Minipulse JFet explanation needed.

[kt315]

nonsense. use BF245. and pls turn off your fantazy.

[asolone]

For singles & duals I like the Vishay DG series. They are cheaper than the Analog Devices ADG series and most are 3V logic compatible. The DG family also has some high voltage (44V) switches which can be handy.
There are also TI's TS5A and TS12A family. And there are the 74XX1G and 74XX2G series that several companies make. And Maxim has their own series. I've pretty much all of these. In SMT layouts they make routing much easier than trying to use a 4066 or 4053. But for breadboard and thru-hole use I stick with the 4066/4053.

can i modify the circuit by replacing the fet j113 with an analog switch type DG403? what advantages would I have?

[6666]

For singles & duals I like the Vishay DG series. They are cheaper than the Analog Devices ADG series and most are 3V logic compatible. The DG family also has some high voltage (44V) switches which can be handy.
There are also TI's TS5A and TS12A family. And there are the 74XX1G and 74XX2G series that several companies make. And Maxim has their own series. I've pretty much all of these. In SMT layouts they make routing much easier than trying to use a 4066 or 4053. But for breadboard and thru-hole use I stick with the 4066/4053.

Been down the rabbit hole looking at the DG series, did not know about them thanks.

[Carl-NC]

My statement is about NEWER better analog switches.

For singles & duals I like the Vishay DG series. They are cheaper than the Analog Devices ADG series and most are 3V logic compatible. The DG family also has some high voltage (44V) switches which can be handy.
There are also TI's TS5A and TS12A family. And there are the 74XX1G and 74XX2G series that several companies make. And Maxim has their own series. I've pretty much all of these. In SMT layouts they make routing much easier than trying to use a 4066 or 4053. But for breadboard and thru-hole use I stick with the 4066/4053.

[waltr]

The 4051 and 4053 chips are dips is there a problem using them ?

No problem using them. My statement is about NEWER better analog switches.
These are still 4000 series although there are newer ones like the HCT version as Carl suggests.

I think the full pn is 74HCT4053.

[Carl-NC]

The 4051 and 4053 chips are dips is there a problem using them ?

The 4053 is the most common demod switch used in VLF designs. If you get the HCT variety they can be directly driven from a 3V micro.

[6666]

There are many newer, better analog switches available in quad, dual and even singles. However, most if not all are surface mount only.

The 4051 and 4053 chips are dips is there a problem using them ?

[surfdetector]

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.

I have done videos of all of the steps for the MPP Rev E which might be of more help for those doing the latest boards.

https://www.youtube.com/playlist?list=PLgBlcFVgN7pQrkCgX23x32pI5m-di2GTf

[waltr]

Aha, The 4066 chip always is a big headache to route the tracks on a pcb. Especially if utilizing all four switches. A nightmare for the hobby man.

Yes, it is. That is why I used JFETs when I built the Hammer Head.

There are many newer, better analog switches available in quad, dual and even singles. However, most if not all are surface mount only.
For a PI with a differential integrator Duals would work in the layout.

[dbanner]

Aha, The 4066 chip always is a big headache to route the tracks on a pcb. Especially if utilizing all four switches. A nightmare for the hobby man.

[Carl-NC]

The down side mentioned is a reason to use an analog switch chip like a 4066. Internal circuits negate these issues.

The 4066 has parallel N & P channel devices, so as the voltage level changes one of them is always low-impedance.

[waltr]

Excellent description of using a JFET for a switch.

The down side mentioned is a reason to use an analog switch chip like a 4066. Internal circuits negate these issues.

[Carl-NC]

The Vgs cutoff voltage is the same as the Vp rating. Transition from linear to saturation occurs at Vgs-Vp. Vp is determined when Vgs=0, and when Vgs=Vp there is no Ids regardless of Vds.

There is an inherent risk in using a JFET for a demod switch. If the source is connected to a fixed voltage (say, virtual ground in an inverting opamp integrator) then the Vgs control will be consistent regardless of the input signal on the drain. But if the JFET source voltage varies with signal level as it does with a differential integrator then Vgs will also vary with signal level, and so will the channel resistance. It may not be a Big Deal in a PI design, and might even be a benefit as the integrator gain will tend to decrease for large signals. It's something to consider, especially if you see weird behavior with large signals.

[dbanner]

There is a difference between Vgs cut off voltage and "pinch off" I think. But more in depth study is required to fully appreciate the characteristics of this ubiquitous voltage controlled device.

[surfdetector]

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

Thanks for that. You presented a very clear explanation.
Cheers