That is correct. The article talks about the output impedance in band.

please note, that Ao has a complete frequency dependent form that is usually estimated as a 2 or 3 pole System (the opamp itself is a 2 or 3 pole System). This means, Ao(w)=(Ao*w1*w2*w3)/(s+w1)(s+w2)(s+w3). All this stuff means that as frequency increases, Ao greatly decreases and eventually the output impedance of the opamp becomes very high. All the calculations in the above post only applies to DC voltages. If you build a regulator circuit with an opamp for example to work on 50kHz, you might want to have a look at the output impedance at 50kHz and check what the voltage drop is inside the opamp that causes an error on the input of the next stage.

The output resistance does not depend on the input source, being it DC or sinusoidal. It is a fixed value. This article shows a method to calculate the output resistance, when feedback is present. Because Rout does not depend on the input source, the input is connected to ground for this calculation.

Actually, the equations are correct. Figure 3 shows that if * R1 + vd = 0 (with the current sense I chose for if)

Therefore if = -vd / R1.

The same Figure 3 shows that Vout = if (R1 +R2).

Equation (11) shows Rout which is correct. Ro/(R1 + R2) cannot be negative, as you say, because that would make Rout negative. A negative resistor means that it has energy (has a power source) which would not make sense in this case.

Uhmmm…No.

if=Vd/R1 (Passive sign convention, IMHO, that’s an error in the calculation…)

Vout = -if (R1+R2) , again, passive sign convention.

Then, Vout = -(Vd/R1) * (R1+R2), that’s anyway correct.

The fact that if = Vd/R1, without the minus sign, lead to an error in the final Rout formula where the part (R0/R1+R2) has to have a minus sign.

Look at Figure 3. The feedback current if flows through R2 and R1. The voltage drop on R1 is equal with vd, but negative, due to the Kirchhoff law on the loop made by the non-inverting input, the inverting input, R1 and ground. So we can write the feedback current as being

if = -vd / R1.

Also, on the loop made by vout, R2 and R1 we can write the equation

vout = if * (R1 + R2).

And since if = -vd / R1, the equation can be written as

vout = -vd/R1 * (R1 + R2).