Voltage follower troubles

[ryouden]

voltage follower

I am building the above circuit at the moment. The basic operation is this: The two analogue voltages induced across the photodiodes are compared by IC17A whilst IC7A passes the highest value of the two photodiodes onto the next stage of the circuitry. Originally when I built this circuitry I did not have the voltage followers and filters on the input to the analogue multiplexer (IC7). The purpose of this filter is to stop noise when the two input voltages of IC7 are very similar, this caused high frequency switching between the output states and made the system very noisy.

Anyway my problem at the moment is with the voltage followers. The photodiodes generate a voltage of around 180mV with no light and 400mV fully illuminated. The output stages of the voltage followers is around 700mV. This doesn't make any sense to me. I am using LM339s for the comparators and voltage followers.

Does anyone have any suggestions.

Thanks in advance.

 

[LvW]

comparator as voltage follower

Hi ryouden,

what was the reason for you to use the LM339 as unity gain amplifiers ?
This IC is not a classical opamp - it is rather a comparator !

You should use one of the various unity gain stable opamps available from each manufacturer.

 

[dfullmer]

lm339

It looks like your trying to use a comparator as an op amp? Is this right? You will need an op amp for the voltage follower. The 700 mv looks like the saturation value of the comparator.

dfullmer

 

[FvM]

30 amp voltage follower

There are more problems with your circuit, apart from the fact that LM339 will never operate as a stable amplifier:
- LM339 has an open collector output, it doesn't work without a load respectively pull-up resistor
- A photodiode is basically a current source that is usually operated with zero voltage bias, or sometimes with negative bias. You can use it open circuit, as you do, but then the voltage variation is rather small (60 mV for a 10:1 light intensity change). Unless you want to utilize this logarithmic transfer function to achieve a very high dynamic range, you should better use a different circuit.