Using transistors to drive load with op amp

331

04:38:58

Here, using negative feedback, we use a normal op-amp, a common MOSFET (draws 10mA at 1A output) and a power BJT to make a high power voltage follower.  
What's special about this follower is that it utilizes the high gain of the op-amp, the infinite input impedance of the MOSFET, and the power-handling characteristics of the BJT.  The op-amp outputs zero current due to the insulated MOSFET gate, meaning it doesn't matter how little power it's rated for.  The total gain of this circuit is even higher than the gain of the op amp itself, meaning the output voltage will precisely match the input.
With this circuit, you could operate a huge fan or a desk lamp from an arduino, with precise control over speed or brightness.

NOTE: If circuit can't find a solution, open the switch, start simulation, then close it.

published 10 years ago

jpoulin0901

10 years ago

The input impedance of a mosfet is not infinite. It is roughly equal to the capacitive reactance of the gate. Typical power MOSFETs have gate capacitance ~1nF, so Xc becomes significant at radio frequencies or where rise times must be ultra short.

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