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  • Author Author: Jan Cumps
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Simple analog JFET switch - part 1: schema, simulate and test

Jan Cumps

A simple low signal / audio "soft" switch with a p-channel JFET.

It's very easy to make, with common components. Low power.

image

The circuit is published on neatcircuits.com

 

I've ordered a set of transistors to play with. One of them is a ON Semiconductor (Fairchild) J176.

This is a small signal P-channel JFET, designed for analog switching.

 

Let's first look at the circuit in the DC domain, with the switch open.

Both drain and source are kept to ground by the 330K resistorsat input and output.

The gate is pulled to the supply voltage (5 - 15 V). The transistor is open (very high resistance).

 

When the switch is closed, the gate is 1 diode drop above ground and above drain and source.

The transistor is closing and input signal can flow to the output.

 

Typically, you'd use an open collector type digital output instead of a mechanical switch.

Attention: The circuit is a bit rough here, because the full charge of the 1 µF capacitor (used to silence the switch) is dumped through diode and switch circuit.

 

In the AC domain, let's check what's possible:

We can derive the amplitude of the signal that can be switched from the datasheet - a subject for a follow up post.

In this post, let's compare what I see on an oscilloscope with the switch closed (yellow is input, blue is output) with a simulation in LTspice (blue is input, green is output):

 

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.... and with the switch open:

 

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This switch operates in the linear region of the FET.

That's a small part of the operational region where the relationship between VDS and IDS acts like a voltage controlled resistor.

We're using that region in its two extremes: maximum resistance and minimum resistance.

 

Preview for the next post: limits of the input signal amplitude. This is what the system looks like with an input amplitude of 1.2 V:

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The LTspice project is attached.

 

Related blog
Simple analog JFET switch - part 1: schema, simulate and test
Simple analog JFET switch - part 2: linear range and limitations
Attachments:
softswitch_j176_20200621.zip

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  • Today I went to the local brick-and-mortar electronic shop.

    It's the place where I order my element14 components (in Belgium you can only order at Farnell online directly if you have a business - and I love to support local business - and the markup is often lower than shipping costs).

     

    I got some ammunition to do more discrete semiconductor investigations:

     

    image

    Perf board with isolated islands (no patterns. Just holes with a copper pad).

    And a set of 100 bipolar transistors.

    • BC547 / 557 (200 mA), 28 / 28 pieces
    • BC337 / 327 (800 mA), 12 / 12
    • BC517 / 516 (darlingtons), 6 / 6
    • BD139 / 140 (1 A), 4 / 4

     

    I'm not reviewing those transistors - I'm trying to focus on J- and MOSFETs.

    But I do need them in the surrounding circuit. As current sources, current mirrors, ... switches, ...

    For my last project14 basket (I've been on a roll the last months), I ordered a few pure copper clad boards, and more FETs. They will be the devices under test.

     

    A few things i want to achieve, that were underplayed during my education:

    • FETs as analog devices. 90% of the theory and labs were focused on bipolar.
    • P-devices. Anyone and any book shows N-devices almost always. Then they say that P-devices are just the opposite. But my brain does not work like that.
Comment

  • Today I went to the local brick-and-mortar electronic shop.

    It's the place where I order my element14 components (in Belgium you can only order at Farnell online directly if you have a business - and I love to support local business - and the markup is often lower than shipping costs).

     

    I got some ammunition to do more discrete semiconductor investigations:

     

    image

    Perf board with isolated islands (no patterns. Just holes with a copper pad).

    And a set of 100 bipolar transistors.

    • BC547 / 557 (200 mA), 28 / 28 pieces
    • BC337 / 327 (800 mA), 12 / 12
    • BC517 / 516 (darlingtons), 6 / 6
    • BD139 / 140 (1 A), 4 / 4

     

    I'm not reviewing those transistors - I'm trying to focus on J- and MOSFETs.

    But I do need them in the surrounding circuit. As current sources, current mirrors, ... switches, ...

    For my last project14 basket (I've been on a roll the last months), I ordered a few pure copper clad boards, and more FETs. They will be the devices under test.

     

    A few things i want to achieve, that were underplayed during my education:

    • FETs as analog devices. 90% of the theory and labs were focused on bipolar.
    • P-devices. Anyone and any book shows N-devices almost always. Then they say that P-devices are just the opposite. But my brain does not work like that.
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