Transresistance amplifier question

Status
Not open for further replies.
P

paulmdrdo

Full Member level 3
Joined
Jan 17, 2014
Messages
183
Helped
1
Reputation
2
Reaction score
2
Trophy points
18
Activity points
1,394
Guys I need help on the detailed derivation for this circuit. I need to know the relationship between Vo and Is. TIA!
 

As per my derivation it should be is = VoR2/[(R1R2) + R3(R1+R2)].
 

"As" is the transfer function of the OpAmp, ideally it is infinite.
 

Attachments

  • transres_amp.png
    17.5 KB · Views: 68
Last edited:

mjuneja said:
As per my derivation it should be is = VoR2/[(R1R2) + R3(R1+R2)].
Click to expand...

The current source is an unchangeable constant. How can it be dependant on Vo?

Ratch
 

Ratch said:
The current source is an unchangeable constant. How can it be dependant on Vo?

Ratch
Click to expand...

In that case it can be written as.

Vo = is[(R1R2) + R3(R1+R2)]/R2.

Thanks for pointing out.
 

Ratch said:
The current source is an unchangeable constant. How can it be dependant on Vo?
Click to expand...

"is" is not unchangeable. It is just an equation, divide both sides with "is" and "Vo", then calculate the multiplicative inverse of both sides. You will see how Vo depends on is then.
By the way, a minus sign is missing from mjuneja's formula I think.
 


Yes since it's a negative feedback therefore negative sign is a must.


Vo = - is[(R1R2) + R3(R1+R2)]/R2.
 

Actually not because of the feedback, because of the figure rather. For example, if you swap the pins of 'is' on schematic, the formula should have plus sign despite of the negative feedback.
 

The simple voltage divider rule leads to frankrose equation (2) in 1 single step (assuming As-->infinite).
 


Is represents a source of current that is unchangeable. Yes, you can calculate what Is should be if Vo is a certain value, but the principle is that Is makes the dog bark, not the other way around.

Ratch
 

hello! the circuit above is assumed to be ideal op-amp

my books says that it should be I still don't get how it arrived to this form.
What I did is to first draw an equivalent model of the circuit, then using the ideal op-amp characteristics I came up with the diagram (2) but then if that was correct Vo = 0v.

Can you please tell me how it arrived with the solution given by the book. Is my diagram above the right equivalent circuit for the problem in question?
 


That is an easy question to answer. Take frankrose's equation for Vo and find the limit as As goes to infinity. That should give you the same answer your book shows. Remember, an ideal opamp has infinite gain. I cannot make sense out of what you are doing in your attachment.

Ratch
 


Is my equivalent circuit correct?
 

paulmdrdo said:
Is my equivalent circuit correct?
Click to expand...

Equivalent to what? You show a differential resistance between the two opamp inputs and an output resistance for the opamp. Neither your book or frankrose does that.

Ratch
 

You have 2 circuits, both are something but seem bad. You model an ideal OpAmp, it is unnecessary a bit and makes no sense. Ideal OpAmp is just a voltage controlled voltage source with infinite gain.
 

I just discovered, that I can arrive at the exact form of equation as my book provided by just using the equivalent circuit of the one in my #1 posted attachment.


I performed nodal analysis and kvl without dealing with limits and stuff, just pure nodal analysis and kvl. So, this model is useful after all.
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…