# Designing an amplifier with a single op amp

#### TL314

Joined Mar 29, 2015
64
Using a single op amp, design an amplifier with a gain of V2/V1 = 3/4, input resistance of 8kohms, and sero output resistance.

#### TL314

Joined Mar 29, 2015
64
my resister is suppose to be 8kohms not just 8 ohms

#### MikeML

Joined Oct 2, 2009
5,444
Well, it has an output impedance of zero, a gain of zero, and an infinite input impedance. One out of three ain't bad...

#### TL314

Joined Mar 29, 2015
64

#### Papabravo

Joined Feb 24, 2006
14,701
Why would you ground the output of your amplifier? It's a good thing most opamps cannot supply much current or you'd have a welder.

#### TL314

Joined Mar 29, 2015
64
Why would you ground the output of your amplifier? It's a good thing most opamps cannot supply much current or you'd have a welder.
trying to figure this out, trying different things

#### Papabravo

Joined Feb 24, 2006
14,701
How does that get you 8 kΩ input impedance?

#### TL314

Joined Mar 29, 2015
64
How does that get you 8 kΩ input impedance?
ah your right, crap ok ill try again

#### Papabravo

Joined Feb 24, 2006
14,701
are you referring to the first or second design?

The first design

#### Papabravo

Joined Feb 24, 2006
14,701
It would help if you showed your work. That way we can show you where you went wrong.

#### TL314

Joined Mar 29, 2015
64
if so is there another way to write this circuit out?

#### Papabravo

Joined Feb 24, 2006
14,701
For the circuits you have drawn, how would you compute the input impedance? If you can show us how to compute it we can see if that configuration can possibly produce the desired result. If not we need to look for an alternative. Trust me you will learn more if you do this than if I just give you the answer.

Look at page 41 in the following:

http://zebu.uoregon.edu/~rayfrey/431/notes9.pdf

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#### TL314

Joined Mar 29, 2015
64
Zinput = R * (V2/(V1-V2))?

#### Papabravo

Joined Feb 24, 2006
14,701
So from your drawing what numbers would you put in to get 8 kΩ? Don't you have to pick a particular device to do that?

#### TL314

Joined Mar 29, 2015
64
2 kohm resister in series with a 6 is 8kohms input resistance is it not?

#### Papabravo

Joined Feb 24, 2006
14,701
Yes but that is not the whole picture. It involves the input imepdance of the amplifier, the forward gain, and the feedback gain.
I don't think you can get there with your chosen configuration. How might you modify it?

FYI: The input resistance of a TL081 is 10^12 Ω

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#### TL314

Joined Mar 29, 2015
64
so from my last drawing I do have a input resistance of 8kohms with a zero output resistance correct?
i just need a gain of v2/v1=3/4?

#### Papabravo

Joined Feb 24, 2006
14,701
No you don't. The input impedance is not a function of only the external components. The amplifier itself In the case of the TL081) has an input impedance of 10^12 Ω. That's huge. How do you plan to counteract that?

#### TL314

Joined Mar 29, 2015
64
may I ask how you know the amplifier has a input impedance of 10^12? or is that a constant for amplifiers?

#### Papabravo

Joined Feb 24, 2006
14,701
may I ask how you know the amplifier has a input impedance of 10^12? or is that a constant for amplifiers?
I consulted the datasheet for that particular amplifier. Most of the opamps that you ever see besides all the ones that you won't will have a very large input impedance with respect to 8KΩ. So the question is: how can I reduce a very large impedance down to the level of 8KΩ give or take a small amount?