Does this (textbook) circuit diagram contain a mistake?

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Discussion Overview

The discussion revolves around a circuit diagram purportedly used to solve differential equations. Participants are examining whether the diagram contains a mistake regarding the input signal and its connection to the circuit components, particularly in the context of an inverting summer configuration.

Discussion Character

  • Homework-related
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions the absence of a resistor for the input signal -f(t) before it reaches the inverting summer, suggesting this might indicate a mistake in the diagram.
  • Another participant proposes that the input could be a current input rather than a voltage input, leaving open the possibility of a mistake.
  • A different participant asserts that the circuit must include another series resistor for the rightmost amplifier to function correctly, implying that the current configuration is flawed.
  • One participant challenges the assumption that the circuit is designed to add three signals, asking for clarification on the problem statement.
  • Another participant emphasizes the need for a resistor based on the virtual ground principle, arguing that the circuit would not make sense without it.
  • One participant introduces the concept of a transimpedance amplifier, suggesting that the input is acting as a current input and referencing Kirchhoff's Current Law (KCL) for further understanding.
  • Another participant reiterates the transimpedance amplifier concept, arguing that a "current input" is essentially a voltage input with a large source resistance, questioning the practicality of the circuit's intended operation.

Areas of Agreement / Disagreement

Participants express differing views on whether the circuit diagram contains a mistake. Some assert that a mistake exists due to the lack of a resistor, while others propose alternative interpretations regarding the nature of the input signal. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants reference principles such as the virtual ground and transimpedance amplification, indicating that the discussion involves nuanced technical reasoning. The assumptions regarding the input type and circuit functionality are not universally agreed upon.

kostoglotov
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Homework Statement


[/B]
This circuit solves some differential equation, the question is asking for the equation based on the circuit diagram

b1npQSK.png


imgur link: http://i.imgur.com/b1npQSK.png

Homework Equations

The Attempt at a Solution



I refer to the input terminal for -f(t)...the signal from it doesn't seem to pass through any resistor on its way into the inverting summer, so I've no way (I think) of finding how the signal from -f(t) is scaled in the summer.

Is this a mistake on the diagram? If not, what am I missing?
 
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Could be a current input rather than a voltage input... or a mistake.
 
Yes - I am sure, it is a mistake. The right most amplifier must add up three signals (voltages) - and for this purpose another series resistor is necessary.
 
LvW said:
The right most amplifier must add up three signals (voltages)...

Where does the problem statement say that? Did you leave something out?
 
Tom.G said:
Where does the problem statement say that? Did you leave something out?
It is the purpose of the circuit to "solve differential equations".
Hence, it needs an input. Such an input (time domain) is shown as "-f(t) directly at the inv. terminal of an opamp, which - at the same time - receives the sum of two other voltages. However, this addition works only if f(t) also is connected through another resistor (remember the virtual ground principle).
Thus, without such a resistor the whole circuit makes no sense.
 
LvW said:
...this addition works only if f(t) also is connected through another resistor...

That input is acting as a transimpedance amplifier. It is a current input. You can use KCL to help wrap your head around it.
https://en.wikipedia.org/wiki/Transimpedance_amplifier
 
Tom.G said:
That input is acting as a transimpedance amplifier. It is a current input. You can use KCL to help wrap your head around it.
https://en.wikipedia.org/wiki/Transimpedance_amplifier
A "current input" is nothing else than a voltage input with a corresponding large source resistance (see my answer).
On the other hand - do you really assume that such a circuit (analog computer circuitry for solving differential equations) is intended to work with current in and voltage out? This would make no sense at all.
 

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