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High frequency photodiode transimpedance amplifier design

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ConnerP:
Greetings, I'm a physicist way in over my head with some circuit design and could use some help. We want to retrofit a high speed photodiode amplifier into an existing atomic force microscope. The present amplifier has a bandwidth of 900 kHz and but we are interesting in signals up to 10 MHz. I have already taken a stab at it but could really use some help.

Constraints:
Available power: +-15V
Signal fed to 10V ADC
Diode sensitivity: 0.36A/W
Diode capacitance: 45pF (0V reverse bias), 9pF (5V reverse bias)
Laser: <=1mW @ 690nm

My attempted solution uses the LT1226, the fastest amplifier with +-15V rails I could find. It is a simple low gain transimpedance amplifier for the first stage. The gain has been set low so that the compensation doesn't wreck the bandwidth. The second stage is pure gain to boost the expected max signal (roughly) up to 10V, no compensation needed since the 80R shields it from seeing any capacitance (I think).

My main concern is, will this thing be stable? So far as I can tell, yes, but maybe I missed something. But as I mentioned, I am a total noob in this area, so I am looking forward to hearing any and all feedback y'all have to offer. If you have tips on how to potentially go faster, I'm all ears.


tggzzz:
There is useful information in The Art of Electronics 3, plus in Phil Hobbs' book Building Electro-optical Systems: Making it All Work.

moffy:
Download LTSpice from the Analog Devices website. They have models built in for the LT1226 and you can test stability. Got a feeling the first stage will oscillate because the LT1226 is only stable for gains of 25 or higher.
The THS4031 or something similar might be a better choice for the first stage. It is low noise and unity gain stable.
Also look at the LTC6268 data sheet. It has a 100MHz BW transimpedance amplifier with a 20k feed back resistor. It is only +/- 2.5v supplies but might be worth a couple of voltage regulators to get the performance.

David Hess:

--- Quote from: moffy on June 12, 2020, 12:39:25 am ---Got a feeling the first stage will oscillate because the LT1226 is only stable for gains of 25 or higher.
--- End quote ---

If Cin/Cf >= 25 then it will still be stable.  Cf is required to compensate for the effects of the input shunt capacitance which would otherwise lower stability even with a gain of 1 amplifier.

Marco:
Can't you just use the LTC6560? A completely integrated solution sidesteps some problems with high bandwidth opamps.

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