So I have done a lot of reading about jfets, but it has confused me more. Considering n channel all the literature says that the gate voltage needs to be negative to control Ids.
I get that with Vgs at zero, the channel is open and Ids is at max. Im now thinking of a single 0 to 5v circuit. When we say negative, do we mean with respect to Vdd or 0v? This point is crucial to my understanding!
I can see how the former would be useful, I.e. bringing to gate to some value between 0 and 5v would control Ids. So at say 2v which is 3v negatjve w.r.t. Vdd of 5v, a smaller current flows. If I have to take this to (in my mind) a real negative voltage of -2.5v ie 7.5v below Vdd it seems like a have to have a split supply and I cant see much use for the n channel device in single supplies.
I cant get my head around this. Should I be thinking of p channel devices? What would the value of Ids be if I just leave thegate floating? What would happen if I tied gate to Vdd ?
I have bought a handful of j113 for £1.50 to play with and understand properly, but I dont want to start a fire or waste my time with a circuit that will never work because its wired wrongly!
There seem to much fewer circuits thna for bjt or mosfet and jfet seem harder to find, is this beacuse they have few specific uses or are they becoming obsolete? Is thre some killer task that only a jfet will work for, or can they always be replaced with mosfet?
I think I generally "get" bjt and mosfet, and I have designed and built successful circuits with both pnp/npn bjts and p and n mosfets, but for some reason I just cant "see" a practical/useful circuit in a single supply situation with jfets. The circuits I have seen appear to use n jfets without split supplies, so I just dont understand how they work.
Help!
I get that with Vgs at zero, the channel is open and Ids is at max. Im now thinking of a single 0 to 5v circuit. When we say negative, do we mean with respect to Vdd or 0v? This point is crucial to my understanding!
I can see how the former would be useful, I.e. bringing to gate to some value between 0 and 5v would control Ids. So at say 2v which is 3v negatjve w.r.t. Vdd of 5v, a smaller current flows. If I have to take this to (in my mind) a real negative voltage of -2.5v ie 7.5v below Vdd it seems like a have to have a split supply and I cant see much use for the n channel device in single supplies.
I cant get my head around this. Should I be thinking of p channel devices? What would the value of Ids be if I just leave thegate floating? What would happen if I tied gate to Vdd ?
I have bought a handful of j113 for £1.50 to play with and understand properly, but I dont want to start a fire or waste my time with a circuit that will never work because its wired wrongly!
There seem to much fewer circuits thna for bjt or mosfet and jfet seem harder to find, is this beacuse they have few specific uses or are they becoming obsolete? Is thre some killer task that only a jfet will work for, or can they always be replaced with mosfet?
I think I generally "get" bjt and mosfet, and I have designed and built successful circuits with both pnp/npn bjts and p and n mosfets, but for some reason I just cant "see" a practical/useful circuit in a single supply situation with jfets. The circuits I have seen appear to use n jfets without split supplies, so I just dont understand how they work.
Help!