# JFET as an inverter

Discussion in 'General Electronics Chat' started by Tree1, Mar 27, 2012.

1. ### Tree1 Thread Starter Member

Nov 30, 2010
46
0
Can a JFET be used an inverter ? For a BJT, in saturation(when IC= Isat) VCE is near zero and in cut off VCE is near VCC. Which makes it ideal for use an inverter.

However, for a JFET when ID= IDSS, VDS is either equal to the pinch off value or more. Pinch off value can sometimes be very high according to the JFET. So for example, if a pulse was supplied to the gate that alternated between VGS(off) and zero. At cutoff VDS( output) would equal VDD and at VGS= 0, VDS= VP. So if I was using the output of a JFET as input to another device ( let's say a device that requires at least 3V turn on and assume that Vp = 4), I am assuming that my device will always be on ? I could be very wrong. Please explain. Let's say VDD was 10V

2. ### Audioguru New Member

Dec 20, 2007
9,411
896
Most Jfets are depletion type. They are fully turned on when the gate-source voltage is zero and are cutoff when the gate-source voltage is what is listed on the datasheet.

The VDS-on voltage depends on the load resistance and how much current the Jfet conducts when its gate-source voltage os zero.
Its VDS-off voltage is high when the gate-source voltage is at the cutoff voltage shown on the datasheet.

The pinchoff voltage is the gate-source voltage (not VDS) that causes the Jfet to be cutoff.

Tree1 likes this.
3. ### Tree1 Thread Starter Member

Nov 30, 2010
46
0
Thank you Audiguro for your input, but I am still confused because... Floyd Electronic Devices book describes Pinch-oof voltage(Vp) as the value of VDS at which drain current becomes constant and is equal to IDSS and is always measured at VGS = 0V.

The terminology that uses to refer to the VGS voltage that causes the JFET to be cutoff is the is called VGS(off).

It also says that VGS(off) and Vp area always equal in magnitude but opposite in sign. For example, IF VGF(off) =-5, then Vp= 5V.

This is how I became confused.

4. ### Audioguru New Member

Dec 20, 2007
9,411
896
The gate-source of a Jfet is a junction diode. It is NEVER forward-biased.
Pinch-off is when the drain-source channel is pinched off by gate-source bias voltage so that no current flows.

5. ### Tree1 Thread Starter Member

Nov 30, 2010
46
0
So the book is wrong ? Interesting. Thank you. Thank you very much for the clarification.