Re: Transimpedance Amplifier - Photodiode Reciever Circuit


generous.boy@xxxxxxxxx wrote:
Hi all,
Need help with component choice!
Trying to build a photodiode recieving circuit. Came up with the
circuit below after reading a few things and scavenging come circuit
designs.
please refer link for circuit schematic
http://generous.boy.googlepages.com/photodioderecievingcircuit



VCC
+
| VCC
.-----. |
| | |
| | .--------o
.-. | | |
56k| | | | |
| | | | |
'-' | .-. |
| |/ 120k| | |
|---| | | |
| |> '-' |
| | | |
| | | |
| .-. | |
110nf --- | | | | __68k
--- | | 47 | .-----)--|___|---.
| '-' 22nf | | | |
| | | | ' |
'-----| || | | |\| |
o---------||-----)--o---|-\ |
| || | | >-------------
| o------|+/
photodiode - | |/|
^ .-. |
| 120k| | |
| | | |
| '-' |
| | |
o----------------o--------'
|
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

The op-amp converts the current signal to a voltage signal.

The circuit worked fine with an AD817AN as the amplifier. But the
AD817AN is too expensive, $13. I tried just pluggin in a few other
amplifiers that were around.. None give me a clean output.

The photodiode(BP104) is reciving pulses (square wave with "on" time of
10us and varyin "off" times)
the frequency of the incident pulses can vary from 14KHz to 50KHz.
What criteria should i take into consideration while choosing the
op-amp?
I thought i would be lookin at the Gain Bandwidth Product(GBP) but dont
know if i am lookin at unity gain (becuase it is a transimpedance
amplifier) or for gain of 68k (the feedback resistor).

I skimmed through www.rsnewzeland.com, but could not find something
with the same spec's as the AD817 for a lesser cost. Do i need such a
high spec op-amp? I am running it of single supply 5v

BP104- photodiode http://www.jaycar.co.nz/products_uploaded/ZD-1947.pdf

Just tried chip TL071, it seems to work. But the output is squiggly.
Tried supplying it woth 10 v single supply.. still no luck!
Please advise of some way to rectify this squiggly output and get a
clean square wave

Thank you Jamie

But decreasing the impedance on the feedback would also mean decreasing
my gain. i want to have a fair gain.

With the 68k feedback resistor i have at the moment, i lose signal when
the transmitter is 5 cm away. I cannot have any lesser gain than i
already have.

Any other suggestions? Is this chip not suitable for this impedance?


Jamie wrote:
generous.boy@xxxxxxxxx wrote:

Just tried chip TL071, it seems to work. But the output is squiggly.
Tried supplying it woth 10 v single supply.. still no luck!
Please advise of some way to rectify this squiggly output and get a
clean square wave

it depends on the Slew/Skew timing you're looking for?

you could lower the impedance on the feed back and input!
that may lower the miller effects ect...


--
"I am never wrong, once i thought i was, but i was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Hi, GB. Got some problems here for sure.

First off, I'm not sure where you got that quote of $13 for an AD817.
DigiKey has the DIP package in stock for $3.60 USD in unit quantities.
Although they don't have the SOIC in stock, their prices on quantity
are $2.02 to $2.88 USD. I think you can do better than $13, for sure.

The AD817 is a good part -- just look at the data***:

http://www.analog.com/UploadedFiles/Data_Sheets/AD817.pdf

It's unity gain stable, and also has a high gain-bandwidth product,
slew rate, and settling time. These are all important. Your 22nF cap
is a short at HF AC and an open at DC. Your application means it needs
to be stable when there's no signal, and very fast when there is one.
Also, the AD817 is specified for single supply operation -- the TL071
isn't.

http://focus.ti.com/lit/ds/symlink/tl071.pdf

The guaranteed input voltage range of the TL071 swamps out your 5V
supply, too. TL071 -- bad choice here. I'm kind of amazed you got
much of anything out of it.

If you feel that the lower price of the AD817 is still too high, you
may be able to get away with a lower gain-bandwidth single supply op
amp at your lower frequency (10us pulses at <50KHz). But you're not
going to get good results with a gumball part like an LM358. The best
you'll be able to do is save about half the $2.02 USD cost of the AD817
-- you'll probably not be able to spend much less than a buck unless
your quantities are very high.

Since you're new to this, you should know that it's considered good
form to bottom post. Please check out Google Groups Help Topic "What's
good 'netiquette' when posting to Usenet?"

http://groups.google.com/support/bin/answer.py?answer=12348&topic=250

Good luck
Chris

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