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Electronics => Beginners => Topic started by: The13thParish on September 08, 2020, 12:38:37 am

Title: Frequency-Dependent Transimpedance Amplifier Behaviour
Post by: The13thParish on September 08, 2020, 12:38:37 am
Dear all,

I am building a transimpedance amplifier for the purpose of converting a photocurrent from a silicon photodiode into a voltage which will then be fed into a microcontroller to actuate some further circuitry.

My circuit is based on the very basic schematic attached from The Art of Electronics as a starting point. I am using an LF411 op amp and a wide-area silicon photodiode (5 mm x 5 mm) which I found in a box and for which I do not have a data sheet. The feedback resistor is 1 Megaohm and I am not currently using a capacitor in parallel with Rf to dampen oscillations caused by photodiode parasitic capacitance.

I can't seem to amplify a DC signal but I can amplify AC signals; that is to say, what I want is to take a constant light source in the visible region which gives rise to a DC current in the circuit, which then I can convert to a DC voltage. I could use an optical chopper to make the light an alternating source however in principle op amps should be able to convert and amplify a DC signal, so I want to improve my understanding.

For troubleshooting purposes, it's easier by eye to find a 100 KHz AC-modulated signal on a scope rather than a DC offset from a background (i.e. room lights), so this is where I started. If I modulate the output of a white light LED using a function generator (sine used) as the LED voltage source, my circuit works at converting sinusoidal photocurrent to a sinusoidal voltage at the same frequency as the light. If I try using a constant light source I notice zero voltage conversion, as seen by zero difference from the oscilloscope baseline. Any tips?

All the best,
Ryan
Title: Re: Frequency-Dependent Transimpedance Amplifier Behaviour
Post by: Jay_Diddy_B on September 08, 2020, 01:29:07 am
Hi,

Can you show the supply connections to the LF411 opamp?

What power supply rails are you using?

Jay_Diddy_B
Title: Re: Frequency-Dependent Transimpedance Amplifier Behaviour
Post by: The13thParish on September 08, 2020, 01:31:13 am
@Jay_Diddy_B,

I am using +/- 15V directly connected to the op-amp power pins VCC+ and VCC- respectively. I am not biasing the photodiode.

All the best,
Ryan
Title: Re: Frequency-Dependent Transimpedance Amplifier Behaviour
Post by: magic on September 08, 2020, 05:30:52 am
Are you sure LED brightness is the same in both tests? What frequency did you test? Start turning it down and see what happens. Maybe you just hit a gain peaking sweet spot.
Try 10MΩ perhaps. Do you know how large is the photovoltaic current supposed to be?