P 116 B Schmitt Triggers and Oscillators E

P 116 B – Schmitt Triggers and Oscillators E. Prebys

Review: Op amp as a comparator We can use an open-loop op amp as a threshold comparator* The problem is that in the presence of noise or parasitic feedback, this can be unstable near the threshold + *This is an inverting comparator. I could make it non-inverting, but this will make sense in a minute This can be solved by using feedback to modify the threshold when the output changes. P 116 B - Schmitt Triggers and Op Amps, E. Prebys

Schmitt Trigger* • The Schmitt trigger uses feedback to change the Initially: Simple: - Vin must rise above this value of VT to change the state of Vout, at which point + Vin must now go to a lower value to change the state of Vout back. *Sorry if you’ve already had the lab P 116 B - Schmitt Triggers and Op Amps, E. Prebys

“Hysteresis” in Schmitt Trigger curve Output depends on the “history” of the input The purpose is to improve the stability of the output. P 116 B - Schmitt Triggers and Op Amps, E. Prebys

General Schmitt Trigger Evaluate current at positive input + Relative size of R 2 and R 3 determine width of hysteresis “gap” P 116 B - Schmitt Triggers and Op Amps, E. Prebys Examples:

Oscillators • In on oscillator, there is a time-delay or “phase shift” in the signal that is fed back to the input, such that the output is not stable. • Simple example + Delay (t) • In practice, we can accomplish this delay with resistors and capacitors P 116 B - Schmitt Triggers and Op Amps, E. Prebys

Relaxation Oscillator • Combine Schmitt Trigger with RC Delay + P 116 B - Schmitt Triggers and Op Amps, E. Prebys

Frequency of Relaxation Oscillator General solution: Boundary conditions: Solve for: P 116 B - Schmitt Triggers and Op Amps, E. Prebys

Triangle Wave Oscillator V 2 will flip whenever V 1 crosses 0 + + Varies linearly, flipping sign when V 1 crosses 0 Extrema will occur at point V 1 = 0 P 116 B - Schmitt Triggers and Op Amps, E. Prebys

Frequency of Triangle Wave Generator P 116 B - Schmitt Triggers and Op Amps, E. Prebys