In article <4onrqd$4ra@news02.deltanet.com> jlundgre@delta1.deltanet.com (John Lundgren) writes: > >The pieces of the puzzle below are missing. What transistor for Q1? An >NPN or PNP? It might be possible to tell from the circuit, if the emitter >and collector were labeled. But since they are not, then it's anyone's >guess. > >John Whitmore (whit@hipress.phys.washington.edu) wrote: >: In article <4o4qnr$m3a@news.flashnet.it> fly@rm3.flashnet.it (Stefano Mosca) writes: >: >Hi world, >: >I need a simple circuit to use a photodiode to detect an IR beam ... >: To get broadband response from a phototransistor, it is >: necessary to use it with a low load impedance: a grounded-base >: transistor amplifier is ideal. > >: +V----+--------->GND >: | >: R1 >: | >: +---------> oscilloscope input >: | >: | >: | >: \ +----C1----+ >: | | | >: Q1 +-------+----R2----+---- (-V) >: | | >: / R2 >: | | >: \ +V >: | >: Q2 + (phototransistor or photodiode) >: | >: / >: | >: -V > >: using a small-signal transistor as Q1, a 9V battery for +V/-V, >: R2 circa 100kohm, C1 circa 100 pF, R1=50 ohms > Since I specified 'grounded-base amplifier', it WAS in fact unambiguous that the Q1 had to be an NPN transistor. I'd recommend a general-purpose type, like PN2222A. Things might work better with more complex biasing (like a parallel RC in series with the Q2 collector, to ensure current limiting), but ASCII art is SO limiting... John Whitmore
Date: 3 Jun 1996 16:53:53 GMT
Original Subject: Re: Help: need simple circuit to use photodiode