[SOLVED] Can you please explain how this N-Channel JFET with Diodes work?

[inklen]

Hi!

I've run into this N-Channel JFET with Diodes - IFD89. Can somebody please explain how it works and used? I don't get it why there are 2 diodes in that configuration.

Thank you.

 

[wwfeldman]

a place to start: n channel j fet:
http://www.learningaboutelectronics.com/Articles/N-channel-JFET

the diodes are an opportunity to protect the gate
for example, consider the SOT353 case:
pin 2 is the gate.
by connecting pin 1 to "something", the gate is "clamped" to no more than
a diode drop from the "something"

if you connect pin 1 to the source, pin 5, the gate cannot be more than 1 diode
drop above or below the source voltage

note that on page 2 of the spec sheet, the gate diode voltage is 0.4 to 0.8 V
so if pin 1 is connected to "something" the gate will not go more that 0.4 to 0.8 V above "something"
nor will it go more that 0.4 to 0.8 V below "something"

i don't see any use for the diodes.
perhaps someone wiser than i has a better answer

i suggest you contact InterFET and ask

 

[schmitt trigger]

My first thought would also be gate protection.

The datasheet mentions that "....and is optimized for low power audio applications. The integrated back to back diodes offers clamping and additional gate leakage."

Not sure why one would want leakage.

 

[FvM]

Not sure why one would want leakage.

The diodes may be also used as replacement for a high ohmic bias resistor, e.g. in an electret microphone or pyroelectric IR sensor amplifier.

 

[inklen]

I actually got it from here https://youtu.be/niZizzHBanA?t=436 about 2 months ago. And I didn't understand anything what was said in that video. So I came back to it to see if now I understand it better... Unfortunately, no. I guess I need more time to learn things.

 

[FvM]

To understand the basic amplifier operation, you can place a high ohmic resistor instead of the diodes.

 

[schmitt trigger]

The diodes may be also used as replacement for a high ohmic bias resistor, e.g. in an electret microphone or pyroelectric IR sensor amplifier.

Makes sense....just a minuscule amount of bias!