# VCVS circuit using standard 1%

Discussion in 'Homework Help' started by Hassanzain, Sep 26, 2016.

1. ### Hassanzain Thread Starter New Member

Feb 7, 2016
4
0
Hi,

This is my first post here, and I'm looking for help with this question

Design a non-inverting VCVS circuit using standard 1% resistance values to provide a voltage gain of 22. Use resistance in the range of 1 Kohm to 100 Kohms

2. ### crutschow Expert

Mar 14, 2008
13,488
3,372
Any particular voltage and current?

3. ### Hassanzain Thread Starter New Member

Feb 7, 2016
4
0
No, what I was doing is that Acl=22, I come up with Rf=42, Ri=2
Required value Rc=Ri || Rf ==> 2K || 42k = 1909 ohms

Not sure where to go from here

4. ### RBR1317 Active Member

Nov 13, 2010
265
54
Since this is a non-inverting VCVS, the control voltage must be applied to the '+' input of the op amp while the negative feedback that sets the gain needs to be applied to the '-' input of the op amp.

5. ### crutschow Expert

Mar 14, 2008
13,488
3,372
Do you know how to calculate the gain of a non-inverting op amp circuit?
(Hint -- it's not the same as an inverting amp, which is what you calculated).

6. ### MrAl Distinguished Member

Jun 17, 2014
2,554
515

Hi,

This question does not seem to be posed correctly.

A voltage controlled voltage source can provide any gain you want from 0 to a zillion without using any resistors. The gain comes from the gain specification of the VCVS, and needs no other adjustment and no resistors at all to achieve that gain.

Could it be that they want you to sue a VCVS with a specific gain, such as 100, 1000, 10000, or even infinity?
There really has to be more to this question. Perhaps you can state the exact problem the way it was presented to you, exactly.

7. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Uh, the person DESIGNING a VCVS can't just say, "Oh, the design spec is for a gain of 22, so nothing further needs to be done."

The TS has been told to DESIGN a VCVS that has that gain.

It appears that they are doing so (or attempting to do so) using a non-inverting opamp circuit. This is fine as long as the control voltage for the VCVS is single-ended. Usually it is assumed that a VCVS input is differential.

8. ### MrAl Distinguished Member

Jun 17, 2014
2,554
515
Hi there,

as meaning to design a circuit with a VCVS in it to provide a gain. But yes, if they have to actually design a VCVS then that's different. I always viewed a VCVS as a basic circuit element not something to be designed, and the fact that there is no mention of an op amp how do we know it is to use an op amp or transistors or whatever? Maybe they dont care which is used, but really i dont feel comfortable doing anything without hearing more about the requirements for this circuit, if any are available.

9. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
I think the proper way to read that sentence is:

1) Design a non-inverting VCVS circuit.
2) Design it using standard 1% resistance values.
3) Design it to provide a voltage gain of 22.

The TS in Post #3 gives a pretty good clue as to how they are attempting to do it (what they provide is really only consistent with using an op-amp circuit, though what they present is actually applicable to an inverting configuration), though I don't know what their Rc is all about -- and perhaps that indicates that they are NOT using an op-amp, but rather a transistor topology.

I agree that the TS has left a lot of things unstated that force us to make lots of assumptions. Wish this were an outlier in that regard, but of course it is closer to being the norm instead.

What I have never understood is how so many people think that it is reasonable to throw out things that are nowhere defined (such as this Rc business) and expect strangers to know what they hell they are talking about. Now, in their defense, if you are under the impression that there is exactly one topology for a VCVS (in this case) and you have always seen component labels that lead you to believe that Ri and Rf and Rc can only mean one thing, then it is probably not too unreasonable for them to think that everyone in the world will automatically know what the hell they are referring to. But we see this "read my mind" stuff WAY too much for that to be a reasonable excuse in very many cases.