TIA Photodiode Bootstrap at 10MHz
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From: Artist on 2 Nov 2009 20:39 I need to bootstrap a photodiode in a TIA circuit similar to the way it is done as shown on page 18 of: http://cds.linear.com/docs/Datasheet/6244fa.pdf This example is much too limited in bandwidth. I need a 10MHz bandwidth. The bootstrapping is needed because of the low impedance of the photodiode. This is 150pF in parallel with 1 Kohm. The problem is one of designing a 10MHz unity gain amplifier with high impedance input, low noise, negligible phase change, and unity gain. Does anyone have any ideas? I am not sure it can be done. -- If you desire to respond directly remove the "sj." from the domain name part of my email address. It is a spam jammer.
From: John Larkin on 2 Nov 2009 20:55 On Mon, 02 Nov 2009 17:39:35 -0800, Artist <Artist(a)sj.speakeasy.net> wrote: >I need to bootstrap a photodiode in a TIA circuit similar to the way it >is done as shown on page 18 of: >http://cds.linear.com/docs/Datasheet/6244fa.pdf >This example is much too limited in bandwidth. I need a 10MHz bandwidth. > >The bootstrapping is needed because of the low impedance of the >photodiode. This is 150pF in parallel with 1 Kohm. The problem is one of > designing a 10MHz unity gain amplifier with high impedance input, low >noise, negligible phase change, and unity gain. > >Does anyone have any ideas? I am not sure it can be done. Do you have Phil Hobbs' book? That is Step One for issues like this. That opamp is a little noisy. John
From: Jamie on 2 Nov 2009 21:48 Artist wrote: > I need to bootstrap a photodiode in a TIA circuit similar to the way it > is done as shown on page 18 of: > http://cds.linear.com/docs/Datasheet/6244fa.pdf > This example is much too limited in bandwidth. I need a 10MHz bandwidth. > > The bootstrapping is needed because of the low impedance of the > photodiode. This is 150pF in parallel with 1 Kohm. The problem is one of > designing a 10MHz unity gain amplifier with high impedance input, low > noise, negligible phase change, and unity gain. > > Does anyone have any ideas? I am not sure it can be done. > That's funny. Brings back memories of years ago trying to make a photo receiver for a specialized light wall. It worked for what I had to do at the time how ever, the next version I made was with a cluster of 4 small body photo diodes into a 4 channel op-amp. I then summed the results. That generated a cleaner output.. P.S. I was only doing 500 khz and it was a digital stream with a little hysteresis in the circuit.
From: George Herold on 2 Nov 2009 22:16 On Nov 2, 8:39 pm, Artist <Art...(a)sj.speakeasy.net> wrote: > I need to bootstrap a photodiode in a TIA circuit similar to the way it > is done as shown on page 18 of:http://cds.linear.com/docs/Datasheet/6244fa.pdf > This example is much too limited in bandwidth. I need a 10MHz bandwidth. > > The bootstrapping is needed because of the low impedance of the > photodiode. This is 150pF in parallel with 1 Kohm. The problem is one of > designing a 10MHz unity gain amplifier with high impedance input, low > noise, negligible phase change, and unity gain. > > Does anyone have any ideas? I am not sure it can be done. > > -- > If you desire to respond directly remove the "sj." from the domain name > part of my email address. It is a spam jammer. Cool circuit, thanks for the link. (I don't quite understand bootstrapping.... something for me to work on.) The gain is due, in part, to the changing of input C from 3nF of the PD to 10pF of the JFET+opamp+stray. Bootstrapping a 150pF PD will give you less improvement. But still perhaps enough. Do you have enough photocurrent to reduce the TIA resistor from 1 Meg to 1k? I would then 'naively' expect a bandwidth improvement of sqrt(R) so a factor of 30... X 350kHz... something near 10MHz may not be out of the question. I've never built PD circuits this fast though...... George H.
From: Phil Hobbs on 2 Nov 2009 22:22
Artist wrote: > I need to bootstrap a photodiode in a TIA circuit similar to the way it > is done as shown on page 18 of: > http://cds.linear.com/docs/Datasheet/6244fa.pdf > This example is much too limited in bandwidth. I need a 10MHz bandwidth. > > The bootstrapping is needed because of the low impedance of the > photodiode. This is 150pF in parallel with 1 Kohm. The problem is one of > designing a 10MHz unity gain amplifier with high impedance input, low > noise, negligible phase change, and unity gain. > > Does anyone have any ideas? I am not sure it can be done. > One method is to connect the PD directly to the input of a nice quiet 50-ohm amplifier. If you have at least 200 uA of photocurrent, this will work very well--you can get to the shot noise limit that way. At lower photocurrents, life gets a bit harder. Your particular problem gets quite difficult below about 20 uA--at that point you have to start trading away SNR or reducing that capacitance. The best Si PIN diodes have a capacitance of 40-100 pF/cm**2 when reverse biased, so if your PD isn't at least a half inch square, you can reduce the capacitance by choosing a different PD and/or reverse biasing. So how big a photocurrent are you expecting, and what's your SNR target? Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net |