Schmitt-Trigger with Simulink

Design will generate dual frequency signal, but due to asymmetric nature it will change duty cycle more and it would be hard to obtain high hi/lo frequency ratio (precise parts needed).

There are other ways.

1. Use additional capacitor switched by mosfet. It may me parallel or serial to main (parallel shown).

^{simulate this circuit – Schematic created using CircuitLab} 2. Use alternating oscillator topology.

^{simulate this circuit}

Here on schematic second 'slave' oscillator work in two modes: when 'master' is high it is two invertor oscillator (NAND is working as second invertor); when 'master' is low NAND gate is blocked and is is acting as single schmitt oscillator.

Second schematic was discovered while playing with CD4093 quad NAND schmitt. Drawback of it that its frequency ratio hardcoded by schmitt thresholds.

Possibly cost, when opamps were expensive. Transistors are cheap enough, but the costs of extra passive components and more complex assembly and testing quickly add up.

Possibly speed : you can find transistors faster than opamps (then you need careful design to achieve that potential high speed).

Definitely voltage tolerance : you can find transistors rated at several hundred volts more easily than high voltage opamps.

If none of these special cases apply, opamps will be much easier to design with, and give a more accurate and predictable design.