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Comments on What is the difference between differential amplifier and differentiator?

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What is the difference between differential amplifier and differentiator?

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Since am interested in how a delta sigma modulator works, I need to know what is the difference between differential amplifier and differentiator if there is a difference of course.

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A differential amplifier and a differentiator are two completely different circuit blocks.

Differential Amplifier

A differential amplifier has two inputs and one output. It takes the difference between the two inputs, multiplies that by the gain, and makes it the output.

    Out = (V1 - V2) ⋅ Gain

Image

In this example, the gain is A/B.

Differentiator

A differentiator takes the derivative of a signal. In other words, its output is proportional to how fast the input is changing.

Note that the gain is not dimensionless, as it is for a normal amplifier. For example, the gain can be the output Volts divided by the input Volts/second, which comes out to units of seconds.

Image

In this example, the gain is proportional to -R1⋅C1.

Why do you think the gain could not expressed as V/V?

I'll assume this is referring to the differentiator, since the gain of the differential amplifier is a voltage divided by a voltage, resulting in a dimensionless value.

For a differentiator, the output is the change in the input. Just dividing the output voltage by the input voltage doesn't yield anything meaningful. For example, you get 0 V out for any steady input voltage. Saying you get 0 V out for 10 V in, but also 0 V out for 3.97 V in (or any other voltage), isn't very useful.

What about Acl=-Aol/(1+Aol*beta) with beta=1/(1+sR1C1)?

Since you didn't define any of your terms, nor the context, it's just meaningless characters.

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LvW‭ wrote almost 4 years ago · edited almost 4 years ago

OK - I was of the opinion that in a short comment it would be appropriate to use the well-known abbreviations for the open-loop gain Aol and the closed-loop gain Acl. The quantity beta was defined using the symbols shown in the drawing. Again, I like to point out that for sinusoidal signals it is, of course, possible to define a dimensionless gain (V/V). For control systems (control loops) It is common practice to define the gain in the frequency domain (PD or PID or PD-T1 blocks).