# Question about design of difference amplifier

#### William.A.

Joined Dec 12, 2020
8
Hello guys I am working on a simulation lab to design a difference amplifier to realize the transfer function = −10(V1) + 0.5(V2). (Hint: R1 = 10 kΩ and R3 = 100 kΩ) . By the general equation, I got the value of R4=4.762kΩ and R2=100kΩ.

Then I set the input V1 as a sinusoidal signal with a 2v peak to peak voltage and V2 is positive 15v.
I think the output is weird because it shows a result around 14.06vwhich doesn't satisfy the transfer function.
why the output isn't in the range of -2.5v to 17.5v?
Is there any wrong or misunderstanding in my calculation and design?

#### ericgibbs

Joined Jan 29, 2010
16,784
Hi William,
Welcome to AAC,
E

#### William.A.

Joined Dec 12, 2020
8

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#### crutschow

Joined Mar 14, 2008
31,115
By the general equation, I got the value of R4=4.762kΩ and R2=100kΩ.
What "general equation"?

#### William.A.

Joined Dec 12, 2020
8
What "general equation"?
Hello crutschow,

This equation is provided by my professor in his lecture ppt.

#### Papabravo

Joined Feb 24, 2006
19,573
Do you know how that equation was derived?

#### William.A.

Joined Dec 12, 2020
8
Now the problem is solved. I just drew the whole circuit again and this time it showed the result I expected. Thank you guys.

#### Papabravo

Joined Feb 24, 2006
19,573
Now the problem is solved. I just drew the whole circuit again and this time it showed the result I expected. Thank you guys.
I find that result....surprising.
The schematic in post #1 is missing critical information.