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Thursday, June 16th, 2011

Current Monitor

Categories: [ DIY ]

current_monitor

This schematics of a current monitor can be found in many datasheets as examples of applications of an op amp. I wanted to find out the relationships between RS (the shunt), R1 and R2. Here's how it goes:

  • V1 = A·(V+ - V-) (1) (A is the open-loop gain of the op amp)
  • V+ = VSS - V·R1/R2 (2) (Common emitter transistor setup, see more particularly this image)
  • V- = VSS - RS·iload (3)
  • V = V1 - 0.65 (4) (voltage drop between the base and the emitter of a common transistor)

Combining (2), (3) and (4) into (1), we get

V + 0.65 = A·(VSS - V·R1/R2 - VSS + RS·iload),

from which follows

V·(1 + A·R1/R2) + 0.65 = A·RS·iload.

We assume A is very large, therefore A·R1/R2 >> 1, so it simplifies into

V·R1/R2 + 0.65/A = RS·iload.

We assume A is very large, and thus 0.65/A << V·R1/R2, leading to

iload = V·R1/(R2·RS).

[ Posted on June 16th, 2011 at 09:40 | 1 comment | ]

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