Maker Pro
Maker Pro

Simple ideal op-amp

kimbokasteniv

Jan 26, 2012
6
Joined
Jan 26, 2012
Messages
6
So I have the circuit below:
412Circuit.png


And I have the equation which I believe describes the circuit, as well as the same equation solved for Vo:
412Equation.png


But, if you try the numbers in my solved equation you will find that they do not agree with the values multisim has calculated. I guess that means my original equation is wrong or I messed up my algebra somewhere. However, I have been looking at this for over an hour and have been unable to see anything peculiar.

Any suggestions on where I messed up?

Oh, and Vcc on the op amp is 15V.
 

GreenGiant

Feb 9, 2012
842
Joined
Feb 9, 2012
Messages
842
First off ideal implies that there is no Vcc for the op-amp, it will give you an exact amplified output based solely on gain, no rail to rail or anything like that.

Second, you may want to take a look at the voltage supplied to Probe 2

Third, if this is a Differential amp you appear to be missing a resistor on the positive side of the amplifier.

Where is the 404 coming from in the second formula?
 

kimbokasteniv

Jan 26, 2012
6
Joined
Jan 26, 2012
Messages
6
Hey GreenGiant, thanks for the response.

My book lists a Vcc votlage for the op-amp, and it also does suggest that it is in an ideal op-amp circuit. So I guess they are stating the Vcc just so I know what would be past saturation.
The voltage at probe 2 is equal to the voltage at Vn, which according to ideal op amp rules is equal to Vp, and Vp is equal to 100mV.
In my top equation I substituted Vn with 100mV. So, I believe that is all correct, was there something else you were getting at?

As for the 404:
steps.png
 
Last edited:

GreenGiant

Feb 9, 2012
842
Joined
Feb 9, 2012
Messages
842
Ok, I was mentioning the Vcc because in Multisim an Ideal Op-Amp has no Vcc, it is limitless. it is referring to perfect gain.

I get a slightly different result than 404, you may want to check your math

Even so, I am not sure where you are getting these formulas, can you take a picture of exactly what your book shows for this problem?
 

kimbokasteniv

Jan 26, 2012
6
Joined
Jan 26, 2012
Messages
6
Shoot, the coefficient of 0.1 should be 101. Those numbers work. That solves my problem, thanks GreenGiant.

For the virtual/ideal op-amp in multisim, there is the swing voltage parameter, which I assume reflects a Vcc voltage.

Otherwise, I got those formulas by doing KCL at probe 2.

I can scan in the question if you are still curious though--I'm just away from home right now. Let me know.
 

GreenGiant

Feb 9, 2012
842
Joined
Feb 9, 2012
Messages
842
Otherwise, I got those formulas by doing KCL at probe 2.

I get where you are coming from now, I wasn't sure where you were at in the stages of learning about electronics, being such a simple circuit I thought you were very early on, but working with KCL and all that about circuit analysis it all makes sense now
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Joined
Jan 21, 2010
Messages
25,505
1/2k + 1/200k is (2k + 200k)(2k*200k), not (2 + 200)/(2k*200k)

You've dropped a 1000's multiplier. And the value you get is 1/1980, not 404

I can get 404 if I type it into my web browser though. :)
 
Top