Can I do a JFET level shifter?

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In another thread, I was wondering if this fairly standard level shifter circuit:

 

mosfet_level_converter

could be made with a JFET instead. Why? No practical reason, just curious. Unfortunately I don't have JFETs around, so I can't build the circuit and test it:

 

JFET_level_shifter

So, anyone has any opinion about this?

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What gate voltage (relative to source terminal) is required to turn the JFET off?

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The MOSFET designs use BSS138, which has a threshold of 0.8 to 1.5V, input capacitance 27 pF, output 13pF.

I looked up a few JFETS, PF5102 seems fine, it turns of at -0.7 to -1.6V, seems to have similar capacitance and can handle a few mA current. So it should be more or less equivalent, no?

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As shown in post #1, the 2N7000 MOSFET has an intrinsic diode from source to drain.

Thus when the Hi side is at ground, it pulls the Lo side down to logic low.

 

For the JFET circuit, when both sides are high and then the Hi side goes to ground,

how does this pull the Lo side to ground?

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Well, some JFETs are symmetrical, so it doesn't matter. Or I could add a diode.

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Your NJFET gate must have higher voltage with respect to it's source and to be above threshold voltage level

to form it's channel and conduct. Because after NJFets, the MOSfets produced, they have better characteristics and so used.

What kind of "shifting" do you want to do? At what frequency?

 

 

 

 

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Ntano wrote:

Your NJFET gate must have higher voltage with respect to it's source and to be above threshold voltage level

to form it's channel and conduct. Because after NJFets, the MOSfets produced, they have better characteristics and so used.

What kind of "shifting" do you want to do? At what frequency?

 

JFETs conduct when gate = source, and close when gate < source. They are "normaly on devices", "depletion mode", they do not work exactly like MOSFETs.

I don't have any problem I need to solve, actually. I already built a "normal" level shifter with MOSFETs and it works fine at 500kHz, the frequency I need. So, problem solved.

I'm just curious, I know MOSFETs are always used and want to know why. They are cheaper, that's certainly one reason. People say they perform better. Maybe. I want to test it or hear it from someone who has.

 

I understand most people here are engineers and think in terms of problems to be solved, but I'm an academic, some times I want to test things that are not actually practical problems, just want to see what happens.

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Note: you can get mosfets in enhancement or depletion mode varieties. The more common are enhancement. The configuration is a common gate amplifier. A bipolar transistor would work except for the b-e junction diode.
Get yourself a spice simulator and see for yourself. Ltspice seems a popular choice.

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Ok, so I decided to make a simulation. I prefer Tina TI instead of LTSpice, so here are the results:

 

1) standard circuit with MOSFET, this design uses BSS138. I'm simulating an open drain/collector signal source with a voltage controlled switch:

 

MOSFET_level_shifter

 

Simulation results with 1us pulses:

 

MOSFET_level_shifter_simulation

 

 

 

2) Equivalent circuit with 2SK2394 JFET:

 

JFET_level_shifter

 

Simulation results with 1us pulses:

 

JFET_level_shifter_simulation

 

 

Everything looks fine with the simulation, in fact much better (faster switching) than the BSS138 standard circuit (I'm sure there must be faster MOSFETs). But simulations are not reality, and I don't have suitable JFETs to build this and try. Suitable JFETs need 2mA minimum "on" current and no more (less, actually...) than -2.0V cutoff voltage.

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That looks like a very nice result.

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