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Question 8 An opamp is ideal except that it has a finite openloop gain
. The opamp is
connected in an inverting configuration using two resistor
and . (a) Show that the closedloop gain
is ( ) (b) Determine ⁄ such that the closedloop gain is
the openloop gain is
? (1 5 points) . (b) What is the closedloop gain if Solution
Part (a)
Consider the inverting amplifier shown. KCL at the inverting input gives (
⁄ Note that with finite gain,
so that
or , but since , . Thus, the KCL equation becomes
⁄ ⁄ (
⁄ Solving for the closedloop gain )( yields the desired expre ssion: (
Part (b) Set
Part (c) Using and
⁄ ) and solve to find
and yeilds 9 )
⁄ 55:041 Electronic Circuits. The University of Iowa. Fall 2011.
Question 9
In the noninverting opamp amplifier shown, the opamp is ideal except
for a finite openloop gain
and differentialmode input resistance
(a) Draw an equivalent model that uses a voltage controlled voltage
⁄ . (b)
source and use the model to derive an expression for
Use the model to derive an expression for the input resistance
(c)
Calculate and for and
(1 5 points) This problem is poorlyposed and will not be graded 10 55:041 Electronic Circuits. The University of Iowa. Fall 2011.
Question 1 0 The circuit shown is a representation of the
commonmode and differentialinput signals to a difference
amplifier. One can write the output voltage as
, where
is the differentialmode gain and
is
the commonmode gain. Assuming an ideal opamp, one can
show that the commonmode gain is
( ) ( ⁄ Further, if
, and
one can show that the differentialmode gain is  
circuit above,
, and
. If the tolerance of each resistor is
determine the minimum CMRR (in dB). ⁄ . For this
, Hint: consider using tools such as Excel or Matlab to search for the resistor combination that result in
minimum CMMR.
(1 0 points)
Solution

This will have minimum value when 
spreadsheet the largest
is when  is the largest 
(
(
(
( 
 is the smallest. Using an Excell (
(
(
( Then
(
(
Further...
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This note was uploaded on 10/26/2013 for the course ECE 55:041 taught by Professor Kruger during the Fall '11 term at University of Iowa.
 Fall '11
 Kruger
 Amplifier

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