Calculating V in an Op-Amp Circuit with Constant Input: Schmitt Trigger Question

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Discussion Overview

The discussion revolves around calculating the input voltage (V) in an op-amp circuit configured as a Schmitt trigger, given a constant output voltage of +2.4 V. Participants explore various approaches to derive V based on the circuit parameters, including resistor values and the op-amp's gain characteristics.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents an initial calculation for the reference voltage (V+) and expresses confusion about the relationship between V and V+.
  • Another participant suggests assuming a finite gain (A) for the op-amp to derive an expression for the output voltage based on input voltage.
  • A participant proposes a formula for V based on the resistor values and output voltage, but questions the negative sign in the equation.
  • Multiple participants discuss the implications of positive feedback in the circuit and how it affects the relationship between V+ and V-.
  • One participant calculates a gain and attempts to derive Vin, expressing uncertainty about the results and the implications of their assumptions.
  • Another participant emphasizes the importance of analyzing the circuit symbolically without assigning numerical values to the gain initially.
  • Some participants highlight the distinction between the op-amp's open-loop gain and the closed-loop gain determined by the circuit configuration.

Areas of Agreement / Disagreement

Participants express differing views on the assumptions regarding the gain of the op-amp and the implications of positive feedback. There is no consensus on the correct approach to calculate V, and multiple competing views remain regarding the analysis of the circuit.

Contextual Notes

Participants note that the circuit's behavior may not be straightforward due to the presence of positive feedback, and there are unresolved questions about the assumptions made regarding gain and the relationships between voltages in the circuit.

Who May Find This Useful

This discussion may be useful for students studying operational amplifiers, particularly in the context of feedback circuits and Schmitt triggers, as well as those seeking to understand the complexities of circuit analysis involving op-amps.

  • #31
jim hardy said:
was i off track much ?

hope i didnt hurt you on this one.

old jim

Absolutely not, my stress levels have been sitting nice and high but you have been great in terms of helping me understand the subject more. I find my study material never supplies anywhere near enough information so places like these forums are invaluable!

Many thanks!

Just to confirm though, there is no equation for an accurate input voltage of a schmitt trigger?
 
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  • #32
StripesUK said:
Just to confirm though, there is no equation for an accurate input voltage of a schmitt trigger?
Input voltage that'll cause what condition?

It is easy to calculate the voltage at which the schmitt will switch output states.
It is not physically possible to apply an input voltage that will hold its output in between those two states, in its linear region , as i interpreted your original post.
you can calculate such a voltage but it is not practical to measure let alone control to the required precision
because changing the applied voltage in the last digit by just one
upload_2016-2-29_16-50-23-png.96630.png

will start the positive feedback loop on its merry way to saturation.

I'm not dodging your question just trying to answer it precisely.
 
  • #33
Understood. Thanks for all your help and patience!
 
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  • #34
StripesUK said:
Understood. Thanks for all your help and patience!

Feedback behavior does not come intuitively to some people, myself for instance. I struggled for years.

Pardon the un-academic next paragraph
i learn a lot just watching everyday stuff around me, like burgers on the charcoal grill.
Ever heard the expression "The fat's in the fire now! " ? It means something calamitous is imminent.
Burgers- The fat drips down and burns hot, flames erupt causing more out-juicing which feeds more fat into the flames and you wind up with charcoal-burgers.
That's positive feedback.
Positive feedback usually results in a departure from linear behavior and often destructive failure like that galloping bridge in Tacoma... or a runaway fire in your barbecue grill...
It makes your schmitt trigger circuit nonlinear.
Sometime buy a completely lean round or sirloin steak and have it ground to hamburgers and grill them. You'll see immediately the effect of removing positive feedback. If you add water to those ultra-lean burgers you'll see that negative feedback calms a system..

In early days of radio we had "Regenerative receivers" that employed positive feedback to increase gain of the RF stages. They operated on the verge of unstable oscillation , that's the "whistling" you hear in old movies when somebody is tuning a radio.

Little stupid experiments like that help you work systems in your head, which for a plodder like me is necessary to understand the math.

Work that schmitt in your head. Also conventional negative feedback 'operational amplifier circuits' which lend themselves beautifully to math..

old jim

old jim
 
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