How JFET Shunt and Series Switch Works

JFET(Junction Field Effect Transistor) can be used in variety of application such as source follower or building JFET amplifier. Another popular application of JFET transistor is as a analog switching device and choppers. Here it is explained with worked out example what a JFET switch is, how a JFET switch works and how one can use JFET as a switching device. 

Watch the following video that shows animation of working operation of a JFET shunt switch and JFET series switch.

A JFET switch is an electronic switch which either allows transmission of signal or blocks the transmission of a signal. It works by turning on and off the JFET by driving it either into the ohmic region or in the cutoff region. This is done by applying voltage to the gate such that the gate to source \(V{GS}\) is 0V to block the transmission or above \(V_{GS(off)}\) to allow the transmission of signal. 

The following shows the JFET operation region for using JFET as a switch.


JFET operation region when used as switch

 There are two types of JFET switch configuration called Shunt Switch and Series Switch. 

JFET Shunt Switch

The JFET shunt switch circuit diagram is shown below.

JFET shunt switch circuit diagram

For the shunt switch, the JFET transistor used is 2N5459. The input analog signal \(V_{IN}\) is applied via the drain resistor \(R_D\). The output \(V{out}\) appears at the load resistor \(R_L\) on the other side of the drain resistor \(R_D\) as shown in the above figure. The JFET switch is turned on and off by applying 0V or voltage more negative than \(V_{GS}\) to the gate. 

When 0V is applied to the gate the JFET, the JFET operates in the ohmic region. When in the ohmic region, the JFET switch is ON, that is the switch is closed. When the switch is closed the signal is blocked from transmission. The output voltage is given by,

\(v_{out}=\frac{R_{DS}}{R_D+R_{DS}} v_{in}\)

where \(R_{DS}\) is the drain to source resistance.

Thus to faithfully block the input signal, the drain resistor value \(R_D\) must be much higher than the drain to source resistance \(R_{DS}\). That is,

\(R_D >> R_{DS}\)

In the above circuit a square wave signal is applied to the gate which is high at 0V and low at -4V voltage. The JFET 2N5459 used here has \(V_{GS(off)}\) of -1.2V. So higher negative bias voltage of -4V is used to ensure that the JFET is in cutoff region. 

When the gate is applied with -4V the JFET operates in the saturation region. When it is in saturation region the JFET switch if OFF, that is the switch is open and all the input signal gets to the output and appears in the load resistor \(R_L\). In this case,

\(v_{out}= v_{in}\)


 JFET Series Switch

 The circuit diagram of a JET series switch is shown below.

JFET series switch circuit diagram

In case of series switch, when the gate is applied with 0V the JFET is in ohmic region and the switch is ON or closed. When the switch is closed the input signal \(v_{in}\) appears at the output at the load resistor RL. Thus in this case,


 When a negative bias of -4V is applied to the gate then the JFET is in saturation region and the switch is off or open. In this case the input signal \(v_{in}\) is blocked and the output signal \(v_{out}\) is approximately zero.

The series JFET switch is more effective than a shunt JFET switch. Also a JET switch is more effective than a BJT switch but BJT switch is faster.

In the above circuit diagram we have shown a square wave signal to control the gate of the JFET. But any signal source that can output two level of DC signal can be used for controlling the gate that is controlling the JFET switch. The gate signal for example can be applied using Arduino or any other microcontroller. So in this way we can use JFET as a switching device.

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