# Can someone help get started with this...

Discussion in 'Homework Help' started by ///m3dave, Jun 16, 2010.

1. ### ///m3dave Thread Starter New Member

Dec 8, 2009
23
0
a. Determine the value of R1 for the OP Amp
circuit shown. AV(dB) = 33dB.
Thanks

2. ### hgmjr Moderator

Jan 28, 2005
9,030
215
What ideas do you have to solve the problem?

hgmjr

3. ### ///m3dave Thread Starter New Member

Dec 8, 2009
23
0
I know that the answer is 223 ohm. But I can't figure out how to get that. From what I've been reading in my book the only thing I can come up with is to divide 33dB by 10 to get Vo, then multiply that by 10k ohm?

4. ### hgmjr Moderator

Jan 28, 2005
9,030
215
The factor of 10 is normally associated with power gain. The problem statement here appears to be requesting voltage gain. In that case you would use 20 rather than 10.

Go to www.allaboutcircuits.com or your textbook and study the information on decibels.

The first step that I would suggest is that you convert the 33db voltage gain into the Voltage-out/voltage-in.

hgmjr

5. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
And make sure you know exactly wich is the gain of the inverting assembly.

6. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,418
903
It is an impractical circuit because its input resistance is very low because the resistor values are very low. Its voltage gain is 33dB only when it is fed from an extremely low impedance.

7. ### Ghar Active Member

Mar 8, 2010
655
73
So then it's practical for a low impedance source and useful otherwise if you don't need the full 33 dB... what's the issue?

8. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,418
903
If the opamp is non-inverting then it could have a high or very high input impedance and still have plenty of gain.

9. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,661
1,528
Impractical or not ... this is some professors idea of an analysis. If we argue the practicality of the circuit, the reason for the problem is lost, which doesn't fulfill the OPs inquiry.

Your issue is not with the OP but their professor and I'm sure the OP can message you with the Professor's email address so you can take the practicality discussion offline or invite the professor to a new thread discussing the practicality of the professor's assignment.

10. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,418
903
If the professor insists on studying an inverting opamp then maybe its feedback resistor should be 1M instead of only 10k.

11. ### Ghar Active Member

Mar 8, 2010
655
73
What if you're building a low noise circuit and there is a lot of unavoidable current noise from some devices?
There's plenty of reason not to use huge resistors if you don't have to.

Jun 17, 2010
26
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13. ### tskaggs New Member

Jun 17, 2010
26
3
Nvm, just change the 10 in my solution to 10k.