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Testing the Bandwidth of Photodiode Transimpedance Amplifier

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emranshd

New Member
Hi!
I am working on the design of Photodiode Transimpedance Amplifier. Can anyone tell me how can i check the Bandwidth of Photodiode Transimpedance Amplifier without having any spectrum or network analyzer?
 

crutschow

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Rapidly switch on an LED to generate a light pulse for the photodiode. Measure the risetime of the transimpedance amp output. The bandwidth can be calculated from this.

Depending upon the speed of the amplifier, the LED risetime could be a factor. You need to drive the LED from a fast pulse such as from a pulse generator. What is the anticipated bandwidth of the amp?
 

emranshd

New Member
Thanks crutschow for your very simple idea. The Photodiode TIA is to be designed to have 100 MHz bandwidth.
 

crutschow

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You may have a problem then with the LED risetime, since your amp would have an anticipated risetime of 3.5ns. Not sure what a typical LED risetime is but I suspect it may be more than that. You may have to find a fast laser diode for your source, such as those used for measuring distance, or use such a range finder as your signal source.

That's a rather high bandwidth transconductance amp. Have you simulated the design with typical parasitic capacitances?
 

emranshd

New Member
You may have a problem then with the LED risetime, since your amp would have an anticipated risetime of 3.5ns. Not sure what a typical LED risetime is but I suspect it may be more than that. You may have to find a fast laser diode for your source, such as those used for measuring distance, or use such a range finder as your signal source.

That's a rather high bandwidth transconductance amp. Have you simulated the design with typical parasitic capacitances?

Can i use any CW laser diode instead of Pulsed laser diode to generate 10ns wide laser pulses with 10 KHz rep rate to check the bandwidth as i don't have any pulsed laser diode.

I haven't simulated the amplifier yet but will try soon. Any idea in this regard will be highly appreciated.
 

crutschow

Well-Known Member
Most Helpful Member
Can i use any CW laser diode instead of Pulsed laser diode to generate 10ns wide laser pulses with 10 KHz rep rate to check the bandwidth as i don't have any pulsed laser diode.

I haven't simulated the amplifier yet but will try soon. Any idea in this regard will be highly appreciated.
How would you generate the short pulses with your CW laser diode?

If you don't have any simulation tool you might try LTspice which is a free program. TI and Analog devices also offer free spice programs to simulate their devices.

Make sure you include estimated stray capacitances in your circuit simulation, particularly at the input nodes, since even a few pF can have a significant effect on a circuit's performance at 100MHz.
 

emranshd

New Member
How would you generate the short pulses with your CW laser diode?

I mean can CW laser diode be drived with Pulsed laser diode driver (generating narrow pulses with high rep rate) without demaging the CW laser diode.

Make sure you include estimated stray capacitances in your circuit simulation, particularly at the input nodes, since even a few pF can have a significant effect on a circuit's performance at 100MHz.

By Stray capacitances you mean opamp input common mode and differential capacitances and stray capacitance of feedback resistor? or anyother stray caps
 

crutschow

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I mean can CW laser diode be drived with Pulsed laser diode driver (generating narrow pulses with high rep rate) without demaging the CW laser diode.



By Stray capacitances you mean opamp input common mode and differential capacitances and stray capacitance of feedback resistor? or anyother stray caps
As long as the pulses have a fast rise-time, that should work. Edit: They don't necessarily require a high rep rate.

Stray capacitances are any parasitic circuit capacitances including from, op amp terminals, wires, resistors, circuit board traces, etc.

Incidentally, for best minimum capacitance you do not want to put the critical summing nodes of an op amp over a ground plane, so you want to remove any ground or voltage planes in that area.
 
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emranshd

New Member
Incidentally, for best minimum capacitance you do not want to put the critical summing nodes of an op amp over a ground plane, so you want to remove any ground or voltage planes in that area.

What do you mean by this. I can only have a single sided pcb. I want to put my SMT Opamp ic (OPA657) at bottom (soldering) side containing the ground plane and circuit traces also and all others components at the top side. Does this scheme correct?
 
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crutschow

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What do you mean by this. I can only have a single sided pcb. I want to put my SMT Opamp ic (OPA657) at bottom (soldering) side containing the ground plane and circuit traces also and all others components at the top side. Does this scheme correct?
That should be OK. Just keep the ground plane away from any of the summing junction node traces and components.
 
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