# i have a small problem with amplifier !

Discussion in 'Homework Help' started by tallo, Mar 10, 2011.

1. ### tallo Thread Starter New Member

Mar 4, 2011
12
0
hi engineers ..

i have a small problem with the following question

how can i determine the input impedance for the two types of amplifiers

thank you

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
290
What do you know about inverting terminal impedance vs non-inverting terminal impedance?

3. ### tallo Thread Starter New Member

Mar 4, 2011
12
0
Zin(I) = Ri

Zout(I) Almost equal to 0

:::::::::

Zin(NI = (1+Aol B ) Zin

Zout(NI) = Zout/(1+Aol B)

where B + Ri/(Ri+Rf)

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
659
Are the op amps ideal?

Mar 4, 2011
12
0
yes they are

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
659
With ideal op amps, your equations will give you the answers. That is, if you know the values of Ri and Aol for an ideal op amp.

7. ### tallo Thread Starter New Member

Mar 4, 2011
12
0
But i don't have the values of Aol, Zin & Zout
So what can I do in this case ?

8. ### tallo Thread Starter New Member

Mar 4, 2011
12
0
suppose that the op amps in practical model

what will be the solution ?

9. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
659
You would have to look up Ri and Aol in the datasheet.

Mar 4, 2011
12
0
I'm waiting

11. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
659
What are you waiting for?

12. ### tallo Thread Starter New Member

Mar 4, 2011
12
0
i am very confusing

if these amps are ideal , what will be the solution

and, if they are in practical model

what the solution will be ?

13. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
659

Many op amps don't specify input resistance, but they all specify voltage gain.
This is homework. We don't do homework for you. Work on it until you get stuck, show us your results and how you got them, and we will try to help.

14. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,418
903
You cannot calculate the gain of the inverting opamp unless you know the input impedance of the source which is in series with the input resistor.
Can you assume it is zero ohms?

15. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
659
OMG! He's confused enough already. You've just opened a whole new can of worms.

16. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,418
903
An inverting opamp is a can of worms because its gain is determined by the input resistor value in series with the source impedance.

Make a non-inverting opamp circuit with a very high input impedance instead.