Engineering Why is a differential amplifier considered the same as a subtractor?

AI Thread Summary
A differential amplifier is often considered the same as a subtractor because it computes the difference between two input voltages, V2 and V1, scaled by a gain factor determined by resistor ratios. The output voltage equation Vout = (R2/R1)(V2 - V1) indicates that while it performs subtraction, it also amplifies the result based on the resistor values. A true subtractor would require equal resistor values to achieve a gain of one, resulting in a direct output of V2 - V1. However, the differential amplifier's ability to scale the output makes it versatile in analog applications. Thus, while they serve similar functions, the key distinction lies in the gain factor of the differential amplifier.
altruan23
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Homework Statement
So i was learning about the subtractor op amp circuit and saw that everyone is saying that a subtractor is the same as a differential op amp.
Relevant Equations
superposition, voltage divider , op amp
So after using superposition and setting the ratio R2/R1 = R4/R3 the same or R2=R4 and R1=R3,i come to the eq. for output voltage Vout= R2/R1 * (V2-V1) or R4/R3(V2-V1). And in the book foundations of analog and digital electronics by agarwal and lang, they are saying that this circuit is a subtractor.
but isn't that just an differential amplifier, as it amplifies with the ratio R2/R1 the difference (V2-V1)? Schouldnt a subtractor be Vout= V2-V1 so that means i need to have R1=R2=R3=R4. why is everyone saying that a differential amp. is the same as subtracter?? i simulated both circuits and as i tought, all resistors must be the same to really have a subtractor.
The first picture is R2/R1 = R4/R3 as in the book.
1648029983564.png


So as you can see, 7V-3V schould be 4 according to a real subtractor, but because the ratio R2/R1 or R4/R3 is 2, Uout is 2*4=8.
And if i set all the resistors the same i truly get a real subtractor.
1648030074744.png

So why is everyone saying that a differential amp is the same as subctractor?? i mean a subtractor is a differential amp with gain 1, but a differential amp with some other gain isn't a subtractor.

the book is here: https://neurophysics.ucsd.edu/courses/physics_120/Agarwal and Lang (2005) Foundations of Analog and Digital.pdf
online book page 883
 
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altruan23 said:
So why is everyone saying that a differential amp is the same as subctractor?? i mean a subtractor is a differential amp with gain 1, but a differential amp with some other gain isn't a subtractor.
If the term (V2 - V1) is present then it is subtracting two voltages, by computing a difference. The gain is a bonus since the output can be scaled without adding more resistors or another op-amp. In analog electronics everything is scaled to use the available common mode range.
 

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