Hysteresis of Relaxation Oscillators

227

03:53:17

Relaxation oscillators use two principles to work. First is hysteresis where two activation thresholds are used to differentiate the switch on input level of the op amp and the switch off level. The other principle used is integration through an RC chain. This sets a frequency reference for the op amp to use to move between the two threshold levels. The RC chain has a given time constant at which the capacitor can charge/discharge through the chosen resistor to reach the two threshold levels - this alternation between the two threshold levels sets the frequency of oscillation. The hysteresis can also be used to set a timebase for the Oscillator. The op amp finds role as an open loop gain (ideally infinite) level switcher to feed the integration chain and make the oscillation process work. 

published 7 years ago

kiani

7 years ago

I see negative resistance effect here... The red and green arrows, in the +ve terminal of opamp and the +ve f/b resistor are in oposit directions. The current is comming from the o/p of opamp in reverse. (-ve resistance effect). how to explain that if what we see is not the case.!

hurz

7 years ago

Rwlaxation oscillator needs two different threshold levels to switch at them. A hysteresis is a way to generate these levels. A positive feedback works like a negative resistance, but is not a must to have. Two voltage levels can be made on a different way, check 555 which uses a tripple 5kOhm voltage divider to make these two levels.

kiani

7 years ago

Think i get it. what yiu are saying is that in relaxation osc, .. The negative resistance effect is a side effect and not the driver, it is a passanger.

hurz

7 years ago

Its a nice effect to make to two different trigger levels

kiani

7 years ago

Without +ve feed back a schmit triger cct. Works but it won't oscillate, will it!? This negative rrsistance thing, i don't think should have been called that at all,. It is just a look alike. Not really negative resistance at all. Just a change in the slope of current. The slope can change in kick action or slowly following. ( not 100% sure yet have to check). What about a multi level references ( threshold) levels that changes randomly each cycle.? Gona try and see if can be done...

kiani

7 years ago

By the way thnx. @thebugger for cct. Beneficial. Plus Ahurz input too.

thebugger

7 years ago

No probs. You would've got there by yourself in good time, trust me

hurz

7 years ago

@thebugger you are not there, so what you are takling?

gbhmage

7 years ago

Sweeeeeet.

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