On Saturday, September 23, 2023 at 11:02:03 AM UTC+5:30, Jan Panteltje wrote:
> On a sunny day (Fri, 22 Sep 2023 05:00:49 -0700 (PDT)) it happened amal
> banerjee <daku...@gmail.com> wrote in
> <5dafb884-2b86-4c19...@googlegroups.com>:
> >> What is the photo diode reverse current with light on say a 12V supply?
> >The 'heavy' version that you have is somewhat congested. If you are saying =
> >TR3(the transistor whose base
> >is connected to the photodiode anode) be a Darlington pair, I have tried th> >at already in the SPICE simulation,
> >although without the 100k (R1). The simulation results were not good - both> > the output current through and
> >voltage across the 50 Ohm load were worse compared to the single transistor> > case. I will definitely try
> >out it out again with the 100k(R1). I did think about the emitter follower > >at the TR3 output, but after
> >I examined the single TR3 output(across the 50 Ohm) I did not pursue it. Th=
> >e last question
> >about the reverse bias current for photodiode I am not sure, because so far> > these are all based on
> >SPICE simulations. The candidate photodiode I have in mind is BPW31|34(the =
> >Google query
> >for BPW31 datasheet returns the one for BPW34). The datasheet for BPW34 sta> >tes that the open
> >circuit voltage is 440 mV -i.e., the reverse biased diode case.
> I am looking at the BWP34 datasheet fig 3 reverse light current vesus irradiance
> it is as much as 100 uA for 2 mW / cm^2
>
> For a beta of even as low as 100 that gives 10 mA Ic in the first transistor
> With say a 220 Ohm resistor that makes 2.2V
> Add an emitter follower or any other buffer (PNP reverse is better) to get 50 Ohm driving capability,
> and it should work.
> Less light less output, higher collector resistor more output, higher beta more output.
> The collector resistor of the first stage should not be too high if you want 100 MHz (effects of Cce Ccb).
> You will need to specify light source , pulse width, etc
> The only spices I use are pepper, chili and salt.
>
> Mr Hobbs who post here has written a book IIRC about driving an opamp into the invering input
> using feedback?
> Whatever you do, you need a low impedance termination for your photo-diode in reverse mode
> as the impedance is high and any capacitance will limit speed.
>
> You can also type
> BWP34 circuit diagram
> in google, then select 'pictures' from top screen,
> and plenty of examples and I even see the circuit I described :-)
>
> Often faster than waiting here for replies.
Which Web site did you find the circuit diagram for the BWP34 ? I got my datasheet from "www.alldatasheets.com"
I have used the op-amp based transimpedance amplifier(with resistive feedback) some years ago for a low
frequency(3.5 MHz) automotive application. A low value capacitor(some pF), parallel to the feedback resistor, curbs
the effects of the capacitance of the reversed biased photodiode.

On Sat, 23 Sep 2023 00:17:58 -0700 (PDT), amal banerjee
<dakupoto@gmail.com> wrote:

>On Friday, September 22, 2023 at 6:19:12?PM UTC+5:30, Phil Hobbs wrote:
>> amal banerjee <daku...@gmail.com> wrote:
>> > On Friday, September 22, 2023 at 4:01:34?PM UTC+5:30, Phil Hobbs wrote:
>> >> amal banerjee <daku...@gmail.com> wrote:
>> >>> On Thursday, September 21, 2023 at 10:54:43/AM UTC+5:30, Jan Panteltje wrote:
>> >>>> On a sunny day (Wed, 20 Sep 2023 21:28:49 -0700 (PDT)) it happened amal
>> >>>> banerjee <daku...@gmail.com> wrote in
>> >>>> <21592d6c-93a9-4852...@googlegroups.com>:
>> >>>>
>> >>>>> On Wednesday, September 20, 2023 at 8:51:50/PM UTC+5:30, John Larki> >n wrote:
>> >>>>>> On Wed, 20 Sep 2023 08:08:08 -0700 (PDT), amal banerjee
>> >>>>>> <daku...@gmail.com> wrote:
>> >>>>>>
>> >>>>>>> Could some electronics guru please help ? I am looking for a very low(fe=
>> >>>>> w milliVolts) VTO commercially available NMOS, and its corresponding PMOS.=
>> >>>>>
>> >>>>>>> I am trying to use a reverse biased photodiode(very low output current) =
>> >>>>> to
>> >>>>>>> trigger a PMOS. The output voltage could then be used to trigger ideally=
>> >>>>> a
>> >>>>>>> BJT. Any hints, suggestiosn would be greatly appreciated. Thanks in adva> >nce.
>> >>>>>> What is VTO?
>> >>>>>>
>> >>>>>> I don't think such a part exists. A photodiode might just barely turn
>> >>>>>> on a high-beta bipolar transisor.
>> >>>>>>
>> >>>>>> What's the open-circuit voltage of the photodiode? What's the current?
>> >>>>>> Is a power supply available? If the pd is back-biased by a supply,
>> >>>>>> you'd have lots of voltage available to turn on a mosfet.
>> >>>>>>
>> >>>>>> A lithium battery might power a micropower comparator for decades.
>> >>>>>
>> >>>>> Sorry for the confusion. 'VTO' means threshold voltage for the mosfet.
>> >>>>> The candidate photodiode is BPW31|34. The datasheet contains the following> >
>> >>>>> data:
>> >>>>> Open circuit voltage : 440 mV
>> >>>>> In reverse bias, the diode is open circuit, so current should flow. The da=
>> >>>>> rk current
>> >>>>> is 12 pA.
>> >>>>
>> >>>> Think 'current'
>> >>>> the reverse photo-diode could drive the base of for exampe a NPN directly
>> >>>> photo diode current amplified by beta * R1 is output.
>> >>>> Note leakage etc..
>> >>>>
>> >>>> ------------------------ +
>> >>>> | |
>> >>>> --- R1 a few k
>> >>>> / \ |
>> >>>> --- |------------ out, negative going on light input
>> >>>> | c
>> >>>> ---- b NPN beta 200
>> >>>> e
>> >>>> |
>> >>>> ----------------- GND
>> >>> I have tested your suggestion with SPICE simulations using an ordinary
>> >>> BJT(BC547) and two RF BJTs
>> >>> (BFR92A, BFQ790) each biased(as per Vce, Ic values listed in their
>> >>> respective datasheets) for beta
>> >>> values(150-200). I am using a GaAs photodiode SPICE model, which uses a
>> >>> third input node for an
>> >>> input voltage for the incident light. The light is pulsed, @ 1.5 MHz,
>> >>> 15.0 MHz and 150.0 MHz with pulse
>> >>> amplitudes in the low milliVolt(1-2) range. Of the three transistors
>> >>> BFQ790 performs best, but like each
>> >>> of the other two, the output voltage at the output 50.0 Ohm resistor is
>> >>> in the tenths of milliVolt range.
>> >>>
>> >> If you don’t know how much light you’ve got, you’re probably doomed. That
>> >> needs to be in optical terms, like “1.2 ns full width at half maximum, 1-2
>> >> picojoules, 950 nm”.
>> >>
>> >> It also matters whether the light is spatially coherent, because that
>> >> governs how small a photodiode you can use.
>> >>
>> >> If you tell us what you’re actually trying to do, we can be a lot more
>> >> helpful. Generating a sync signal from a laser beam is much easier than
>> >> detecting scattered light from a matte-textured object at a distance, but
>> >> there are tricks to make both easier.
>> >>
>> >> Something like, “I need to detect laser pulses bouncing off a microscope
>> >> sample so that I can do lock-in measurements of the photoacoustic response”
>> >> or “I’m going to bounce laser pulses off my cat” would help a lot.
>> >>
>> >> Cheers
>> >>
>> >> Phil Hobbs
>> >>
>> >> Cheers
>> >>
>> >> Phil Hobbs
>> >>
>> >> For instance
>> >> Find that out, first
>> >>
>> >> --
>> >> Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
>> >> Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
>> > So far all my results are based on SPICE simulations. The candidate
>> > photodiode is BPW34 whose datasheet
>> > states that it is for high speed light detection for visible and near
>> > infra red with half angle of sensitivity +/-65
>> > degree. Reverse light current is 75 mA(typical).
>> > I am experimenting with line of sight light detection at high frequency.
>> > No I would not want to bounce light
>> > off my kitty, esepecially now that it has a kitten to take care of.
>> >
>> >
>> That’s way too vague for me to be able to help much, except to say that the
>> difficulty ranges from trivial to impossible, depending on the details.
>>
>> The BPW34 isn’t great for bandwidths over 50 MHz or so, but with a better
>> detector and a decent front end it isn’t hard to reproduce nanosecond
>> pulses. Even op amps can have gain bandwidths in the gigahertz.
>>
>> Cheers
>>
>> Phil Hobbs
>> --
>> Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
>> Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
>I know that the BPW34 is inappropriate for very high frequencies, but since I have
>not worked with optoelectronic circuits for a while, I am trying to get my "feet wet"
>before plunging into the real stuff. Yes, I have seen documentation on GHz op-amps,
>but have not used them before.

Phil's book, Designing Electro-Optical Systems, has a lot of good
stuff on the subject.