TIA Photodiode Bootstrap at 10MHz
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From: Artist on 3 Nov 2009 16:36 John Larkin wrote: > 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 > I don't. A book I do have is by Jerald Graeme, "Photodiode Amplifiers, Opamp Solutions" http://books.google.com/books?id=sHV0c5hBW4QC&dq=Jerald+G+Graeme&printsec=frontcover&source=an&hl=en&ei=_pzwStuuMYPYsgOh5LX0BQ&sa=X&oi=book_result&ct=result&resnum=4&ved=0CBMQ6AEwAw#v=onepage&q=&f=false There is a topology for a boot strap amplifier in it. He begins his treatment of them in Chapter 4.3 page 71. He does not give any component values or part numbers for the topology (fig 4.12 page 81). It would take a lot of research for me to figure out if there are any transistors and resistors that can make this topology go up to 10MHz. Is this the book you are referring to? http://www.amazon.com/gp/pdp/profile/AKFB26K3TYMSS I will get back all of you on the expected photodiode current. -- To reply directly remove the sj. from my email address. This is a spam jammer.
From: John Larkin on 3 Nov 2009 19:19 On Tue, 03 Nov 2009 13:36:26 -0800, Artist <artist(a)sj.speakeasy.net> wrote: >John Larkin wrote: >> 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 >> >I don't. A book I do have is by Jerald Graeme, "Photodiode Amplifiers, >Opamp Solutions" >http://books.google.com/books?id=sHV0c5hBW4QC&dq=Jerald+G+Graeme&printsec=frontcover&source=an&hl=en&ei=_pzwStuuMYPYsgOh5LX0BQ&sa=X&oi=book_result&ct=result&resnum=4&ved=0CBMQ6AEwAw#v=onepage&q=&f=false > >There is a topology for a boot strap amplifier in it. He begins his >treatment of them in Chapter 4.3 page 71. He does not give any component >values or part numbers for the topology (fig 4.12 page 81). It would >take a lot of research for me to figure out if there are any transistors >and resistors that can make this topology go up to 10MHz. > >Is this the book you are referring to? >http://www.amazon.com/gp/pdp/profile/AKFB26K3TYMSS > >I will get back all of you on the expected photodiode current. This one: http://www.amazon.com/Building-Electro-Optical-Systems-Making-Applied/dp/0470402296/ref=sr_1_1?ie=UTF8&s=books&qid=1257293909&sr=1-1 John
From: Artist on 3 Nov 2009 20:13 Phil Hobbs wrote: > 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 > The peak current is expected to be 1 uA. The latest value for the capacitance I have is now 30pF. I do not have a choice on photodiodes. The detector I have been assigned to make work for this project is not actually a photodiode in the conventional sense. It is a custom made photoelectromotive force detector for use in a laser ultrasonics application. This device cannot be reverse biased like a PIN diode. A major concern about the low series resistance is that it will create a high gain noninverting amplifier with the feedback resistor for the equivalent input noise on the inverting input. This gain will also reduce the bandwidth of the opamp circuit. The zero the capacitance will make is another reason I am looking to bootstrap this. -- To reply directly remove the sj. from my email address. This is a spam jammer.
From: Phil Hobbs on 3 Nov 2009 20:32 Artist wrote: > Phil Hobbs wrote: >> 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 >> > The peak current is expected to be 1 uA. If there's a way to make that 10 uA, your life will be much easier. > > The latest value for the capacitance I have is now 30pF. > > I do not have a choice on photodiodes. The detector I have been assigned > to make work for this project is not actually a photodiode in the > conventional sense. It is a custom made photoelectromotive force > detector for use in a laser ultrasonics application. This device cannot > be reverse biased like a PIN diode. > > A major concern about the low series resistance is that it will create a > high gain noninverting amplifier with the feedback resistor for the > equivalent input noise on the inverting input. This gain will also > reduce the bandwidth of the opamp circuit. > > The zero the capacitance will make is another reason I am looking to > bootstrap this. > Bootstraps have the same noise multiplication problem as TIAs, for the same reason: they put their own noise voltage across the PD capacitance. With equivalent devices, you can get a 3 dB improvement by using both, but bootstrapping is not a slam dunk. One good thing about it is that you can AC-couple the bootstrap, which means it can be single-ended rather than differential. You can get the same 3 dB improvement by putting a TIA on each end of the PD. If it's a photoacoustic measurement, you may not need DC-10 MHz. What's the actual measurement bandwidth? 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
From: George Herold on 3 Nov 2009 21:43
On Nov 3, 8:32 pm, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > Artist wrote: > > Phil Hobbs wrote: > >> 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 > > > The peak current is expected to be 1 uA. > > If there's a way to make that 10 uA, your life will be much easier. > > > > > > > > > The latest value for the capacitance I have is now 30pF. > > > I do not have a choice on photodiodes. The detector I have been assigned > > to make work for this project is not actually a photodiode in the > > conventional sense. It is a custom made photoelectromotive force > > detector for use in a laser ultrasonics application. This device cannot > > be reverse biased like a PIN diode. > > > A major concern about the low series resistance is that it will create a > > high gain noninverting amplifier with the feedback resistor for the > > equivalent input noise on the inverting input. This gain will also > > reduce the bandwidth of the opamp circuit. > > > The zero the capacitance will make is another reason I am looking to > > bootstrap this. > > Bootstraps have the same noise multiplication problem as TIAs, for the > same reason: they put their own noise voltage across the PD capacitance. > With equivalent devices, you can get a 3 dB improvement by using both, > but bootstrapping is not a slam dunk. One good thing about it is that > you can AC-couple the bootstrap, which means it can be single-ended > rather than differential. > > You can get the same 3 dB improvement by putting a TIA on each end of > the PD. > > If it's a photoacoustic measurement, you may not need DC-10 MHz. What's > the actual measurement bandwidth? > > 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 nethttp://electrooptical.net- Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text - "> You can get the same 3 dB improvement by putting a TIA on each end of > the PD." Hey, I remember that idea! No ones ever tried it though, have they? Say, will your cascode circuit work for photo detectors other than photodiodes? I have this silly idea that electrons from photodiodes are born at half a volt or so, and are thus able to do a bit of work before they are detected. George H. |