Driving a JFET from a micro

[dbanner]

Can a jfet switch be driven adequately from a microcontroller directly, or is level shifting required as is the case with a mosfet gate.
Reason why I ask is because I always see transistor drivers ( I presume for level shifting?) between the micro and the 4066 switches, but in this schematic, I see old pic driving the jfets directly. I assume this is logic level 3.3 or 5V.

So I suppose Vdd on the micro is connected to gound and Vss is at -5V ??? to allow pinch off of the n-channel jfet.

What's going on here?

[6666]

I think it can, as long as the micro is powered by gnd and -5volts, been going down this path and trying to find out myself, there are a couple of 5v analogue circuits that appear to be able to convert to micro. Thanks for cs7 cct, only seen the old ones with 4017 switching the fets

I just noticed on your cct it says U6 pin5 = -5 v, pin 14 ground

[dbanner]

I just noticed on your cct it says U6 pin5 = -5 v, pin 14 ground

Yes, I noticed that as well. The schematic shows plainly the micro driving the J113's directly, But that's an old pic micro.

[dbanner]

I think I have to look at the Vgs cut off voltage for the j113. At any rate, the Cscope power supply is interesting, the way they set it up.

[Carl-NC]

The CS7 places the battery (+) terminal as ground and then uses a voltage inverter to generate +5V. I think Eric designed the CS7 and this is a pretty common element of his designs. Since the micro is powered from ground to -5V, the I/Os can directly drive ground-referred JFETs. I did something similar with the PI circuits in ITMD.

[6666]

Dbanner I am also getting interested in CD4051, simple addressing, no need for transistor switch's on address lines

[dbanner]

I never thought of it. Only CD4053, this multiplexer one I see all the time.

How would it be wired up?

[waltr]

CD4053 and others have Logic level controls. So a 5V signal will turn the switch on/off.

Check the data sheet.
http://www.ti.com/lit/ds/symlink/cd4...=1588849439700

[ivconic]

I never thought of it. Only CD4053, this multiplexer one I see all the time.

How would it be wired up?

In case micro controller is supplied "regularly" between +5v and GND; than 4053 is supplied between +/-5v and Inh, Vee goes to GND while Vss goes to -5v.
Otherwise if micro controller is supplied between GND and -5V; than i see no problems to switch jfets.
Coincidentally i am working on exactly the same thing these days, so i already checked this.
At present my micro controller is supplied "regularly".
So i am thinking now to make things simpler, supply it between GND and -5V and switch jfets instead 4053, since two fets occupies less space on pcb.
Not sure yet which way is better.

[6666]

I only make PI, and at this point I am preferring the 4051 over a 4053, because its a bit more easier to work with, I have attached a sample schematic of the Atiny4 PI, it uses the 4051 , a micro, and a n channel IGBT

[6666]

Hello Ivconic looks like we have been working on similar ideas.

As I mentioned I only do PI, but looking at different circuits such as MPP and XL500, they both use Jfets and the gate logic IC's (4538's) are powered by ground and minus 5 volts
so I cannot see why a micro cannot be used and powered with ground and minus 5 volts for jfet gate logic and TX pulses. At least that's my theory.

[dbanner]

Yes,well I don't see any better performance to be had. Only a very elaborate way of doing simple things. The chip is underutilized. Ideally you want to use every chip to it's maximum potential, while reducing the number of chips to the minimum required to get the job done.Sometimes you see extra unused opamps on a schematic, or extra switches which are not used.The surf pi is an example of good utilization of chips with no wasted or left overs.Of course this cannot always be achieved, but it should be a design goal to make most efficient use of components in an elegant design with good performance.Achieving your design objectives with minimum components will keep the price point down. This is obviously a consideration on the commercial end of things.

[dbanner]

Let me give you an example of a circuit I revisited recently.

A quad bilateral switch was used in the schematic. Two switches were used for the sample gates, the third was arranged as an audio oscillator, the last switch used to chop back into the audio amplifier.
All four switches were used, none wasted. Now that's what I call a frugal design.

[dbanner]

There's a reason why you rarely see 4051 in these circuits, it is the least elegant and utilitarian way of doing the job.

[ivconic]

Hello Ivconic looks like we have been working on similar ideas.

As I mentioned I only do PI, but looking at different circuits such as MPP and XL500, they both use Jfets and the gate logic IC's (4538's) are powered by ground and minus 5 volts
so I cannot see why a micro cannot be used and powered with ground and minus 5 volts for jfet gate logic and TX pulses. At least that's my theory.

I agree, micro should work quite alright when supplied between GND and -5v.