Here's a way to do capacitor multiplication...
limited only by OpAmp GBW product, and no resistive components until the OpAmps run out of steam.
Mathematically, this is the same way I did behavioral models for crummy capacitors such as X7R and Y5U, except I had ideal amplifiers, and equations and/or tables to describe the changes versus voltage and temperature.
Larkin's "Maybe I'll do this:"
does not fare so well. Do the math. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Cap multipliers work but have practical limitations with leakage current and high output impedance.
Straightforward application of Miller's Theorem...
Now THAT is innovative modelling.
2.5x the parts (including a GHz opamp), maybe 50x the noise. It would destroy the low-noise transimpedance amp that I'm doing. You have just invented the world's noisiest capacitor. Well, you probably "invented" it 40 years ago.
And I need a floating capacitor, not a grounded one. And I need about 0.4 pF, which would call for a capacitance divider, not a multiplier.
Net idiotic.
-- 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
It's just Miller capacitance, C' = C * (1+G) , so you never invented it.
-- 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
Larkinus Bloviatus Fartus Maximus:
I asked, "Do the math."
Larkin didn't.
I did.
Though any kid fresh out of school would look at this...
And note, by simple observation, several things...
It's not a two terminal network.
The impedance looking into the node on the left (marked arrow-style by Larkin) looks like that resistor from the arrow to IN- of the OpAmp (until the OpAmp runs into the GBW stop).
The impedance looking into the right arrow is, in essence, a voltage source, zero AC impedance, again until the OpAmp runs into the GBW stop.
If that's a _floating_ capacitor, I'm the King of Siam.
But Larkin's sickophants should be happy, their low-information mentality, and need for name-calling, has been satisfied. They will now face toward San Fransicko and chant... >:-} ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Okay, maybe I'm wrong, but I thought the intent was simply to simulate a tiny variable cap across the feedback resistor. If that is the presumed intent, and Xc >> Rpot/4 ...
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
You somehow missed the point entirely. I have an opamp circuit that includes a cap, roughly 0.4 pF, but I want to tweak it. I want to trim that cap up/down a bit until the circuit works the way I want it to, in a breadboard. I could solder in various caps, which has obvious problems. Or I could use a varicap diode, a trimmer cap, or a gimmick.
Or I could use this circuit with a 1 pF cap and maybe a 500 ohm pot, replacing the fixed capacitor. I can turn the pot and vary the opamp response without bending anything or soldering. Even better, I can measure the resistances of the turned pot with an ohmmeter and easily calculate the value of the equivalent single capacitor, which none of the other methods do.
The pot is a "cap divider" that, in my circuit, adds trivial amounts of noise or phase shift and really lets me simulate a ~~0.4 pF wideband variable capacitor.
Your circuit is cool, except for not being original, and having almost everything wrong with it.
What is this "fresh kid out of school" obsession that you keep having? Make sure they're at least 18.
-- 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
Right. Maybe a 500 ohm pot. The highest impedance the wiper looks is 125 ohms, which is a 125 ps time constant, out of my range of interest. That 125 ohms adds a teeny amount of Johnson noise to my breadboard, not a concern. Once I figure out the best cap value, I can design that into the PCB, probably two caps in series to give high tweaking resolution, like if I need 0.35 pF.
Saves a lot of soldering, and I can calculate the equivalent C, which I can't with a gimmick. I could even use this, 1 pF cap + pot or two resistors, in production, so I can hit small oddball values.
-- 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
Depends on your definition of "floating" ;-) ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
I thought you were trying not to dish that stuff out?
Using a voltage divider on a small compensation cap is a pretty common approach in TIAs, though IME it's usually a capacitive divider rather than a resistive one. It works OK up to the point where the summing junction loading is important. You clearly don't want to try synthesizing 1 pF using a 1000:1 voltage divider and a nanofarad in series.(*)
Using a pot, you have to worry about the current noise of the low resistances getting into the summing junction. (I posted a TIA fragment a week or two back where I had to short out a 1G resistor with a relay to avoid its Johnson noise current dominating the circuit performance above a few hundred hertz.)
However, given that the input capacitance is probably nearly 2 pF to begin with, something like 0.5 pF and a low-value pot isn't necessarily a silly way of making an adjustable 0.3 pF for that sort of use.
Cheers
Phil Hobbs
(*) Back in the early part of my career, I actually managed to make an op amp integrator oscillate all by itself. Fast op amp with high Zout and some phase funnies in its compensation, small R_in/large C_F.
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
Actually defining the total problem would have been a more useful exercise.
Nowhere in Larkin's multiple posts was "capacitor immersed in an integrator" mentioned.
When I show a true capacitor multiplier, I get his usual ration of shit... and bloviations about "floating". ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Oscillation isn't an unusual event; stability is!
I sim'd the pot thing (yes, you prefer algebra) and it had an inconsequential effect on noise. I'd only do this on the breadboard. I like being able to calculate the effective capacitance after I twiddle the pot; I can't think of any other trimmer that easily allows that.
-- 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
Other people understood what I meant; I want a sub-pF variable cap to trim a circuit, and neither end is grounded. I posted the actual circuit.
If you had posted "your" cap multiplier with some neutral comment, like "take a look at this cap multiplier", you'd look somewhat less stupid. Or if the context wasn't clear, you could have asked. But you misunderstood the situation and posted a challenge. How did you get to be so rash and insecure?
You're sure not aging gracefully.
-- 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
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