Say I would like to amplify small signals at 100kHz, the opamp has GBW=3MHz, so I set gain to 20 in order to have bandwidth of
150kHz. What *in practice* would happen to a 500kHz signal? The sim and a bench test show substantial attenuation, which is desired, but is it a general rule or just a property of a particular opamp? Could it cause any instabilities/distortion in the baseband?Best regards, Piotr
I mean, it's only single pole, so it's not a great filter. Not accurate either, GBW varies. But yeah, it's there.
The major downside is, with loop gain ~= total gain (in the stop band), there's no feedback to fix distortion.
Slew rate limiting is also relevant up there, depending on amplitude. But, well, it's distortion, it depends on amplitude, that's the point, and the problem. :-)
Given all those drawbacks, if it's still acceptable for your application, sure, why not?
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:
The catch comes from the non-linearity of the input stage when it's driven outside it's linear range - perhaps 50mV for a bipolar long-tail pair input.
Feedback stops that from happening for frequencies low enough for the output to keep the input from seeing big voltage excursions, but at higher frequencies big excursions look as if they are being rectified.
FET and MOSFET inputs have a much bigger linear range - closer to a volt - so the quick and dirty solution to a lot of RF-pickup problems can be to swap out the bipolar input stage by switching to FET-input op amp.
-- Bill Sloman, Sydney
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** Yep = about 12dB.** That is exactly how millions of popular op-amps are used in hi-fi audio circuits, resistors set the gain ( rarely more than 20 for a single state ) and long as the upper -3dB limit well exceeds 20kHz all is fine.
.... Phil
regards, Piotr
The application is a fast ADC driver. There is everything needed to aim at high processing gain (oversampling like crazy + a lot of preexisting dithering noise), so only a simple antialiasing filter would be fine. I thought to turn the pretty low opamp's GBW to an advantage and have a preamplifier-lowpass combo with no additional parts. The input is sufficiently band-limited already by an LC tank circuit, so it is there just to administer the final blow. Does it make sense?
Best regards, Piotr
Check the slew rate of the opamp. You're probably OK with letting the amp do your soft anti-aliasing, but if it ever slew limits accuracy will go to hell. A faster amp and an RC or RLC filter would be safer.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
The Soviet Polivoks synthesizer used altering the gain-bandwidth product of op-amps in the signal path (injecting more or less current into the LTP on a type that had a pin for that) to act as the control element in a resonant low-pass filter:
Why not use an inverting configuration and use a small capacitor in parallel with the feedback loop to set the bandwidth? Then there would be less anxiety. Invert when you can, non-invert when you must
I would buffer the input signal _first_, and then use an inverting stage with small-value resistors and feedback cap to add gain and roll off the response.
Trying to buffer, add gain, and roll off the response (if that is what OP is trying to do) all with one stage is IMO not a good plan
Yup. If the signals are sufficiently small, it can work OK, but generally you're a lot better off with a separate filter.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net https://hobbs-eo.com
Yikes, just the sort of application that you don't want to use a slow amp for.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net https://hobbs-eo.com
Whether a circuit in the frequency range he's describing is susceptible to interference from the low end of the AM broadcast band is kind of critical and I wouldn't be relying on a parameter with a large spread to ensure that it wasn't
If I were messing around with freq near the AM broadcast band I don't think I'd be leaving anything to chance.
Oh, one other gotcha: most fast ADCs kick out a bunch of charge from their sample/hold front end. That can really freak out an opamp, especially a slow opamp. An RC, or better yet an RLC, directly into the ADC input is a good way to block that charge spike.
What ADC are you using?
-- John Larkin Highland Technology, Inc lunatic fringe electronics
No. Its a truly shit thing to do.
With no loop gain, the input voltage across the diff input will be huge. A bipolar diff pair will be in limiting with about 50mV across it. With 10mV, its distortion will be greater than 10%, i.e. about 3 bits.
To drive a fast ADC accurately, you need an op-amp with a huge GBW so that you get enough loop gain at the frequency harmonics of the signal you are digitising. In fact, a decent ADC driver is often a fundamental bottleneck to the accuracy of the system.
-- Kevin Aylward
It seems that Piotr is still designing the DCF77 (77.5 kHz) receiver without resorting to proper radio design practices.
A fast ADC needs a fast follower to feed it, to get a low feed impedance.
-- -TV
The 12-bit one from PSoC5LP (actually, both of them, interleaved to get ~2Msa/s), ditto the opamp. One opamp is used by a DAC, so I have two spares.
Best regards, Piotr
Exactly, the second version of it, also capable of using the English
60kHz time signal. The first works, but I'd like to maximally simplify it, using as many resources of the PSoC as possible. I'd also like to learn the switching capacitor filtering techiques, etc. But for some reason this approach doesn't work, I have all kinds of oscillations.What do you mean by "proper"? A classic direct conversion receiver, what's wrong with that?
The opamp has GBW of 3MHz and 25mA output current capability, which should be good enough in follower mode.
Best regards, Piotr
Now I have a separate filter (8th order Bessel based on LTC1562) and would like to get rid of it. The chip is way to good and I can increase the sampling frequency to ~2Ms/s and filter the debris digitally. But I am afraid of the receiver's dynamic range, the level of EMI is huge here.
Best regards, Piotr
My goal is to end up with the PSoC chip, its bypass capacitors and a loopstick antenna with its resonant capacitor. I'm extremely curious how close to this target I can get.
Best regards, Piotr
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