So this is an interesting read this morning. I may refer back and forth to this for some investigations, thank you.

Would you recommend me increasing the 2M resistance or the changing the 10k resistance to increase the gain of my opamp and also, do you think that 00.8v on the AC measuring shows an oscillating output?

I haven't been following this thread too closely. When you say AC measuring output, do you mean to say that you're using an analog-to-digital converter (ADC) that you then read into your microcontroller?

 

No, this is with a multimeter connected monitoring AC Voltage on the output pin. However if i do connect it to a MCU with a ADC, it shows the same voltage as the AC monitoring multimeter BUT fluctuates from 0.8v to 0.22v sometimes spikes up too 1v too.

 

Assuming there is oscillation from my output/non-inverting input terminal on the OP AMP I have been doing some research on this matter.

It turns out that there are two ways to reduce/remove this effect;
1. Lower the feedback resistor values to reduce the load a little on the OP AMP.
OR
2. Add a feedback capacitor from output to the non-inverting input of the op amp.

It would seem that you can't take the feedback resistors too low due to making the load on a OP AMP makes it even more unstable apparently, so i would prefer to go the feedback capacitor route. However, this is also something that i'm unable to calculate the needed capacitance due to not having a oscilloscope to visually see the oscillation/noise gain on the terminals.

Do i need to be exact with the capacitor value?

 

Try anything from, say, 100pF up to 1uF (non-polarised). It's not an exact science. You may or may not notice any improvement.

 

I will experiment, I need an improvement, my ADC can't monitor it smoothly.

@Alec_t - Is this how you would go about remove oscillation on the output then?

 

It's one way. Applying negative feedback via a capacitor has the effect of reducing the amplifier gain for high frequencies, thus reducing the likelihood of oscillation.

 

Its now sitting with a bias of 0.1-0.2v (There may be still some oscillation, i will experiment more with more capacitance). The feedback resistance is 2M and then 5k7 connected to gnd, with the cap there, it may need a little more gain on the op amp but i will keep experimenting with that too. On the plus side, we're getting closer to the desired configuration!

I would like to thank everyone for their help up to this point.

 

Hello,

You gain will be about 350 times.
When using high gains in one stage instability may occur.
When you break it up in two stages with each a gain of about 20, you will have a gain of 400.

Bertus

 

Hello,

You gain will be about 350 times.
When using high gains in one stage instability may occur.
When you break it up in two stages with each a gain of about 20, you will have a gain of 400.

Bertus

I'm sorry, i'm not too sure what you mean by one stage and two stage. Do you mean using two op amps in a configuration of having a low amplified first op amp and then another op amp amplifying the first output?

 

Hello,

I do mean with two stages, two opamps with each a gain of about 20.

Bertus

 

I never gave this a thought if i'm honest with you. So with two op amps (in a double package) with a gain of 20 on each it would equal around the 350 mark overall. Interesting concept for this application, i will give that a look at on the breadboard and report back with my findings,

Thank you @bertus

 

Hello,

I do mean with two stages, two opamps with each a gain of about 20.

Bertus

Could you explain how to connect these two op amps to make this circuit please?

I have attempted to but i have very basic knowledge on op amps currently, It is one area i'm concentrating on at the moment. I have a feeling the two have to have the same non-inverting input voltages, so split the connected to the light transistor and have it feeding both op amps maybe?

Below is what i have and obviously it wont and is not working correctly.

 

Hello,

You have created two amplifiers with a gain of about 178 each.
The total gain will be about 178 * 178 = 31684 times, wich might be to much for your application.
Raise the value of the 5K6 to about 47K to have a total gain of about 440 times.

Bertus

 

I'm sorry, i didn't correct the schematic, those 5k6 resistors are infact 47k resistors. Silly mistake on my behalf on the schematic notation.

 

Hello,

The circuit is a pure DC amplifier.
With a gain of 440 and a supply of 5 Volts, the amplifier will hit the 5 Volts at about 11 mV input voltage.
Is that what you want to have?

Bertus

 

I think it is indeed what i want to have. It would appear that it is oscillating pretty bad right now. Reading 8V AC volts on the output of op amp 2. Would it be acceptable to put a cap on the output and the non-inverting input on both of the op amps, or would this make it worse? Where would you apply capacitors to stop the oscillation?

If i had a scope or a logic analyser i would be probing around to see where the oscillation is originating, but i don't.

 

Hello,

What is the output voltage when you are shorting the input of the first opamp (disconnect the photo transistor)?

Bertus

 

3.84v with the two inputs of the first op amp shorted together. Measuring AC its giving me 8v still. is this actually oscillation that is happening do you think?

 

Hello,

Sorry, I meant to put pin 3 in your schematic to ground.
That way we can see if the opamp has an offset.

Bertus

 

0.2V on the output of op amp 2 with pin 3 shorted to ground and 0.01V on pin 1 (Output of op amp 1)