# DC currents through Rd and Rs in JFET amplifier

#### Vikrama Chandar

Mar 12, 2017
4
Hi i'm an engineering student and i have a doubt regarding the JFET amplifier circuit. I have attached the circuit along with this. My doubt is when the actual DC currents through Rd and Rs are calculated, they are same. How they are same? And is these currents calculated using normal ohm's law or is there any special formula for that?

#### Attachments

• amp14.gif
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#### BobK

Jan 5, 2010
7,682
They are the same because the only other connection to that network is the gate which is very high impedance. This applies only if there is no load connected at the drain. If there is a load, its impedance would have to be taken into account.

Ohms law applies, in that the current through the source and drain resistors causes a voltage drop define by I*R.

The N channel jfet can be thought of as a voltage controlled current sink. When Vgs is at zero volts, a certain amount of current will flow, largely independent of Vds. Negative Vgs reduces the current, positive Vgs increases it.

Bob

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
That circuit is typical of an NPN (bipolar) biasing arrangement.
Since that's a class A JFET amplifier I'm curious why R1 is in there? It would seem counter productive to the negative (GS) bias voltage, needed for an NCHJFET and produced by Source current flowing through RS.
On the other hand there's always a chance that I'm missing something here? I have lots of recipes for Crow.

Chris

#### Audioguru

Sep 24, 2016
3,656
I agree that the positive biasing of the gate must not be used with a Jfet that is a depletion device. The positive biasing is used with an enhancement device like a bipolar transistor and a Mosfet.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
That biasing is fine. Assuming values are chosen appropriately, the source voltage will rise until Vgs is an appropriate negative voltage (source more positive than gate) at the operating point.

Given that all these devices are voltage controlled current sources, it's not surprising you can use similar biasing.

#### Audioguru

Sep 24, 2016
3,656
Electronic-tutorials.com got the schematic wrong but the text is correct.
EDIT: I changed my attachment a little.

#### Attachments

• Jfet common-source biasing.png
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Last edited:

May 8, 2012
4,960
Ditto!

Chris

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