LMV794 Amplifier Design....

cool_buddy

Other Parts Discussed in Thread: LMV794

I m using LMV794 as amplifier circuit for photodiode BPW34S.In first stage i put 5.6M feedback 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 for feedback? I need rectangular pulse... pl sumone help how to calculate the values of both feedback resistor and capacitor!!!!! The frequency used is 4Khz

LMV794_amplifier.pdf

over 10 years ago

0 Samir Cherian over 10 years ago

TI__Genius 12631 points

Hello,

As a 1st step I would isolate the 1st transimpedance stage and make sure that it is working as expected. Once the two stages are isolated you can then focus on stabilizing the transimpedance stage(TIA). Couple of things I would do to simplify my analysis even further is to remove the input capacitor to the TIA stage. If you havent already done so you can download TINA Spice from the TI website to run some simulations to fine tune the circuit. Also, what is the BW you need for your application? You will also need to know the parasitic capacitance from your phot-diode in order to model your TIA stage properly. Please go through links attached below for information on how to compensate TIAs properly.

Regarding the 2nd stage: you have the input to R10 (VREF) coming from the resistive divider. You really want to buffer this voltage before providing it to subsequent stages so that you have a low impedance output going into R10. Also, for the 2nd stage you are ac coupling from the previous stage and there is no real way of knowing the correct dc bias point for amplifier U1-B. You are better off using the 2nd stage as an inverting amplifier stage. My recommendation for the 2nd stage is as follows:

1. Put VREF to the non-inverting input (pin 5) of U1-B. You dont need to buffer VREF here since you are going into a very high impedance input.

2. AC couple the output of U1-A through C8 and R10 into the inverting input (pin 6) of U1-B.

Please see attached Application Note on compensating TIAs:

http://www.ti.com/lit/an/sboa122/sboa122.pdf

Also, please see attached post on how to simulate a Transimpedance Amplifier for stabiltiy:

http://e2e.ti.com/support/amplifiers/high_speed_amplifiers/f/10/p/333642/1164455.aspx#1164455

Regards,

Samir

0 cool_buddy over 10 years ago in reply to Samir Cherian

Prodigy 115 points

Thank you Samir....I regret for replyin late.I ve done the modifications you told.Sorry that i didnt attach the latest circuit.Now the problem is in the Second Stage the voltage is around 3.5v.I just need around 3.So inorder to do so I should change both feedback resistor and capacitor.Could you plz guide me how to proceed and wat formula should i use???? Herewith I attached latest circuit....

amplifier_latest.pdf

0 Samir Cherian over 10 years ago in reply to cool_buddy

TI__Genius 12631 points

Hello,

Do you mean the output voltage is 3.5V even with no input to the photodetector? If so there is something wrong in the circuit, because the ouput voltage with no input to the PD should be 33/133*3.3 = 0.82V. Have you simulated the circuit in TINA? If not I would advice you to start there.

Samir

0 cool_buddy over 10 years ago in reply to Samir Cherian

Prodigy 115 points

No sir, With input to photodetector,the output voltage from the circuit is 3.5v.How can i simulate in Tina Ti? What component sud i use in the place of photodiode to simulate? How you are tellin 0.8v ll come without input to photodiode? My doubt is the 3.5v comin out from circuit must be reduced to 3v.What changes sud be made? Really a very big thanks for your Quick reply sir......