Explain to me this solved part of exercise please


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
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Here is the second figure enter image description here

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 Rin of 766K.

Assuming µ is really 104 (not 104), I solved for the voltage at the inverting input of the opamp as a function of V1, and came up with V- = V1 × 6.6555. This means that the effective resistance between the inverting input and ground (the real value of R11) must be 6.6555 × Rid, or 665.55 kΩ. Rin is the sum of these two resistances, or 765.55 kΩ.

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