Bandpassfilter amplifer

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JamesGon

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Hi.
I am designing a Bandpass transimpedance amplifier. But I unfortunately I have trouble understanding some basic stuf. The circuit I am planning to use is shown below. It is a reversed biased photodiodes.

I know that the lower and upper frequencies in a active bandpassfilter is found by the following equations:
flow = 1/2piR2C2 (highpass) and fhigh = 1/2piR1C1 (lowpass). I want my flow to 600kHz and fhigh to at 700kHz, so a 100kHz bandwidth. But I am not getting close to this, when doing a AC-sweep. Can someone tell me, what I am doing wrong.

 

Presumed the photo diodes can be assumed as current source with sufficient low capacitance (<< 35 pF), the circuit does have the said high-pass/low-pass characteristic. But you shouldn't expect significant selectivity from the first order filter.

Better specify the intended filter characteristic with pass- and stop-band parameters and determine the required filter topology and order.
 

Most opamps do not have much gain at such high frequencies.
The first-order filters are so simple that their corners and slopes are very gradual and affect each other.
 

Hi,

I agree.

Maybe you should look for a higher Q (resonance) circuit.

Klaus
 

HI
Okay, I am confused. Can you guys elaborate that with an example or proper literature?
By the way, what is the gain? Usually in a tranimpedance amplifer you have A/V gain. Is the gain here decided by R1 and R2.
And How do you do a proper simulation – that simulates the current pulses generated by the photodiode?

Best regards
 

Hi,

read about passive or active bandpass filters with Q>1
Did you try: https://en.wikipedia.org/wiki/Band-pass_filter

Gain:
the gain in V/A of a transimpedance amplifier is determined by the feedback resistor R1,
For AC: the feedback impedance: R1 || C1.

But in your case you have additional circuitry: R2, C2 .. for sure they influence the gain, too.

For sure you can all do this with just one OPAMP, but maybe it´s easier and more obvious if you divide it in a true TIA and an extra OPAMP for the bandpass.

It depends if you do this just for fun or for a research, or if you design a mass product.

Klaus
 

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