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op amps using specific power supply configuration eg:+8v/-5v why?????????
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dmills:
BF245 is a deletion mode jfet, it is on with Vgs = 0, and cut off with negative Vgs.

You could build a single supply rail metal detector of course, but I doubt it would save you many parts and may actually be more expensive.

Lots of the doings are AC coupled so for a 'single supply' design you would need a reference somewhere above the negative rail to allow the AC amplifiers to work.

There are (as usual) plenty of ways to design such things, (I might have gone for a 'bus switch' as a synchronous detector for example in something resembling a H mode mixer, or maybe a cmos analogue switch), and often design with this sort of thing comes down to whatever the original designer had kicking around at the time.

Regards, Dan.
ifrenide:
okmy brother . lets consider that bf245 will be active when vgs=0 . when it will be off so , the answer +5v or lets say +8v , then the output of the comparator wich drives bf245 must alternating between 0v and +8v, and then if the output of the comparator alters between these two valuses , its negative voltage supply -5v what it does  :-\ how to get 0v at its output while the negative volage supply is -5v, and this is a part of the problem without talking about the op amps of this detector .
best regards
dmills:
No, with the source of the BF245 at ~0V it will be off when the gate is at -5V, and on when the gate is at ~0V, the gate never goes positive because that would forward bias the intrinsic diode.

The comparator output stage is a NPN transistor with the emitter connected to the comparators negative supply pin (-5V), when this transistor is off the jfet gate is pulled to ground by a resistor.

Regards, Dan.
ifrenide:
yes that's rigth .I cheked that thanks
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