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What is the idea behind this circuit?Howland Current Source for Grounded Load 

Idea: Keeping a constant current by adding a current. As we already know, the simple current source VR is imperfect as the load affects the current. The voltage drop VL across the load (suppose again that it is a varying resistor RL) is harmful as it enervates the excitation voltage V. As a result, the current I decreases with the value VL/R. So far, we aided the exciting voltage source by using an additional supplementary voltage source. Now, we will try to aid the voltage source injecting an additional current Is = VL/R by another "helping" current source. We may build it by connecting a "helping" voltage source Bs through another resistor Rs = R to the load. 
Implementation: Using an additional opamp helping current source. Тhis genius idea is implemented in the famous Howland current source. In this clever circuit, the excitation voltage V and the resistor R form a basic current source. It produces a current I = V/R  VL/R, which passes from the left hand side through the load RL. The noninverting amplifier R1R2OA and the resistor R3 form a helping current source. It produces a current Is = VL/R, which passes from the right hand side through the load RL. As a result, a steady total current IL = V/R  VL/R + VL/R = V/R flows through the varying load RL. It is a genius idea, isn't it? 
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