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7th October 2011, 18:49 #1
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Mathematical Formula for given circuit ?
Hi,
Can somebody give me the mathematical formula for the following circuit.
Where R1≠R2≠R3≠R4, R5 is variable. Calculate Vout for any V1 value.
Regards,
Sameer Dhiman
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7th October 2011, 19:08 #2
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Re: Mathematical Formula for given circuit ?
To make it a simple network calculation problem you have to assume an ideal OP with infinite input resistance and gain. Then Vout will be always controlled so that the input OP difference voltage becomes zero. This gives you the oportunity to apply a basic network calculation method of your choice.

8th October 2011, 10:23 #3
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Re: Mathematical Formula for given circuit ?
Hi FvM,
I simulated the circuit under LTSpice with the following values.
Ideal OpAmp:
.subckt idealopamp 1 2 3
G1 0 3 1 2 100Meg
.ends idealopamp
R1=10K, R2=270, R3=1K5, R4=560 and R5=27.5, V1=1Vpk1KHz, Vout=0.198133
Calculation Used:
NonInverting Vninv = V1×[R2/(R2+R1)]×[1+(R4/R5)], So Vninv = 0.561649
Inverting Vinv = V1×(R4/R3), So Vinv = 0.373333
Therefore Vout = Vninv + Vinv, Vout = 0.188316
What I am missing ?Regards,
Sameer Dhiman
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8th October 2011, 10:52 #4
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Re: Mathematical Formula for given circuit ?
Vninv = V1×[R2/(R2+R1)]×[1+(R4/R5)]
Your "ideal OpAmp" is an OTA, by the way. You should use a voltage controlled voltage source to model an OP. But both can work in this circuit.
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8th October 2011, 11:46 #5
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Re: Mathematical Formula for given circuit ?
Thank you so much FvM
Vninv = V1×[R2/(R2+R1)]×[1+{R4/(R5R3}]
 Post added at 16:16  Previous post was at 15:51 
May I know why we put R3 parallel to R5 ?Regards,
Sameer Dhiman
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8th October 2011, 12:09 #6
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Re: Mathematical Formula for given circuit ?
Superposition method involves that you're supplying input voltage to +ve and ve branche seperately. The other input is connected to zero input voltage. Zero input voltage means paralleling both resistors.
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8th October 2011, 13:30 #7
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Re: Mathematical Formula for given circuit ?
Thanks
Regards,
Sameer Dhiman
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