From Newsgroup: sci.electronics.design

Hi. In Roberge's Op Amp book (2007 2nd ed) there is this practice
problem:

Assume that the operational-amplifier connection shown in Fig. 1.10
satisfies the two conditions stated in Section 1.2.2. Use these
conditions to determine the output resistance of the connection (i.e.,
the resistance seen by the load).

With the indicated conditions being:

1. A negligibly small differential voltage applied between the two input terminals of the amplifier is sufficient to produce any desired output voltage.
2. The current required at either amplifier terminal is negligibly
small.

Here is a reproduction of the diagram, basically just a non-inverting amplifier:

```
ro+\
ro!roCroCroCroCroCron+\
ro| roe \
Vi roe /ro|roCroCro<
ro+ ro!roCron-/ ro!roCro|roCroCro<
ro| roe ro|/ roeLOADroe
GND roe ro#roCro4roCroCro>
ro#roCroCroCroCroCroCroCroCron
ro|
R
ro+
ro|
GND
```

Is this a more complicated problem than it appears to be on the surface?
The output resistance is just the resistor R, right? Or do I have to
prove that?
--
Christopher Howard
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From Newsgroup: sci.electronics.design

On Fri, 15 Aug 2025 12:07:37 -0800, Christopher Howard <christopher@librehacker.com> wrote:

Hi. In Roberge's Op Amp book (2007 2nd ed) there is this practice
problem:

Assume that the operational-amplifier connection shown in Fig. 1.10
satisfies the two conditions stated in Section 1.2.2. Use these
conditions to determine the output resistance of the connection (i.e.,
the resistance seen by the load).

With the indicated conditions being:

1. A negligibly small differential voltage applied between the two input
terminals of the amplifier is sufficient to produce any desired output
voltage.
2. The current required at either amplifier terminal is negligibly
small.

Here is a reproduction of the diagram, basically just a non-inverting >amplifier:

```
?\
???????+\
? ? \
Vi ? /????
? ???-/ ??????
? ? ?/ ?LOAD?
GND ? ??????
??????????
?
R
?
?
GND
```

Is this a more complicated problem than it appears to be on the surface?
The output resistance is just the resistor R, right? Or do I have to
prove that?

What is the current through the load in terms of Vin and R?
If you change the load by a factor of say x10, how much does the
current through it change?
What does that tell you about the impedance "seen" by the load?
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