charge sensitive Amplifier

[sureshbagewadi]

dears all,

i am designing photo diode detector. i am using LF156 op-amp. with photo diode input circuit is working fine but WITHOUT input circuit will start oscillating at 9KHZ. detector circuit inverting input is connected to 3nF capacitor in series with 10Kohm register and feedback is 500kohm in parallel with 9.1pF capacitor. please suggest me , how to remove those oscillations.

thank you,...

 

[Dominik Przyborowski]

If you have 3nF an input You shouldn't be surprised. Connect cathode of your photodiode directly to inverting input of opamp and bias anode by negative voltage.

BTW: in CS mode amplifier should have wider GBW than 5MHz

 

[KlausST]

Hi,

inverting input is connected to 3nF capacitor in series with 10Kohm register

do you need the 3nF? Try it without this and see what happens.


do you use inverting or non inverting circuit?


Klaus

 

[sureshbagewadi]

Hello,
it is inverting , i am using 3nF capacitor to attenuate high frequencies.

 

[KlausST]

Hi,

it is inverting , i am using 3nF capacitor to attenuate high frequencies.

How to? capacitor on inverting input to GND? Don´t do this.


Show us your schematic

Klaus

 

[sureshbagewadi]

Hi,

hello,
please see attached circuit....

 

[KlausST]

Hi,

i can´t find an attachement

Klaus

 

[RajivConfident]

sureshbagewadi,upload yourcircuit diagram and size of the material used?

 

[sureshbagewadi]

hello,
please see circuit diagram....

thank you

 

[KlausST]

Hi,

now i see...

OK C3 is in series. That´s good.

Btw.: nice schematic with all the RC in the opamp´s supply. Good job.

Without input the opamp should not oscillate. The opamp should be unity gain stable.

What types for VD1 and VD2 do you use? are thy low capacitance?
Maybe a small series resistor (100R ?) between VD2 and IN- could stabilize.

Against VD1 and VD that protects the input, there is no need for VD21, VD22, VD23, VD24. But not wrong.

If it still is oscillating try a 100R at the IN+ connections.


Let´s see what happens.


Good luck

Klaus

 

[RajivConfident]

i am also intrested in your project.you have to increase the size of that capacator.We have to use the capacitor between 2nF to 10nF.By decreasing it is not possible to reduce oscilation so we must use higher.

 

[Audioguru]

With a 10k ohm input resistor and a 500k feedback capacitor then each inverting opamp has a voltage gain of 50 times and the three have a total voltage gain of 125000 times.

Quote= "detector circuit inverting input is connected to 3nF capacitor in series with 10Kohm resistor..... i am using 3nF capacitor to attenuate high frequencies."
No. The series 3nF capacitor and 10k resistor attenuate LOW frequencies. The 3nF input capacitor and the 10k input resistor produce a low frequency cutoff at 5300Hz.
The 500k parallel with 9.1pF produce a high frequency cutoff at 35kHz but stray capacitance will reduce it a little.

I suspect the circuit is made on a solderless breadboard with its connecting wires all over the place and there is capacitive coupling between the wires and between all the rows of contacts.
Then the extremely high gain causes oscillation when there is no input.

 

[KlausST]

Hi,

Audioguru is absolutely right.

that´s why i was confused in post#4
You could attenuate HF but with C to GND. And this can be done with non inverting input, but not with inverting input.

Klaus

 

[sureshbagewadi]

Hello,
yeah it is LOW frequency attenuation only, i just wrongly mentioned. input signal from photo diode is very very less, we need that much high gain.

3nF capacitor also helps to block the photo diode DC Biasing voltage.

But whenever i touch my finger to the input 3nF point then noise will disappear, HOW ???????, why???

How, what all parameters of photo diode effects on the response of amplifier?

please reply soon

thank you,

 

[KlausST]

Hi,

But whenever i touch my finger to the input 3nF point then noise will disappear, HOW ???????, why???

Mybe by touching with fingercthe first stage gets saturated. The signal then is fixed to a supply rail. So noise gain = 0.
The next stages blocknthe DC and all seems to be ok, because output is near zero....but you see less noise as with open input.

Before you spoke of oscillating. This usually is a single frequency, while noise usually is a mixture of a lot random frequencies.

Btw. With opamps you will find voltage noise and current noise.
With an open input the current noise at the output is multiplied with feedback impedance and voltage noise is amplified with 1.
With input connected current noise is still multiplied with feedback impedance, but voltage noise now is multiplied with the opamp circuit gain. In your circuit the gain is very high,. So you will see more noise when the input is connected to anything (even gnd) than when it is left floating.

Klaus