Electronic – Photodiode showing triangle waves instead of square? (re: need op amp?)

infraredoperational-amplifieroscilloscopephotodiode

I'm trying to visualize an infrared signal using a IR receiver diode but my oscilloscope is showing triangle waves instead of square for the carrier frequency:

oscilloscope picture

One of the guys at EEVblog said I need an op-amp because the signal is too fast (38 kHz) but I'm still learning electronics and haven't used any op-amps before.

Can anybody let me know what I need or point me in the right direction?

(I already have a proper 38 kHz IR receiver with demodulation, etc, but I'm trying to visualize the signal for learning purposes using a "simple" photodiode).

EDIT: Some other questions/answers also seem to indicate I need an op-amp with a sufficient slew rate?

EDIT2: Here is my schematic per request: (I'm probing across the resistor)
schematic

Best Answer

The problem is in the parasitic capacitance of the circuit you have implemented. It forms an RC timeconstant with the gain resistor which filters out high frequency signals. There are plenty of resources to help you with this problem

You will need to minimize the capacitance and possibly get a faster opamp with more gain bandwidth product (GBWP). Breadboards are terrible for implementing circuits like this, even on a PCB traces can have enough capacitance (pFs) to make a difference, so short trace sizes are best between sensor and transimpedance amplifier.