Impossible Op-Amp Circuit....Maybe

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Discussion Overview

The discussion revolves around the feasibility of a specific op-amp circuit, particularly focusing on its transfer function and the implications of its design. Participants explore the theoretical and practical aspects of the circuit, including its behavior in simulation software (pSpice) and its alignment with known circuit models.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the possibility of finding the transfer function of the far right op-amp in the circuit.
  • Another participant notes the lack of low-frequency feedback, suggesting that the outputs may be pegged at the supply rail.
  • Concerns are raised about floating nodes in the circuit, with suggestions to add resistors to ground to aid simulation convergence.
  • A participant mentions that the circuit resembles a known GIC circuit and discusses its applications in active filters.
  • There is a debate about whether the components in the schematic can be considered ideal op-amps or ideal circuit elements, with implications for bias current and output impedance.
  • Some participants express skepticism about the circuit's design and its theoretical viability, particularly regarding the behavior of components connected to the op-amp inputs.
  • One participant provides a symbolic expression for the transfer function, indicating finite gain values for the op-amps.
  • Questions arise about the implications of the voltage across certain components being zero, with discussions on ideal versus finite gain scenarios.
  • Several participants emphasize the necessity of negative DC feedback for the circuit to function properly.

Areas of Agreement / Disagreement

Participants express a range of views on the circuit's validity, with some agreeing on the need for DC feedback while others question the circuit's overall design and purpose. The discussion remains unresolved regarding the circuit's theoretical soundness and practical implementation.

Contextual Notes

Limitations include the absence of clear definitions for circuit components, the potential impact of ideal versus finite gain assumptions, and unresolved questions about the circuit's intended function.

  • #31
jim hardy said:
What do you guys think ?

Well, for REAL opamps I don`t think that a transfer function can be given because there will be no operational point within the linear region of the opamps.
Therefore, the node "NODE" will never be at zero volts (this would be valid for working negative DC feedback for both opamps only).
Instead, I think that both opamps will go into saturation.
 
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  • #32
LvW said:
Well, for REAL opamps I don`t think that a transfer function can be given because there will be no operational point within the linear region of the opamps.
Therefore, the node "NODE" will never be at zero volts

Whenever there's capacitors that can hold charge one must specify among his initial conditions their state of charge. Remember "Constant of Integration" from first semester calculus ?

So, start the thought experiment with initial condition of zero volts everywhere including isolated NODE , and ideal opamps.
Any change in voltage at Ralph's +input will propagate through Ralph, through C2 to Node
which will cause Louie to immediately reach around and undo that change .
So Ralph is held at zero output by Louie, the feedback element. Gain of circuit is zero.

It's a little easier to see if you remove R2, all he does is halve Vt.

Ralph is a unity gain voltage follower , transfer function of 1
Louie is feedback with gain and transfer function of ∞
closed loop transfer function G/(1+GH) is 1/(1+1X∞) = 0

I'd call the circuit a "Frustrated Voltage Follower ".
@Bullington ---- See what Professor thinks ?

but i agree it wouldn't work with real parts.

old jim
 
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