm input impedance, without using some tricks.For example, your bjt buffer will have a Zin of <10Kohm, likely in the 1Kohm - 5Kohm range this side of 10Mhz. Your jfet may get you 10x of that, but still nowhere near 1Mohm you are seeking.That kind of input impedance at RF frequencies is just crazy.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 17, 2014, 01:03:23 pm
content: OK. I am going to use it to probe tube amps, but I see what you mean. Even the HP400 series drops to the 3-4K ohms at 2-4 MHz, and their DC resistance in is 10M ohm.

author: pinkysbrein
author_rank: Contributor
timestamp: February 17, 2014, 01:28:28 pm
content: The main advantage of fets is that the probe is easy to make mains safe, you can have large resistance on the input bypassed by a capacitor to limit current at 60 Hz.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 23, 2014, 05:33:50 am
content: This unit will be in an insulated plastic case and not used for mains anyway, but that is interesting.OK, I am such a dumbass, I got the new OP Amps, and they are the teeny surface mount versions. I mis-ordered. I cant use these little fellas.I can barely manage to handle the old standard N8 cases.So now I have 5 ADA4817-ARDZ, 1GHz Op Amps in an unopened anti-static bag. First PM takes them for shipping cost.ADA4817-ARDZI did reorder some more LT1222's though. And I'll try again with those.Meanwhile, I tweaked my "brute force" amp and added a stage. Now I have a quite flat gain of 62 from 500KHz to 4.0MHz, and it works really well to drive the meter movement on a few mV of RF. If I use it in the meter I'm building, It will need some pretty stong attenuation because anything much more than 5 or 6 mV P-P pegs the needle. The battery drain is acceptable but not efficient at it could be. I still will post what I come up with for LT1222 with probably an FET input buffer.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 23, 2014, 12:49:13 pm
content: Quote from: 3roomlab on February 23, 2014, 06:46:34 amI just installed LTS and downloaded the basic tutorial a few days ago and am still trying to figure it out. Thanks for the info.It looks like it will be a time saver--as soon as I figure out what LTS "nodes" are, and how to run a simulation.

author: dannyf
author_rank: Super Contributor
timestamp: February 23, 2014, 01:36:49 pm
content: QuoteSome suggestions:1) you may want to have a buffer in and a buffer out;2) in between, put a gain stage. Run it a little bit cooler than you are;3) I don't know what that 100p capacitor is doing.4) you may want to think about dc coupling the whole thing. Makes it simpler.5) look into mmic. They typically provide 17-18db of power gain and are very simple to use. They do drain more current, however.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 24, 2014, 01:05:33 am
content: Quote from: dannyf on February 23, 2014, 01:36:49 pmThanks, yeah, it's far from an optimal EE job. (It works very well though...go figure LOL)The reason it is how it "landed" is after all the various calculations, there was still a lot of empirical determination of component values and placement. Tweaking, basically...then when a stage was doin as it should another was calculated, added and tweaked until it was ready for moving from the breadboard to the solder PC board. Then some final adjustments.The first stage is a bootstrapped emmiter follower as my buffer, and there are emmtier follower buffers in between the gain stages until the signal strength was strong enough to go with two consecutive gain stages at the end.The 100pF is there to level out the high end of the band where the gain was creeping upward.As far as the cooling it goes, I found that running higher input resistances and higher emmiter/collector currents allowed for a rapid charging/discharging of the interelectrode capacitance and more or less neutralized that. With lower currents or lower base bias resistances inevitably caused wavefor distortion--and/or an uneven gain below 1MHz or above 3MHz.Is it a work of art? No, it's probably like a monkey built it...but I'm learning. Anyway, those are the reasons it is the way it is.Edit: I thought about DC coupling but I don't have any experience doing that. I may do that next one. Thanks for the suggestions.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: March 01, 2014, 06:43:11 pm
content: I am trying again to use a pair of LT1222 op amps for the amplifier unit, but although LT Spice shows a good gain and sufficiently flat response, when it is breadboarded, it breaks into about a 10MHz self oscillation. It this a breadbord issue? Can these devices be used for a gain more than about 10 without acting up?Thanks!RobHere, again, is the datasheet link:http://www.alldatasheet.com/datasheet-pdf/...NER/LT1222.htmlThe schematic:The AC analysis:And the LTspice IV netlist:* C:\Users\XXXXXXXXXXXXXXXXXXXXXX\LT1222 Spice.ascXU1 N007 N002 N001 N009 N004 P001 LT1222V1 N001 0 6V Rser=10V2 0 N009 6V Rser=10R1 N002 0 220 tol=1 pwr=.25C1 N007 In 2.2µFV3 In 0 SINE(0 .005 2meg 0 0 0 100) AC .005 0R2 N002 N004 1200 tol=1 pwr=.25C2 N005 N004 4.7µFR3 N005 0 499 tol=1 pwr=.25R4 N007 0 1MEG tol=1 pwr=.25XU2 N008 N003 N001 N009 N006 P002 LT1222R5 N003 0 47 tol=1 pwr=.25R6 N003 N006 1200 tol=1 pwr=.25C3 Out N006 10µFR7 Out 0 499 tol=1 pwr=.25C4 P001 P003 100pFC5 P002 P004 100pFR8 P003 0 10KR9 P004 0 12KR10 N008 N005 220 tol=1 pwr=.5.tran 0 .00001 0.lib LTC.lib.backanno.end

author: dannyf
author_rank: Super Contributor
timestamp: March 01, 2014, 07:25:21 pm
content: Fast devices are usually very difficult to use, and in my view, should be avoided as much as possible.Having said that, a few things to consider:1) decouple with fast / small capacitors;2) use compensation to active limit the bankdwidth of the amplifier;3) think about signal conditioning. Put a serial resistor on the input, at a minimum.4) hire a good layout engineer.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: March 01, 2014, 09:51:17 pm
content: Thanks...the last option is not for me though. This is just a hobby project.I'm going to go ahead and solder it up to a PC and see what happens. The more I think about it, the more I think the push-in breadboard may be capacitively coupling signal where it shouldn't be.I also bought some J310G N-channel JFETs to experiment with...One thing at a time.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: March 19, 2014, 01:59:59 am
content: I went ahead with the meter without the op amps for now. It works well up to about 3MHz and is fairly accurate. It does all I need it to do and seems to pick up the RF voltage in tube radios without excessive loading.

author: David Hess
author_rank: Super Contributor
timestamp: March 19, 2014, 02:17:08 pm
content: I agree with the earlier posters that what you are looking for is essentially an oscilloscope input stage using a low input capacitance JFET or MOSFET intended for RF applications or an active probe which is basically the same thing. The later can get down into the 1 picofarad range pretty easily. Considerable input protection can be added using low capacitance clamping diodes and a high value bypassed resistor at the expense of additional input capacitance.Since you can use AC coupling, offset and offset drift are not important making the example from Troubleshooting Analog Circuits written by Bob Pease that I am attaching overkill but it shows the basic idea. Linear Technology application note 21 includes a similar design.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: March 19, 2014, 02:28:02 pm
content: Thanks for the circuit idea. I'll keep the picture and probably make a probe I can use with a scope, this meter, or my HP400 series RF meters.Is Q2 supposed to be an N-channel or a P-channel?The reason I'm holding off on the op amps now is that even without DC isolation, I am having trouble getting the gain with a good bandwidth and no self-oscillation.Rob

author: David Hess
author_rank: Super Contributor
timestamp: March 19, 2014, 02:59:54 pm
content: Q2 is an N-channel just like Q1 and they should be the same type of device. I remember noticing the missing arrow before.That JFET configuration is used to compensate for voltage offset and offset drift and while it is not needed in an AC coupled application, the constant high impedance of the lower JFET helps keep the voltage gain of the upper one as close to 1 and constant as possible over the input signal range. The lower JFET could be replaced with a bipolar transistor constant current source as shown in Jim William's design with or without the offset voltage correction operational amplifier.The old school integrated way to do this is with an EL2004 FET buffer which achieves 350 MHz performance. Elantec is part of Intersil now and they do not make an equivalent anymore. My quick search did not find anything like it currently made.

author: dannyf
author_rank: Super Contributor
timestamp: March 19, 2014, 04:29:48 pm
content: Q2 is a CCs. That can be done with a n, p or a resistor.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: March 19, 2014, 08:04:56 pm
content: Thanks guys.