Photodiode Amplifier

Hallo ,

I have designed first stage for Photo diode amplifier (I-V stage) now i am getting mV/uV signals(11.5606 - 11.5611 V) which i want to convert to 0-5 V signal .please help

Are you saying you want to take the 500 uV range between 11.5606 and 115611 volts and provide a TTL compatible digital output?

I am getting after first Stage(I to V ) out put of 930.615 mV - 930.648 mV pulse . which i want to convert into 0- 5 V pulse.

By putting a Tansistor CE amplifier i got 11.5606 -11.5611 V pulse . i want form 0 -5 V pulse

So you want the 33 uV difference to provide 0 or 5 volts.

I suggest removing the DC component by capacitor coupling. The resulting 33 uV AC signal can then be amplified and applied to a comparator. What is the rate of the pulses to be detected?

1 MHz....Ohk Let me try that. My problem is if i amplify , whole Pulse will be shifted to 11 V . i want it to start from 0 and Max 5. with 1 MHz

If you need to detect fast change in the signal it can be AC coupled to remove the DC component.

Let's say you want to detect a change no slower than 1 ms. The filter pole of an RC filter can be set to 300 Hz. For example, a 1 uF series capacitor and a 530 ohm shunt resistor. The 33 uV signal is then amplified, with a gain of 300 and a bandwidth exceeding 1 MHz, and fed to a comparator set to trip at 10 mV.

Thermal noise is a limiting factor and the two things that contribute to added noise are the op amp input resistor of 530 ohms and the op amp itself. The thermal noise of a 530 ohm resistor is 3 nV / rt hertz. Add to this the op amp input noise (let's say it's 2 nV / rt Hz). Add these by RSS and the noise is 3.6 nV / rt Hz or about 20 nV / rt Hz peak-to-peak.

If your circuit has a noise bandwidth of 3 MHz the added noise is 35 uV P-P. Because this exceeds the 33 uV signal to be detected something needs to change. Either the input noise needs to be reduced (reduce the shunt resistor value) or reduce the bandwidth.

Note that a bipolar op amp should be used as long as the impedance driving it is low enough. If the impedance driving it is too high the op amp input current noise may dominate the system noise.

A JFET or CMOS op amp will have high 1/f noise below 1-10 kHz. and they generally exhibit higher thermal noise above the 1/f point. So, that's the reason to use a bipolar op amp that has low input noise voltage.