LMV794 amplifier design...

[cool_buddy]

I m using LMV794 as amplifier circuit for photodiode BPW34S.In first stage i put 5.6M resistor and 10Pf.The supply voltage is 3.3v.The first stage output is aroud 1.5-1.7v.In second stage one input is 0.9v dc and other is the output from first stage.I used 6.8k resistor to obtain 2v finally.But my waveform is like spikes.i dunno wat capacitor value sud be put? I need rectangular pulse... pl sumone help how to calculate both feedback resistor and capacitor!!!! the frequency used is 4khz...

 

[D.A.(Tony)Stewart]

Why no negative bias on PD with C6 blocking DC?
Why no DC bias current for U1-B pin 8 (+) with high R to Vref.

What is 3kHz that you need? What Bandwidth for low and high frequency filters?

What are the spikes from that you dont want? Freq? PW50 amplitude after 1st stage?

Keep in mind wide spectral sensor will amplify CFL and fluorescent light as pulses.

No photo's? of layout and scope readings?

 

[cool_buddy]

4khz is the signal from the emitter.The sensor should sense nly signal in tis range.Actually the output from the first stage is in range of 1.4-1.7v rectangular pulse.i already ve first stage.I ve to design 2nd stsge so that output will be 2v rectangular pulse.Instead of rectangular pulse i m gettin some spike lik waveform. somethin similar to noise.

 

[Audioguru]

The photo diode is missing a bias voltage across it because its anode should have a resistor to ground.

The (+) input of the second opamp is missing the reference DC voltage. Connect the (+) input to Vref with a 100k resistor. It is a non-inverting amplifier so R11 should be replaced by a piece of wire.

 

[schmitt trigger]

"Keep in mind wide spectral sensor will amplify CFL and fluorescent light as pulses."

As incandescent bulbs become extinct and replaced mostly by CFLs, this is becoming a VERY SIGNIFICANT problem for IR communications. Your second stage, besides providing additional amplification, should also provide low pass filtering.

Also...what AG mentions is true. You require to provide a DC bias path to the photodiode and the second stage.