# Homework Help: Does this (textbook) circuit diagram contain a mistake?

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1. Feb 26, 2016

### kostoglotov

1. The problem statement, all variables and given/known data

This circuit solves some differential equation, the question is asking for the equation based on the circuit diagram

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

2. Relevant equations

3. 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?

2. Feb 26, 2016

### Tom.G

Could be a current input rather than a voltage input... or a mistake.

3. Feb 26, 2016

### LvW

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.

4. Feb 26, 2016

### Tom.G

Where does the problem statement say that? Did you leave something out?

5. Feb 27, 2016

### LvW

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.

6. Feb 27, 2016

### Tom.G

7. Feb 28, 2016

### LvW

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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