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non inverting amplifier

PRIYADHARSHINI

Feb 6, 2014
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Hi all,

this is the basic concept..i have designed the circuit to amplify the 0.002v to 5 v..
according to the formula for non inverting amplifier the Rf and Rin values are 1.5M ohms
and 1K..to produce 5v as output..
i have simulated this circuit in Ltspice.
but i get the waveforms like this.what is the reason for this?
can u suggest me where i did the mistake?
thanks in advance
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Several thinks to consider now:

1) What is the gain of the circuit?
2) Are there any limitations on the output of the circuit?
 

PRIYADHARSHINI

Feb 6, 2014
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thanks for ur reply
no limitations..just i want to design the non inverting amplifier with the gain of 2.5 with 2mV input
but i did nt get in simulation.
i want to know the reason..that s it
thanks in advance
 

Harald Kapp

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How do you compute the gain for a non-inverting opamp configuration? Show us your calculation.
Certainly with the values you use you will not arrive at a gain of 2.5 but 1501.
 
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PRIYADHARSHINI

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How do you compute the gain for a non-inverting opamp configuration? Show us your calculation.
Certainly with the values you use you will not arrive at a gain of 2.5 but 1600.
sorry Harald kapp
it produces the correct output only
Vout=0.002*gain=0.002*(1+1.5*10^3)=3V.
thanks for ur replies
 

(*steve*)

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And here was me thinking you wanted a 5V output and a non-inverting amplifier and perhaps a gain of 2.5 (or some combination thereof). Sorry I must have mis-read the original question.
 

PRIYADHARSHINI

Feb 6, 2014
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actually i need the 5v for 0.002v input
so i have to keep the gain as 2500
here i get the output in ltspice
 

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Harald Kapp

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You seem to work by trial and error. Which equations do you use? The standard equations for a non-inverting opamp will not give you 2.49Meg/1k for a gain of 2500.
 

Harald Kapp

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Right, but not what I meant.
What I'd like to know how you calculate gain from the resistors or how you need to calculate the resistors. Provided that gain is given as 2500 and assuming R1=1k, which value for R2 will set the gain at 2500?
 

PRIYADHARSHINI

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i have calculate the gain like this..(1+2.5meg/1k)
is this not a right method?i don t know..if its wrong kindly explain me the right way to choose the resistors.
 

Harald Kapp

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i have calculate the gain like this..(1+2.5meg/1k)
This is correct, so why do you use 2.49MΩ instead of [Edit] 2.499MΩ in your simulation? No wonder gain is lower than expected.

Note that in a real circuit 2.5MΩ is unreasonably large for many (if not most) applications. Also a gain of 2500 will reduce the useable bandwidth of the opamp noticeably. In a real circuit it may be advantageous to split the gain using two separate gain stages (opamps) with a smaller gain for each gain such that gain1*gain2=gain_total=2500.
 
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PRIYADHARSHINI

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This is correct, so why do you use 2.49MΩ instead of [Edit] 2.499MΩ in your simulation? No wonder gain is lower than expected.

Note that in a real circuit 2.5MΩ is unreasonably large for many (if not most) applications. Also a gain of 2500 will reduce the useable bandwidth of the opamp noticeably. In a real circuit it may be advantageous to split the gain using two separate gain stages (opamps) with a smaller gain for each gain such that gain1*gain2=gain_total=2500.
thanks for ur explanation
this means
may split the gain of 2 stages like this
(i.e)gain1=50
gain2=50
so two stages 50*50=2500
is this right?
 

PRIYADHARSHINI

Feb 6, 2014
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here have modified my circuit..please check this
and i know that GBWP=Av * f
this is theoretical explanation
practically how the gain affect the bandwidth of an opamp?
i have made the google search..let me know that there is any link to see this concept in CRO Display
 

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Harald Kapp

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