Re: Why Adding a Voltage Amplifier after a Transimpedance Amplifier reduce BW? Book Claim by 0.64

Started by Phil Hobbs May 14, 2015
On 05/14/2015 11:52 AM, Felipe Jimenez wrote:
> On Thursday, May 14, 2015 at 4:08:04 AM UTC-7, Phil Hobbs wrote: >>> Oh and I'm fairly certain the non-inverting amplifier is wired up >>> correctly. Between switching back and forth from the >>> non-inverting to the inverting configuration several times, I am >>> fairly certain that both configurations were wired up properly. >> >> Hmm... "several times" does make it sound like a solderless >> breadboard, as John suggested. Those things are slabs of misery, >> for sure. >> >> The AD8066 doesn't come in a DIP package, so I assumed it wasn't >> one of those things, but having bad layout strays is one of the >> ways to not build what you think you're building. >> >> Oh, one other thing--did you get your parts from an authorized >> distributor? Otherwise it's possible the op amp is counterfeit. >> >> (And do knock off the 'Dr.' bit--that's in my sig for SEO reasons. >> It's all free-men-and-equals round here.) >> >> Cheers >> >> Phil Hobbs > > Sorry Phil...Still in grad school, so I'm just used to saying "Dr." > if I don't know the person and I see those letters near their name. > > As for all the other replies including your own: > > The pnp C-multiplier guy was drawn wrong. Drew it last night. I know > it was as we measured both +/-10V with a DMM and with a scope to > check for oscillations. > > The whole circuit was built on piece of copper clad board. Made the > job tight. Lifting input pins of the op amps off of the copper clad > board. Used 0603 caps and resistors and 30AWG for connections. I will > tell my co-worker to add another C-multiplier for the photodiode and > to add some beefy caps on the supply pins. And to consider using > single package opamps instead of dual package.
<snip> > > As for his specs: 50nW of light, PD: PC5-6-TO5, BW = 50kHz, light > wavelength = 852nm Okay, good. As a SWAG, I'd say it's still getting in via the negative supply. As John says, the fact that inverting the output polarity (and hence the current required to drive the cable) changes the bandwidth is pretty diagnostic. At 50 kHz, a 6-foot cable (200 pF) will be pulling i_out = 2 pi * 50kHz * 200pF = 63 uA per volt of output amplitude, with a 90-degree phase lead. That volt of output is 1V/45M = 22 nA. To get 22 nA through 10 pF at 50 kHz requires a supply jiggle of v_ripple = 22nA/(2 pi * 50kHz * 10e-12) = 7 mV. That's a very believable number for a lightly loaded cap multiplier. Using a separate one for the PD, and loading it down so that it 's drawing a few milliamps ought to fix it. A 2k resistor from each supply pin to ground will stiffen the supplies a lot. Meanwhile, try measuring the output with a x10 probe! Cheers Phil Hobbs
-- 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 hobbs at electrooptical dot net