Transimpedance Amplifier - Photodiode Reciever Circuit

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:

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...

with out knowing exactly how your applying the Op-amp, i couldn't say for sure how ever, i am kind of fond of the TL082, its also just about the same with latch up free, JFET input, short circuit protect etc.. it's a dual 8 package. you should be able to cascade them..

maybe the approach your taking is reducing your response ?

the last circuit i made that required a high gain response on a pulse stream or carrier with very low signal, i used a 081. i used both +/- inputs in a common mode network to the IR led's via Input resistors. the + input at the Op-amp had a small Cap to common, this way only the high speed pulse would response and force the - input of the op-amp to swing the output to the + side.. because the gain is so high in these units and both inputs being driven via a T-network. it worked like a charm. you may want to put in a pot on one R in the network to balance the threshold of the input. i used my circuit as a comparator.

hope this gives you some idea's

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 datasheet:

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.

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

You know, I'm wondering. The photodiode doesn't seem to be that unusual -- how about just shamelessly swiping a typical application from p.2 of the LM311 data sheet: | | +5V +5V | | | | | .-. | ~ - 5K| | | ~ ^ | | | | '-' +5V | | | | | | | +5V .-. | | | | | |1K | | | LM311| | | | | | |\\ '-' | o------------)-----|-\\ | | | ___ | | >---o---- | o-|___|-o----o-----|+/| | | 1K | | |/ | | .-. --- | | | | 50K| | --- .-.1K |=== | | | .01uF| | | |GND | '-' | | | === | | === '-' GND | === GND | | GND === | GND | (created by AACircuit v1.28.6 beta 04/19/05

This should work fairly well. Your maximum dark current is supposed to be 2uA typ., and with the 50K resistor, that would set a max low level of 0.1V (probably less). The data sheet shows 34uA with 1mW/cm^2, which should show as around 1.7V (probably more). If you set the trigger value of the comparator at about 0.83V (as above), you should be just about done. Of course, the trigger level responds to the average input, so it will be somewhat less (depending on duty cycle), but certainly more than 0.6V with a 50% duty cycle. The only tweak you might add is to provide a bit of hysteresis if a little chatter at the output caused by the slowly rising/falling input signal bothers. Better sensitivity, adequate speed, gumball parts, almost rail-to-rail digital output signal, go home on time for less than a buck. What's not to like?

Cheers Chris

well, that would explain things, the BandWidth is 50..70 Mhz.. so i guess all you would need is a high gain amp with high freq. the TL071 family is only 3 Mhz. that sure would make a different in shape of the wave on the output.

Thanx Chris. I looked up DigiKey.The shipping to New Zealand is horrendously expensive. About $70. I dont think i could afford that.

Thank you for the above information, though i was using the circuit, i was not completely aware of how it was working.

i think this would suit me better cost wise. Thank you very much

GB