T- switch question

[tempus]

From an earlier topic, posted by RG: 

Another option you may want to look at is the T switch. Take two JFET switches, and put them in series. Take a third JFET from the junction of the first two, and have it switch from the junction to a BFC to ground (biased up at the bias voltage, of course). Drive the two series JFETs from one phase of a two phase switch signal, and the shunt JFET from the opposite phase. Now when the control signal is one way, the two JFETs are on and the shunt JFET is off, so you only see the Rds of two JFETs in series. Flip the control signal the other way and you have both series JFETs at some very high resistance, but the shunt JFET sucking down any signal that gets through the series JFETs. You can get really huge isolation this way. It's also called a series-shunt switch.

I'm not sure how to implement this. 1st of all, what do you mean 'in series'? Does this mean connect the drain of one to the source of the other? What's a BFC? Also, would this setup be as a replacement for the usual 1 JFET, or would the bypass signal be on one of the JETS with the effected signal another? Sorry if these questions seem stupid but I'm having trouble picturing this.

Thanks

[R.G.]

I'm not sure how to implement this. 1st of all, what do you mean 'in series'? Does this mean connect the drain of one to the source of the other?

1. A series-type switch is a switch in series with the signal. When the switch is closed (that is, low resistance) the signal goes through. When the switch is open, only leakage goes through, as the switch is high resistance.
2. A shunt switch is a switch to ground from the signal path. In effect, a shunt switch "shorts" the signal to ground.
3. A T switch is two series-type switches, one right after the other, with a shunt switch in the middle between them.

So when I say "in series" I mean "take a JFET series-type switch. Now take another one. Hook them up one right after the other so the signal has to go through both of them. Most JFETs have interchangeable source and drain, so any combination of source to source, drain to source, and drain to drain works. Bias the sources and drains up to a voltage which lets you turn them both off by grounding the gate through the normal resistor/cap/diode circuit. Since the two connected pins of the JFETs are both tied to Vbias with a resistor, you can combine that into one resistor. But otherwise, this is just two JFET series-type switches one right after the other, signal going through both. They are both turned on and off together.

The shunt-type switch is a third JFET, connected with its drain to the junction of the other two JFETs and its source connected directly to a BFC to ground and then through a large bias resistor to Vbias. This switch is driven with a control signal that is the opposite of the signal on the two series-type switches. When it is on, the series-type switches are off. When the series-type switches are on, the shunt switch is off.

What's a BFC?

Big Freaking Capacitor. That is, an AC short to ground.

Also, would this setup be as a replacement for the usual 1 JFET, or would the bypass signal be on one of the JETS with the effected signal another?

This three-JFET circuit is a replacement for only one normal series-type JFET switch. If you want to select effect or dry signal, you need two of these, driven out of phase.

What's important here is that one series-type JFET switch may leak. Putting a shunt switch to AC ground after the series switch shunts any leakage to ground. Putting a second series-type switch after that opens the path and reduces any leakage even further.

There is a subtlety here. Shunt switches are justifiably famous for NOT introducing feedthrough or switching artifacts since they clamp the signal line to effectively ground. This tends to eat any switch pop or noise as long as you are not actually trying to switch between different DC levels. If you turn on the shunt switch a microsecond or two before turning the series switches off, this lowers the impedance of the signal line to both sides and the shunt switch now eats any control feedthrough from the series switches. It has the potential to be very quiet and highly isolating.