We have the exercise :

The figure shows an op amp connected in the noninverting configuration. The op amp has an openloop gain μ, a differential input resistance Rid, and an output resistance ro. Recall that in our analysis of op-amp circuits in Chapter 2, we neglected the effects of Rid (assumed it to be infinite) and of ro (assumed it to be zero). Here we wish to use the feedback method to analyze the circuit taking both Rid and ro into

account. Find expressions for A, β, the closed-loop gain , the input resistance Rin and the output resistance Rout. Also find numerical values, given μ = 104, Rid = 100 kΩ, ro = 1 kΩ, RL = 2 kΩ, R1 = 1 kΩ, R2 = 1 MΩ, and Rs = 10 kΩ.

Now,this exercise is already solved in the book.Here it is

Here is the second figure

**My question is** : We know that Rs=10 KOhm.Rif=777 KOhm. Now,Rin=Rif-Rs=777-10=767 KOhm.As I have underlined in the second picture,why is Rin=739 kohm?

## Best Answer

I think it must be a typo. Using a completely different approach, I came up with a value for R

_{in}of 766K.Assuming µ is really 10

^{4}(not 104), I solved for the voltage at the inverting input of the opamp as a function of V_{1}, and came up with V_{-}= V_{1}× 6.6555. This means that the effective resistance between the inverting input and ground (therealvalue of R_{11}) must be 6.6555 × R_{id}, or 665.55 kΩ. R_{in}is the sum of these two resistances, or 765.55 kΩ.