Output resistance of equivalent op amp circuit

[Ratch]

This seems like a big "if" to me. Getting the transfer function is probably the most difficult part of the problem.

Unless the circuit is very simple, I think is is just as easy to find the transfer function (TF) as it is to find the I/O impedances, After all, you have to bring the output load RL into the equation for the input impedance and the source impedance Rs into the equation for the output impedance. Why do things twice? The transfer equation has to be done only once, and is just as easy to find the TF with a computer equation solver as finding the I/O impedances equation separately. Furthermore, it automatically takes care of dependent sources in a routine systematic manner using node or mesh analysis.

If a homework problem wants various gain functions as well as input/output immitances, then the transfer function will have to be calculated to get those gain functions anyway. Using the GIT is then a small additional effort.

Yes, that is where it really shines.

But if all that is wanted is the input/output immitance, the effort of obtaining the transfer function should be considered part of using the GIT; using the GIT is then no longer a small additional effort.

For pencil and paper and a simple circuit, you are probably right. But for a circuit with many components and using a computer, I don't think so.

Won't one have to put up with the angst of dependent sources in obtaining the transfer function, which we have to do before we can use the GIT?

Yes, but it is easier to implement in a mesh or node computation. The implementation setup is routine, and the computer does all the grit work.

Do you have Dr. Stockman's "Theorem Book": http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6323775 (reference 1).

I am familiar with that document. It is one of the few resources that discuss that method.

Ratch

 

[Ratch]

George2525,

It pains me to say this, but it appears that the 3rd edition of Sedra and Smith, which I own, has a better treatment of the voltage-shunt feedback than your 6th edition. It appears that they have dumbed down that section. I have seen that happen in other texts as well, where authors delete important material in their newer editions. I guess it is to make room for other material. The 3rd edition has two resistors called Ricm (resistor, input common mode) situated on the V-+ and V- terminals, whose analysis is missing in your edition. If you can find the older edition, you might want to peruse it.

Ratch

 

[george2525]

George2525,

It pains me to say this, but it appears that the 3rd edition of Sedra and Smith, which I own, has a better treatment of the voltage-shunt feedback than your 6th edition. It appears that they have dumbed down that section.

Ratch

yes I have the 3rd edition!

I noticed that too