Minipulse JFet explanation needed.

[dbanner]

A quick read up on n-channel jfet indicates that a negative voltage on the gate is required for "pinch off" to occur. This is the voltage at which the source to drain stops conducting.
At zero gate voltage, the source to drain is fully conducting.
Now it's easy to see the action of the N-JFET as a sample switch with the pulse train provided by the 4538's

About the pulse train provided by the 4538's, it pulses between -5v and 0 volts. This is because Vdd(pin16) is at 0 volts and Vss is at -5V.

[6666]

With the MPP you dont have to worry about the pre amp off set as its capacitor coupled, I hate Jfets to

[Brian Deese]

Pretty obvious isn't it, my oversight. Have you built one of these? I have a small offset in the preamp output that I'm not crazy about. If I jumper the rx connection which grounds the input I get about -800 mv on the output.

[dbanner]

A quick read up on n-channel jfet indicates that a negative voltage on the gate is required for "pinch off" to occur. This is the voltage at which the source to drain stops conducting.
At zero gate voltage, the source to drain is fully conducting.
Now it's easy to see the action of the N-JFET as a sample switch with the pulse train provided by the 4538's

[Brian Deese]

I think I must need a multi vitamin,or more turkey sandwiches. My brain wants to clock out at 5pm. Thanks for the pdf I'll check it out tomorrow on the desktop.

[dbanner]

N-channel JFET negative 5 volts on gate, the FET is switched off, and for the duration of the sample at 0V on the gate, the FET is switched on.

I found a PDF document on another mini-pulse thread with scope shots of all the test points. Maybe you have it already.

[Brian Deese]

I have the minipulse plus assembled and I am scoping everything to verify before I connect a coil,and the connection to the preamp. On TP7,and TP8 I have the sample pulses going to the JFets, the pulses go from -5V to 0V the question I have is how are we turning on a JFet with a Positive going pulse? I have no experience with JFets I am sure I am not understanding the way this circuit works.