and 346 are metal oxide semiconductor field effect transistors mosfets that have gates sources and drains although other transistor types may be used an inverting input of a first op amp 348 communicates with a source or second terminal of the first transistor 336 an output of the first op amp 348 communicates with a gate or control terminal of the first transistor 336 the source of the first transistor 336 communicates with a first terminal of a second resistance 350 an opposite terminal of the second resistance 350 communicates with an inverting input of a second op amp 352 the opposite terminal of the second resistance 350 also communicates with a source of the second transistor 338 a gate of the second transistor 338 communicates with an output of the second op amp 352 a drain or first terminal of the second transistor 338 communicates with a drain of the third transistor 340 the drain of the third transistor 340 communicates with a gate of the third transistor 340 a source of the third transistor 340 communicates with the ground potential 328 the gate of the third transistor 340 communicates with a first terminal of a first switch 354 a second terminal of the first switch 354 communicates with a gate of the fourth transistor 342 a source of the fourth transistor 342 communicates with the ground potential 328 a drain of the fourth transistor 342 communicates with the first terminal of the variable resistance 326 a drain of the first transistor 336 communicates with a drain of the fifth transistor 344 the drain of the fifth transistor 344 communicates with a gate of the fifth transistor 344 a source of the fifth transistor communicates with a second supply potential.