the op. amp. doesn't work as it should

[fogl]

I connected the photodiode (current source) as shown:

If i measure the voltage across + and - input of op. amp. i don't get 0V, but the voltage rises proportional to current from current source. How is that possible? Rolling Eyes The op. amp. is TLC2272 and Vcc is 5V. I get 0.35V across at 2.5mA current and 0.46V at 2.6mA.

The aim of this circuit is to measure the solar radiation, which is proportional to short circuit current of photodiode. But if the voltage at - input of op. amp. rises, this is no more short circuit current i am measuring.

 

[deepu_ttc]

Hello friend,

For an op-amp there are many parameters that effect the output out of them
INPUT OFFSET VOLTAGE is one this .Due to this voltage across two terminals will never be zero

 

[Kral]

fogl,
I appears that the OP Amp is running out of output current drive capability. A quick look at the output voltage vs output current capability confirms this. The input current that you supply must be provided by the op-amp output (through the 470 Ohm feedback resistor). When the op amp is asked to deliver more output current than it is capable of providing, one or more of the internal stages saturates, bringing the OP amp out of its linear region. This is the reason that the differential input voltage is greater than the input offset voltage specification. Try using an op amp with more output current drive capability.
Regards,
Kral

 

[fogl]

But tlc2272 is capable of 50mA...which is a lot more than in my case.

 

[Kral]

fogl,
The 50 mA rating is the absolute maximum rating. No output voltage is specified for this condition. Figure 14 of the TI data sheet gives the output voltage vs current curve. According to this curve, the 2272 shouold be able to provide about 4V at an output current of 2.5 mA. However, this curve is for a "typical' device. The sample you are using might not have this capability. If you solve this problem, please post your findings. I imagine others would be interested in what you discover.
Regards,
Kral

 

[yousafzai]

Kindly go through Robert Malvino "Electronic Principles" and u will have a huge sense of relief...

 

[venkateshr]

HI ,

I doubt over the setup or I could not have got that .The only problem is having a stiff Current source in series with the photodiode .what is the purpose of that.

If you can try this attachment it may help you.

 

[Karthikeya]

hi
I agree with Kral...its the problem of current drive capability. One way to overcome the problem is to design for a low output voltage within the op-Amp's voltage vs current curve. Then use a second op-Amp stage to improve the gain and obtain the required output voltage.

regards

 

[ipsc]

If i measure the voltage across + and - input of op. amp. i don't get 0V, but the voltage rises proportional to current from current source. How is that possible? Rolling Eyes The op. amp. is TLC2272 and Vcc is 5V. I get 0.35V across at 2.5mA current and 0.46V at 2.6mA.

The problem might be due to I/P resistance of Voltmeter<I/P resistance of opamp. So rather than measuring the voltage across + and -, try measuring each voltage individually and see the difference between the two.

Hope this helps.