I'm trying to drive an LED with a square wave / pulses and look at the output with a photodiode / transimpedance amp detector. The starting frequency is about 1 KHz but I will eventually go up to about 100 Khz. The problem I'm running into is non-square LED output waveforms being sensed by the detector. In the scope capture, channel 1 is the driving waveform, channel 4 is the signal at the LED and channel 3 is the detector output. My first thought was to try a different LED driver circuit, specifically a shunt driver configuration. I read online that adding some RC components could speed it up (increase the slew rate). I found on another site something similar albeit a digital driver that uses dual inverters as drivers. Could someone explain to me what these RC networks are doing (specifically) and how to determine the appropriate values for them? Thanks in advance.
The capacitor in the RC circuit effectively bypasses the resistor for a short time to provide a "gulp" of current for the device. The setup is reminiscent of a boot strap. Lets take the faster shunt driver as an example. To keep D3 off Q3 is on and diverting current away from D3. This sets up a voltage drop across R7 which is stored in C2. When Q3 turns off the point of C2 which was sitting at ground gets lifted to 5V causing the charge stored in it to push into D3. This push makes available a short burst of current helping to turn on D3 faster.