Ridhwan Avicenna: Elektronika

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nothing is impossible.

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Showing posts with label Elektronika. Show all posts

Showing posts with label Elektronika. Show all posts

Wednesday, March 5, 2014

Instrumentation Amplifier

5:46 PM Elektronika, Semester II No comments

Instrumentation AmplifierOne drawback of the differential amplifier is that its input impedance () may not be high enough if the output impedance of the previous stage is not low enough. To overcome this problem, two non-inverters with high input resistance can be used each for one of the two inputs to the differential amplifier. The resulting circuit is shown below:

The analysis of this circuit is very simple. As the output impedance of the non-inverter is low, the three op-amp circuit can be considered as three independent circuits. The outputs of the two non-inverters are:

$\begin{displaymath}V'_1=V_1\left(1+\frac{R_f}{R_1}\right),\;\;\;\;\;\;\;\;\; V'_2=V_2\left(1+\frac{R_f}{R_1}\right) \end{displaymath}$

The output voltage of the differential amplifier is:

$\begin{displaymath}V_{out}=\frac{R_4}{R_3}(V'_2-V'_1)=\frac{R_4}{R_3}\left(1+\frac{R_f}{R_1}\right)(V_2-V_1) \end{displaymath}$

Of course the two resistors can be combined to become , i.e., , then the output can be written as:

$\begin{displaymath}V_{out}=\frac{R_4}{R_3}\left(1+\frac{2R_f}{R_0}\right)(V_2-V_1) \end{displaymath}$

Alternatively, we consider the current going from to :

$\begin{displaymath}\frac{V'_1-V_1}{R_f}=\frac{V_1-V_0}{R_1}=\frac{V_0-V_2}{R_1}=\frac{V_2-V'_2}{R_f} \end{displaymath}$

From the equation of the first two terms we get:

$\begin{displaymath}V'_1=\left(1+\frac{R_f}{R_1}\right)V_1-\frac{R_f}{R_1}V_0 \end{displaymath}$

From the equation of the second two terms we get:

$\begin{displaymath}V'_2=\left(1+\frac{R_f}{R_1}\right)V_2-\frac{R_f}{R_1}V_0 \end{displaymath}$

Using the equation of the differential amplifier above, we get the same result as above:

$\begin{displaymath}V_{out}=\frac{R_4}{R_3}(V'_2-V'_1)=\frac{R_4}{R_3}\left(1+\frac{R_f}{R_1}\right)(V_2-V_1) \end{displaymath}$

sumber : http://fourier.eng.hmc.edu/e84/lectures/opamp/node3.html

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