# Transconductance Help.

Discussion in 'Homework Help' started by Durzo972, Dec 31, 2012.

1. ### Durzo972 Thread Starter New Member

Nov 16, 2012
8
0
Hello everyone,

I am just attempting this question but it seems like i get stuck at a dead end or don't know what to do with some values, or just dont know if i am doing it correctly. I am not asking for the answer but i just want someone to direct me to where i should begin, superposition?(i attempted this but just didn't know where to go from there), assume the output to the op amp is saturated? anyways i would appreciate the help. Thanks!

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2. ### #12 Expert

Nov 30, 2010
18,078
9,616
First, I don't think the output of the opamp is always saturated becaue that means it can't change. Second, "S" is a Siemen, which is a Mho, which is 1/R. So, invert the .02 to get resistance. That's the "50" that the teacher doesn't want. Now, R=V/I so 50 = change in voltage over change in current.

What Rsense will produce an output of 1 amp per 50 volts of change at the input of the opamp?

Here's a page with the math I don't know. I hope it gets you started, considering everybody else is playing, "Happy New Year" right now.

http://www.ece101.com/2009/03/the-difference-amplifier-or-differential-amplifier/

3. ### WBahn Moderator

Mar 31, 2012
22,862
6,828
First, make sure that you understand what is being asked for. The term "conductance" is the ratio of the current between two points divided by the voltage between those same two points. The term "transconductance" is the ratioof the current between two points divided by the voltage between some other two points.

Q1) In this case, the transconductance in question is the ratio of what current to what voltage?

Q2) In terms of Vin, what is the voltage at the non-inverting input of the opamp?

Q3) Assuming the opamp is in its active region, what is the voltage at the inverting input?

See if you can keep following this line of reasoning out to the point were you can determine the current that is the focus of Q1.

Assuming I didn't mess up my work someplace (I haven't checked it), the answer for Rsense is somewhere between 1Ω and 10Ω.

4. ### MKCheruvu Member

Nov 20, 2012
30
6
I feel the following is the likely solution
Gm= 0.02=[{1+(R2/R1)}{R4/R3}]/Rsense.
Rsense = {1.1x0.1}/0.02=5.5 ohms
Request to conifirm .

5. ### WBahn Moderator

Mar 31, 2012
22,862
6,828
Please don't just work someone's homework problem for them. You aren't doing them any favors. Consider that they have almost certainly already seen similar problems worked out, either in the text or by the instructor. So just having one more worked out by someone else isn't adding much to the mix. They need to struggle with thinking about how to work it themselves. We are here to help spot their errors, correct misconceptions, and given hints to help steer them toward the next steps in the solution. But they need to do as much of the work on their own as possible for that is how they are going to learn.

Ron H likes this.
6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
673
I agree. And I think he got it wrong.