# Inverting Op-Amp HELP!

Discussion in 'The Projects Forum' started by dustinjan10, Sep 16, 2012.

1. ### dustinjan10 Thread Starter New Member

Sep 16, 2012
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Hello,

I was wondering if someone could help. I need an inverting op-amp using a 741. I needs an input impedance of 200 - 2k Ohms and a voltage gain of 6-40 at a frequency of 200Hz-5Khz when loaded with its characteristic output impedance.

Any help would be greatly appreciated!

Thanks

Jun 22, 2012
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3. ### crutschow Expert

Mar 14, 2008
21,146
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Op amps don't really have a "characteristic output impedance", just a maximum output current they can drive. Is that what you mean?

4. ### dustinjan10 Thread Starter New Member

Sep 16, 2012
6
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Something like that, yes. I know how to figure the gain. What I am getting stuck on is the input and output impedance. I need the input impedance to be between 200-2k ohms, then I need to measure the output impedance.

5. ### dustinjan10 Thread Starter New Member

Sep 16, 2012
6
0
Really, I am having trouble with the input and output impedance. I need an input impedance of 200 - 2kohms and then I need to measure the output impedance.

6. ### ScottWang Moderator

Aug 23, 2012
6,554
1,010
I'm not sure what's your purpose for Input resistance of Op Amp?

Because the Input resistance is really high, but you cann't reduce it from inside, so you can do it from outside, you just need to connecting one 1KΩ resistor to the input pin and ground, then the Input resistance will be close to 1KΩ, it's value between 200Ω and 2KΩ, or you want to change it to the value you like.

See page 5.
ri, Input resistance, 25°C, 0.3 2, 0.3 2 MΩ
ro, Output resistance, VO = 0, See Note 5, 25°C, 75 75 Ω
http://www.datasheetcatalog.org/datasheet/texasinstruments/ua741.pdf

Last edited: Sep 16, 2012
7. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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the input impedance is set by the resistor on pin 2, which also sets the gain, so the circuit at the bottom has an input of 1k and a gain of 100.

Last edited: Sep 16, 2012
8. ### dustinjan10 Thread Starter New Member

Sep 16, 2012
6
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Do you mean the two resistors on pin two? This being an inverting Op amp?

9. ### dustinjan10 Thread Starter New Member

Sep 16, 2012
6
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What do you mean the circuit at the bottom?

10. ### #12 Expert

Nov 30, 2010
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The bottom of the page that was given a link in post #2

and the gain is 100. You got it mixed up with the non-inverting formula, Dave.

Dodgydave likes this.
11. ### dustinjan10 Thread Starter New Member

Sep 16, 2012
6
0
Got it! Thanks for the help.