need help op-amp calculations...

[jaymee]

I have been given a circuit on LT spice, and need to calculate the CMRR. I know that the CMRR = Differential Gain/Common mode Gain, but I dont know how to get these from the circuit.

There is a 12v supply rail, 10v input, which splits with a 1k resistor on each branch of the input. It also has a 10k Rf, and 10k Rl.

Is it right that Vout = 182uV? This is my weakest subject, I need help!!

I cant attach the diagram,

 

[y.gh]

please show your circuit.
and redraw your circuit.

 

[LvW]

I have been given a circuit on LT spice, and need to calculate the CMRR. I know that the CMRR = Differential Gain/Common mode Gain, but I dont know how to get these from the circuit.
There is a 12v supply rail, 10v input, which splits with a 1k resistor on each branch of the input. It also has a 10k Rf, and 10k Rl.
Is it right that Vout = 182uV? This is my weakest subject, I need help!!
I cant attach the diagram,

Hi jaymee!

May I give you a general advice?
You have a small problem - and you ask for help. But you must enable us to give you this help. Without any drawing you use abbreviations/symbols like Rf, Rl, Vout.
Do you really expext that we know what your circuit looks like?

 

[jaymee]

thanks for relying

i tried to attach the diagram, but its in LTspice and it won't upload.

how can I draw it here?

 

[LvW]

thanks for relying
i tried to attach the diagram, but its in LTspice and it won't upload.
how can I draw it here?

If it is not too complicated - kindly describe the circuit in words, but without using symbols. Be explicit ! (For example: I suppose it is a differential amplifier. Why don't you give us this information?)

 

[jaymee]

right i'll try my best to describe what i have;

10Volt supply to 2 x 1k resistors in parrallel
one of those resistors (R2) goes to the + input of an opamp (LT1001), there is also a tee'd 10k resistor (R3) to ground from the same branch.

the other 1k resistor (R1) goes to the - input of the opamp.

the op amp has a 12Volt supply rail. (external supply)

the output of the opamp runs to another 10k resistor (R4) which i understand to be a feedback resistor. This connects to the output side of (R1).

I hope this makes sense! I used the 1k and 10k resistors as I was supposed to have a differential gain of 10. Im supposed to run the simulator, and messure the output voltage, I only got 182uV, which I dont think is correct due to it being so low?

Ive then got to calculate the CMRR, but im not sure what to use to do this. the only info I have on the CMRR is that:
CMRR = Differential Gain/Common mode Gain


I hope all this makes sense, Ive tried my best to explain the curcuit, but my lack of knowlege makes it pretty hard,

thanks
Jamie

 

[LvW]

No, sorry. That's no clear discription.

10Volt supply to 2 x 1k resistors in parrallel
one of those resistors (R2) goes to the + input of an opamp (LT1001), there is also a tee'd 10k resistor (R3) to ground from the same branch.
???

the op amp has a 12Volt supply rail. (external supply)

Single supply?

the output of the opamp runs to another 10k resistor (R4) which i understand to be a feedback resistor. This connects to the output side of (R1).

...output side of R1 ?

I hope this makes sense! I used the 1k and 10k resistors as I was supposed to have a differential gain of 10.

Where is your differential input ???

 

[jaymee]

I appolagise for my lack of knowlege, but here is a similar diagram:

http://upload.wikimedia.org/wikiped...erential.svg/725px-Opamp-differential.svg.png

but V1 and V2 are the same source. (10V)

R1 = 1k
R2 = 1k
Rf = 10k
Rg = 10k

there is also a 12V supply going through the op amp, is the Vss on this diagram:
**broken link removed**

hope this is clearer!

 

[LvW]

Yes, now it's clear. The problem was, you have declared the 10 volts source as a supply voltage!
Your circuit is a classical differential amplifier - realized with an opamp.
However, as both inputs are tied together, the gain will be zero!

Calculation: Vout=Vin*(-Rf/R1+(10/11)*(1+Rf/R1)=Vin*(-10+10)=0.

CMRR is determined by the resistor tolerances.
Further questions?

Attention: Do you have single or dual supply?

 

[fg]

As far as I understand, the 10V means a DC supply like the 12V.

EDIT: Posted a corrected drawing.

 

[jaymee]

Yes, now it's clear. The problem was, you have declared the 10 volts source as a supply voltage!
Your circuit is a classical differential amplifier - realized with an opamp.
However, as both inputs are tied together, the gain will be zero!

Calculation: Vout=Vin*(-Rf/R1+(10/11)*(1+Rf/R1)=Vin*(-10+10)=0.

CMRR is determined by the resistor tolerances.
Further questions?

Attention: Do you have single or dual supply?

there are 2 12V supplies yes, as it is a simulation, I have no tolerences do i? I assume by tolerences you mean likegold or silver bands on the resistors? ie +/- 5% or +/- 10%??

is it possible to create a differential gain of 10, by choosing different resistor values?


fg - thats the circuit I have!

thank you.

 

[LvW]

there are 2 12V supplies yes,
..........
is it possible to create a differential gain of 10, by choosing different resistor values?
fg - thats the circuit I have!

* first statement: Two 12 V supplies
* second statement: circuit from fg - that means single supply!

This sounds a bit contradictory.
Jaymee, do you know what I am speaking about?

 

[jaymee]

yeh sorry, the circuit is the same as fg's, but it where the 12v goes to ground, there is a -12v.

 

[LvW]

yeh sorry, the circuit is the same as fg's, but it where the 12v goes to ground, there is a -12v.

In this case, my answer from 17:15 applies.
Gain=0 if R1 and R2 are tied together.
What is the purpose of this circuitry?

 

[jaymee]

thanks, its from an assignment I've been given from college.

I was asked to choose resistor values to give a differential gain of 10, but you say there is no way I can do this as R1 and R2 are tied together.

I need to talk with my tutor!

 

[LvW]

Jaymee,
"differential gain" means that you have a differential input signal which has to be amplified. But when both input pins get the same signal there is no difference at all!