# gain of inverting op amp?

Discussion in 'Homework Help' started by no12thward, Oct 25, 2011.

1. ### no12thward Thread Starter Member

Jan 10, 2011
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0
I'm looking for two resistor values from the table that will give me a gain of -18. Ive tried converting to kilo, milli, micro, nano ohms, etc, then using -(Rf/Ri). Cant seem to find any combo of resistor values on the table that will give me the gain of -18.

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2. ### JoeJester AAC Fanatic!

Apr 26, 2005
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There are two number combinations from your table that will work ... how do you think you should go about and solve this problem?

3. ### no12thward Thread Starter Member

Jan 10, 2011
30
0
i know I'd have to change one of the resistor values from Ωs to a larger or smaller ohm conversion, but I've tried that for R1 as well as R2 resistors. Maybe im missing something?

4. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Just look at your values more carefully. You have available all of the powers of 10 of these values. For example, you can use 1Ω, 10Ω, 100Ω etc.

Does that make it any easier?

These papers you post lately aren't the most strict and best-written exercises, granted.

5. ### no12thward Thread Starter Member

Jan 10, 2011
30
0
Sorry, it doesnt. im not seeing any power of ohm value i can use to get a gain as low as 18. I've used all the conversions

6. ### no12thward Thread Starter Member

Jan 10, 2011
30
0
how can i derive anything above 9.1ohm (10ohm or 100ohm) from the values on the table?

7. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,662
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9.1, 910, 9100, 91000, 910000, 9100000

Keep multiplying by 10.

There are two groups that satisfy your requirements.

8. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Another hit:

If I wanted a gain of -11, I could use a resistor of 11kΩ and a resistor of 1kΩ.

9. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,418
903
Your list is missing the standard resistance value of 1.8, 18, 180, 1.8k 18k and 180k.
A 1k input resistor and 18k feedback resistor or a 10k input resistor and a 180k feedback resistor results in a gain of 18 times.

10. ### steveb Senior Member

Jul 3, 2008
2,433
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I think it is not just missing, but instead it looks like there is a typo where the 1.7 should be 1.8.

11. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Since this is homework, let's assume that the teacher has set the restrictions that he feels they will best test the student's knowledge.

It's rare that homework that eludes the restrictions but applies to everyday rules gets rewarded.

12. ### no12thward Thread Starter Member

Jan 10, 2011
30
0
I'm thinking I could use Rf=2 and Rin=1.1. But then how would I convert the ohm values to get 18?

13. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
The closest possible gain you could achieve with those coefficient would be
$A=-\frac{2*10^1}{1.1}=-18.18$

It is close, but you can do better.

Remember that if $|A|=\frac{R_f}{R_i}$ then $R_f=|A| \cdot R_i$
You can input various values for $R_i$ and see if you have the $R_f$ available.

14. ### no12thward Thread Starter Member

Jan 10, 2011
30
0
I think were allowed a 1% error. My only question would be, with only being able to use these standard resistor values...if I use the 2ohm, the next size up would be a milliohm right? How could I use 2 X 10^1. Wouldn't that exceed my resistor values from the table?

15. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Your don't need to insert error if you can help it.

A mili is equal to $10^{-3}$, so in that aspect you are wrong.

Pay a bit more attention. It has been stated numerous times that the exercise wants you to use any power of 10 you want to multiply your values with. It cannot be done any other way.

I 'll give you another hint: You can set $R_i=1.5K\Omega$. How much will $R_f$ be, according to the formula I gave you previously?

16. ### no12thward Thread Starter Member

Jan 10, 2011
30
0
Would be 2.7?

17. ### Georacer Moderator

Nov 25, 2009
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2.7what? If I assume that you mean 2.7Ω then the gain will be $A=-\frac{2.7 \Omega}{1.5 \cdot 10^3 \Omega}=-0.0018$ which is clearly wrong.

Try once more.

18. ### no12thward Thread Starter Member

Jan 10, 2011
30
0
Sorry. 2.7X10^1Ω

19. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
I gave you $R_i=1.5K\Omega$, not $R_i=1.5\Omega$.

With $R_f=27\Omega$ you will have a gain of $A=-\frac{27}{1500}=-0.018$, once again not acceptable.

20. ### SgtWookie Expert

Jul 17, 2007
22,194
1,761
I think the 1.7 is a deliberate error to keep students from jumping at it.