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Old 8th October 2011, 09:02 AM   #1
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Default Transfer Funtion of the given circuit ?

Hi,

Please give me the transfer function of the following circuit. Where R1,R2,R3,R4 and R5 are not equal.
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Old 8th October 2011, 12:00 PM   #2
DF96 is offline DF96  England
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How many marks will your teacher give me for the answer?
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Old 8th October 2011, 03:42 PM   #3
Ron E is offline Ron E  United States
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Quote:
Originally Posted by sameerdhiman View Post
Hi,

Please give me the transfer function of the following circuit. Where R1,R2,R3,R4 and R5 are not equal.
You should check out a neat little program called sapwin
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Old 8th October 2011, 04:20 PM   #4
SY is offline SY  United States
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Just remember that the voltages at inverting and noninverting inputs are equal, that the current flowing into the inverting and noninverting inputs is zero, and it will be pretty easy to set up the basic equations to solve for Vo/Vi.
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Old 8th October 2011, 08:57 PM   #5
jcx is online now jcx  United States
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it actually "safer" to explicitly include op amp gain "A" and derive the equations

you can always take the limit as A goes to infinity later - but by assuming it from the start you may save a term or two in the calculations or you may have your solution "blow up" from the infinity you are implicitly carrying around

the op amp Howland current source is a favorite nasty problem set for those not understanding the "null injection" implicit infinity
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Old 8th October 2011, 09:24 PM   #6
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What method do you think is best for solving this problem, and how far along did you get?

Dale
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Old 10th October 2011, 03:48 PM   #7
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@DF96
I am not a student in any institute so do not expect any Marks.

@SY, jcx
Thank you, I solved it using superposition theory.

R1 and R2 forms voltage divider for Non-Inv voltage. R3||RD forms Rg and R4 forms Rf. So Non-Inv, Inv output voltage will be

Vninv = V1*[R2/(R1+R2)]*[1+{R4/(R3||RD)}]
Vinv = V1*-(R4/R3)

Vout = Vninv + Vinv
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Old 10th October 2011, 05:22 PM   #8
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Would you not want Vout in terms of V1 ?
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Old 10th October 2011, 05:49 PM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by sameerdhiman
I am not a student in any institute so do not expect any Marks.
Good. Sorry for jumping to conclusions, but your question looked like homework.

I'm not certain that your result is correct. I would expect to see two simultaneous equations involving Vin and Vout, which you then invert to get Vout in terms of Vin. What is RD?
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Old 10th October 2011, 07:17 PM   #10
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Sorry, R5=RD (In practical circuit R5 will be variable resistor e.g. JFET or LDR)

I've verified the equation with LTSpice.

If you any other equation/solution please post that.
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