11/29/2022 0 Comments Led feed back osculator![]() ![]() The absence of a frequency maximum and the higer maximum frequency compared to the normal ring oscillator without reach-through capacitor (orange) suggest that the LED capacitance does indeed act in a similar way as the parallel capacitor. The larger swing of the logic levels also leads to a reduction of propagation delay and hence oscillator frequency. The frequency is continuously rising with the voltage, no maximum is visible within the voltage range.Īn increased operating voltage is expected since the additional voltage drop over the LED increases the turn on voltage of each inverter.Oscillations are only observed at a much higher operating voltage of ~2.2V instead of 1V for the others devices. ![]() Results for a voltage sweep of the PMBT2369 based ring-oscillator are shown above in green, in comparison with previous results. I built two versions: One with PMBT2369 and one with PMBT3904. Please ignore the resistor values, I used 470 Ohm collector resistors in all cases. What started as a quick experiment turned into quite a curious feat, as you will notice later. Since LEDs have a non-negligible internal capacitance, I was also hoping that the LED would act in a similar way as the reach-through capacitors in parallel to the base resistor, a s I tried earlier. There was a bit more rationale behind this, though: The LED will increase switching voltage and should improve noise margin, similar to what I did with " LTL". Why would I do that? Well, first of all the footprint of the LED and resistor exactly matched on the PCB. Out of curiosity I tried replacing the base resistor in my 5-stage ring oscillator with a red LED. ![]()
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