In this post i will show some detail information of JFET self bias. To operate the JFET we need to apply a technique called as biasing.
Now we are going to see the very most popular biasing method called as the self bias.
We know that JFET should be reverse biased at the gate to source junction.
As we know already that JFET is operated in gate to source junction which is always reverse biased. And here we can notice very easily that the requirement of negative Vgs for an n-channel JFET and positive Vgs for p-channel JFET.
So this can only be possible that is using self-bias arrangements which is shown in figure.
Here we can see that gate remains at 0V because gate resistor Rg, does not affect the bias voltage since it has essentially does not contain voltage drop across it.
Now let us consider n-channel JFET which is as shown in figure.
So in figure we can notice that voltage drop is produced by Is across Rs and it does source positive with respect to ground.
Here because we have Is = Id and Vg =0, then we will get The expression like Vs = IdRs
The gate to source voltage is given as:
Vgs = Vg – Vs = 0 – IdRs = -IdRs
Vgs = – IdRs
Now let us consider for p-channel JFET which is as shown in the diagram that is here mainly current in Rs produces a negative voltage at the source.
Therefore Is = Id
Vgs = +IdRs
Now the drain voltage with respect to ground is determined which is as shown below:
Vd = Vdd – IdRd
Since we have Vs= IdRs
Vds= Vd – Vs
= Vdd – Id(Rd + Rs)
Hence in this post we have dealt about JFET self bias.