# IH5011 Fet switch , virtual ground switch?

Discussion in 'General Electronics Discussion' started by Davewalker5, Sep 20, 2014.

1. ### Davewalker5

125
0
Sep 20, 2014
IH5011 FET switch, virtual ground switch it says on the datasheets

How is it a virtual ground switch?

When the FET is ON or OFF the diode is always in a parallel circuit which causes current to bleed and sink

take a look at the hand drawings and schematic

File size:
83.4 KB
Views:
274
File size:
87.9 KB
Views:
221
File size:
116.7 KB
Views:
289
2. ### KrisBlueNZSadly passed away in 2015

8,393
1,272
Nov 28, 2011
Can you provide a link to a data sheet?

125
0
Sep 20, 2014
4. ### KrisBlueNZSadly passed away in 2015

8,393
1,272
Nov 28, 2011
For the benefit of anyone else reading this thread, here are the three pages of the data sheet:

I haven't seen this type of device before, so I'll have to make some guesses and assumptions.

It looks like each element is a P-channel JFET, and a diode. A P-channel JFET is turned OFF by applying a positive voltage to its gate (this voltage must be positive with respect to both the source and drain, which are interchangeable). When the gate voltage is roughly equal to the drain and source voltages, it will conduct. This fits with the description in the data sheet that says "A logic "0" turns the channel ON and a logic "1" turns the channel OFF".

As for what it does, and what the diode is there for, have a look at how it's used in your circuit.

The circuit formed by U13 is an integrator, using C35, which provides the negative feedback path around U13 so that U13 can keep point C at the same voltage as point D. Point D is at 0V, so point C will also be at 0V unless the integrator is saturated.

The input to the integrator is a current applied at point C, which comes from three sources: point A via R113, from another part of the circuit via R110, and when the JFET is ON, from point A via R85, which is ten times lower than R113. So the JFET switch is used to increase the scale of the voltage at point A.

The circuit also has a feedback loop marked F. If you describe what the circuit does, I may be able to explain how it works.

As I said before, the JFET is controlled by its gate voltage at point G, which the JFET takes relative to its source and drain. A JFET is symmetrical and its source and drain are interchangeable. To turn a P-channel JFET OFF, the gate must be made several volts positive relative to both the source and the drain. Therefore if the JFET is to be controlled properly, both the source and drain must be prevented from having a significant positive voltage on them. A positive voltage on either the source or drain would reduce the reverse bias caused by the positive voltage at the gate, and the JFET would start to conduct when it shouldn't.

So both the source and drain must be kept from going significantly positive. The right hand terminal, connected to point C, is always at 0V because of the action of U13. But the left hand terminal, point B, will be brought positive by R85 if point A goes positive. This is the reason for the diode, whose cathode is connected to 0V. It prevents point B from going significantly positive, which would cause the JFET to see less reverse bias on the gate and would make it start to conduct.

The diode doesn't affect the operation of the circuit, because point A is a low-impedance point at the output of U10. All it will do is clip the voltage at point B to about +0.6V. Since R85 is such a high value, this will have no effect on the rest of the circuit.

5. ### Davewalker5

125
0
Sep 20, 2014
What you mean by it's symmetrical?

How are the source and drain interchangeable? what do you mean

So the Diode, is a clamping diode?

If the diode wasn't in the circuit , the JFET would prematurely conduct or turn on?

Is a Diode high or low impedance?

I thought the diode was used to SINK the output of U10 op amps output when the JFET is open , turned off

6. ### davennModerator

13,984
2,015
Sep 5, 2009
its a characteristic of the way a JFET is manufactured
look at this drawing ....

draw an imaginary line right through the device in line with the Gate .... what you see on the top side of the line is the same as what is on the lower side that is symmetry ( think ... a line dividing your face from the top of your head , down through your nose to your chin .... left side is an image of the right side = symmetry)

This means that the Source and the Drain are electrically the same

yes and yes
reread Kris's comment on the diode ....
not from Kris's description

you still haven't told us what the circuit is ( what it is out of) and what it's supposed to do

cheers
Dave

7. ### KrisBlueNZSadly passed away in 2015

8,393
1,272
Nov 28, 2011
Most semiconductors, like transistors, MOSFETs etc, are controlled by a voltage or current between two specific pins. Transistors respond to base-emitter current; MOSFETs respond to gate-source voltage. But JFETs are different. The source and drain are just opposite ends of a symmetrical area called the channel.

What this means in practice is that for a P-channel JFET, to turn the device OFF, the gate must be several volts positive relative to the source and the drain. In this circuit, if the gate voltage is, say, +10V to turn the JFET OFF, and either the source or the drain is taken positive and start to approach +10V, the JFET will start to conduct. So to keep the JFET OFF when it needs to be OFF, both ends of it (source and drain) need to be kept from going significantly positive.
You could call it that, yes.
Yes, if U10's output went positive.
That's not really a valid question. You could say that it's high impedance until it starts to conduct, then its impedance drops.
Well, now you know

8. ### Davewalker5

125
0
Sep 20, 2014
So the channel does what? the channel is low impedance

Since a MOSFET doesn't have a channel , what does that mean?

9. ### KrisBlueNZSadly passed away in 2015

8,393
1,272
Nov 28, 2011
The channel is the part of the FET where the drain-to-source current flows. MOSFETs do have channels. Use Google or Wikipedia to learn more.

10. ### Davewalker5

125
0
Sep 20, 2014
What is the difference between a JFETS channel compared to a MOSFETS channel? are they the same or difference or what is the differences?

8,393
1,272
Nov 28, 2011