Hi. I have a 2 stage op amp design that I have some questions on. The 1st stage output is the output from a photodiode signal. This signal is then fed into a level shifted DC blocking capacitor to block out the background white light from the photodetector. This is is then fed into a second stage op-amp. The output from the 2 opamps are shown in blue (first) and green (second). I was wondering if it is possible, with the configuration I have, to pull the output to ground (it currently goes low to .7V) and tune the duty cycle to 50%. There are 2 more op-amps on this chip (LM324) that I can use. I would rather not add extra components such as flip flops or schmitt triggers for pulse shaping. Is there anyway I can acheive this? Thanks.

 

Hello,

A schmitt-trigger is made using positive feedback.
Take a look at this page of the eBook:
http://www.allaboutcircuits.com/vol_3/chpt_8/12.html

Bertus

 

Could you just put your output thru a diode, and then pull it down with a, eg. 10K resistor? Dropping across the diode would lose ~0.7v. Of course, it will be lopped off the high signal also.

 

Bertus-I put my ouput through a LM311 comparator. The pulse gets cleaned up quite a bit, however the duty cycle is still not 50%. But you are right I can use the on chip opamp as a schmitt trigger to clean up teh pulses also. Will this give 50% duty cycle, or close to it? Since the comparator doesn't I feel the schmitt trigger will not either. I will try though.

 

The input to the second opamp should be through a resistor because the (-) input is a dead short in the inverting opamp circuit.

The (-) input of the second inverting opamp does not need the biasing parts because it is biased by the 15k feedback resistor.

 

You can get the output of the LM324 to swing to ground by adding a pull-down resistor from the output to ground. If the output of the circuit does not have to source significant current, such as if you are only driving a scope probe, the resistor can be many thousands of ohms.

 

Thank you everyone for your input. I will implement and test.