Re: Analog switch methods

In thinking about oscilloscope sections (see
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> background), I want a high bandwidth (out to about 20MHz) SPDT switch: > something a little faster than a relay, maybe do a clean 1MHz with > corresponding rise time (under 50ns would be good). > > So there's the classic diode switch, ala fast sampler stuff. =A0Foward > bias FWB with CCSs and the AC terminals become connected (within > capacity of the CCS). =A0Reverse and it opens up. =A0Easy enough. =A0So p=

in two (that's eight diodes), and alternate their current sources for > the SPDT action. =A0And to do that, I can use diode gates (counting 16 > diodes...) and a complementary square wave, which can easily be made > with reasonable rise time (down to 30ns or so from mere 2N3904s). =A0And > with just a pF or so between the diodes, isolation won't even be too > bad up at 20MHz (figuring a line impedance of 50 ohms I think). =A0Takes > an awful lot of diodes though, and I wonder about linearity -- it's > that first 0.5V that needs to be beautiful. > > JFETs are classic switches. =A0Not usually too conductive, so I'm > thinking moderate impedance (500 ohms-ish?) at which point Cds would > be worth making a "T" switch. =A0And that's fine, the ground part is > switched inverse to the signal parts, and there's two of them (a tee > for each input) that are switched inversely, so I need the complement > anyway. =A0Switching hooks up about the same as for the diodes, maybe > even using current sources (that's six of them...) to supply the gates > for faster response (a current sink on the output compensates for the > gate bias). =A0MOSFETs are just as classic, but transistors with > substrate taken out to a pin are harder to find.

SD214 if anybody is stll making it. The series ran from SD210 to SD214, but haf of them had protection diodes on the gate, which could be inconvenient.

Or there's the BJT switch. =A0Little known, and rarely used (I only know > of one "invertible" transistor, the 2SC2878, of which I have two). > But surprisingly effective, having MOSFET-grade "Rce(on)" specs (C2878 > says about 5 ohms at Ib =3D 10mA), low enough to couple 50 ohm lines. > Capacitance is a bit high though, not really giving enough isolation > at 20MHz, even at 50 ohms. =A0Maybe some smaller transistors would work > here (C2878 is some 400mA Ice(max)).

Any old transistor can be used as an invertign switch, but the 5V reverse voltage rating of most planar transistor base-emitter junctions is inconveniently low. Broadband transistors are even worse at 2V but some of them do have very low capacitances.

-- Bill Sloman, Nijmegen

Looks lovely (although I'd like less Rds(on) -- I suppose it's a compromise between high Cds and high Rds though, so I'd have to cut my losses with a tee switch).

Surprisingly SD214 shows up at Mouser, with "Lead-Time 27 Weeks" and the price, "48: $182.70". No, thanks...

Searching Mouser, I see some transistors with Veb =3D 10V or so, which are rated for switching duty (Ron < 1 ohm at good Ib). I don't know how, but the 150mA type has 20pF Cout at Vce =3D 5V whereas the 500mA type has under 10pF...umm okay... And I suspect the 2SC2878 doesn't exist anymore.

Come to think of it, there's a lot of unused base charge in one of these switches, isn't there? If you're using it at hFE < 1, isn't it going to take a while to stop being a switch? And which BE junction does the charge go into, anyway? It's kind of both, depending on which way the signal is... if it's swashing back and forth, that must make some awful time constants and nonlinearity, right?

Tim