A Faster LM3900 Schmitt Trigger for a Triangle Wave Generator VCO

Engineering Notes for Faster Schmitt Trigger

Recently, after building a current-switching VCO based on the AN-72 topology, the Schmitt Trigger element was discovered to be rather sensitive to touch and AC hum pickup in the pad-per-hold breadboard implementation, and it had speed limitations that limited frequency range. 

It's sort of obvious, but then as demonstrated with a LM3900 Frequency Doubler, use of higher input currents for applications where the LM3900 must switch is more favorable. Linear operation with the LM3900 tends to work well with input currents ≤ 100 µA, but for switching operation input currents ≥ 100 µA are more favorable.

This test circuit was a retooling of the VCO Schmitt Trigger for a higher input current range, and measurement of the Vbe at the inputs in order to understand how to set higher precision of the switching voltages, or trip-points.

The circuit tested is shown in the engineering notes page above. Some representative scope fotos explaining results are follow the engineering notes. 

Basic I/IO with 100 Hz test stimulus designed to elicit 50% duty cycle given the Schmitt Trigger switching points

1 KHz Operation

5 KHz Operation, where 0.5V/µS Tplh vs 20V/µS Tphl with the LM3900 is more visible.

Noting the very unusual switching operation for the Schmitt Trigger at the -Input of the LM3900. The Vbe goes nearly to 0V, but then swings up to a bit more 600mV.

However, now overlaying the +input of the Schmitt Trigger, which changes very little -- It hovers between approximately 627mV and 637 mV!