#82: How to use a Diode as a Switch / Diodes as Switches / Basics of Diode Switches

we recently did a couple videos here that were walkthroughs of the schematic of a couple of qrp transceivers this MFJ cub transceiver and this Heath kit to hw9 one of the questions that I got most often from both of these videos was about the use of diodes as RF switches here's an example where switches were used for the transmit received path with a cub transceiver and then also in this Heathkit there was lots of uses of switches here in the transmit received path between bandpass filters and the low-pass filter there's another one used here as part of the transmit path and some diode switch is used pretty extensively here in the oscillator board to switch the various crystals and resonant circuits into some hfo heterodyne frequency oscillators for the premix oscillators and also to switch in the appropriate bandpass filters for the pre-mixed signal to go into the first mixer so a lot of people have questions about using diodes as switches because most people think about diodes as rectifiers okay like you know following this typical characteristic forward voltage the current flows reverse voltage no current flows so it's a rectifier so how do we use it as a switch so one thing that is apparent from all these schematics that we did is that we were switching relatively small amplitude signals receivables that before they've got amplified it into the PA and the transmitter and the small signals in the receiver so let's take a small signal view of what happens and it's become apparent of how we can use this as a switch so let's a look at this simple little circuit here where I've got a bias voltage and a couple of resistors to limit the current and and you can see if we as we change that bias voltage okay will reach a point where we turn the diode on and so the current starts to flow or we can reverse bias it and no current flows let's say we establish some DC bias point okay that it will establish an operating point on this curve now if we apply a small signal say at the input here that's really going to just represent a small variation okay of the is point okay that small variation that bias point will result in a small variation in current so that small variation in current it will also allow that voltage to kind of appear on top of this resistor here so that essentially that small voltage that we applied basically just appears over here and the only really change that we've made is we've it's been shifted down by the diode voltage but the signal is still there now if we turn this bias voltage off okay see right down at this point and then we applied that small signal again that signal is so small that it doesn't reach the turn-on voltage and therefore there's no change in output current so essentially that signal gets blocked so that's like opening the switch so we can see you know quite clearly even if we reverse bias it same kind of a thing so as long as the signal is small with respect to the bias point we can use this diode as a switch to switch these small signals okay and again that was the application and the qrp transceivers okay let's take a little bit closer look at some of the considerations that you need to look at when you think about this for dealing with RF signals or higher frequency signals okay so typically in RF applications a lot of times rather than using resistors to kind of block the signal from going and being sunk into the bias voltages and things like that a lot of times RF chokes will be used or a resistor or sometimes a series combination because they still might need a resistor to set the current but use an RF choke so that that allows or blocks the RF from going up and down the bias lines okay so a lot of times both will be used and also for RF application typically the signals are EC coupled in so we don't really care about the diode drop okay so many times also the bias through the through the diode might be controlled from one side or the other okay and for these small signal or RF applications and again small signal being where the RF voltage are if current is small with respect to the DC bias of the diode the diodes that can be used are often just just about anything switching diodes like these one and 9:14 a s or 4148 s or even powered it's like a 1n4007 okay they could all be used so let's take a look at that so I've got a signal coming out of my signal generator here at 455 kilohertz I picked that because it's a common if' frequency you'll find in in transceivers that's being coupled into my circuit board here little proto board so that's coming in through this capacitor on top of this diode right here this is a 1n4007 I've got a DC voltage being applied through this resistor I think I picked out of the junk box here what does that look like a 390 ohm resistor okay there's my 1n4007 and there was another 390 ohm resistor okay that's being going to ground I've got another capacitor coupling that signal out to this guy which is going off to the scope all right so if we take a look my my signal line of the signal generator is on okay is it 50 milli volts peak-to-peak so it's a relatively small signal and if I kind of hold the camera right about here I think we can capture both the the meter here which is reading current through the diode I'm on a 10 milli amp scale here so we'll be able to read the meter and look at the the signal on the scope if I reach over and grab the power supply here okay I turn the power supply voltage up we can actually see the signal come up on the scope and I can see I'm just running it there's about two milliamps of forward current and there's my my 455 kilohertz signal on the scope okay as I turn the current up we can see that going up on the meter we can see the amplitude kind of increasing slightly on that but once we turn that diode on it sits pretty well on we get down to very low current so we'll start seeing the amplitude on the scope dropping okay and it'll get distorted because we're just barely turning the diode on at that point but we can now see that once we get the diode turned on to some kind of DC level you know I've got that signal passing through just fine so by simply controlling that power supply to turn the diode off or turn the diode on that's all I'm doing is just adjusting this power supply on and off okay we can essentially switch that RF signal on or off so that works just fine now I mentioned that you can kind of use you know any of these different types of diodes I've got this four thousand seven in here one way to think about it is that the diode and the off state kind of looks like this there's a little bit of resistance and lead inductance but it's pretty small and it's typically a relatively small capacitance when the diode is off that's really what it looks like for these you know the switching diodes like the little nine fourteen or 4148 that capacitance might be on the order of one or two Pico farad's but for a larger like you know power rectifier like a four thousand seven that might be you know forty or fifty Pico farad's or more okay so you might have to worry about that because depending on what frequency you're operating at that might make a difference essentially when we turn the diode on what you have is basically you're essentially shunting this diode with the resistance and that resistance actually is proportional to the current right because the resistance is really just the slope of that IV curve okay is one way to think about it okay for a small signal kind of approximation here so so where can that be a problem in terms of this this capacitance here like if we went to instead of 455 kilohertz here let's say I changed the signal frequency here from 455 kilohertz to say eight point eight three megahertz okay that's another common if' frequency if I look over at the scope or the meter I've still got this thing off there's no current going through this diode right now but you can see I've actually got a small signal going through there okay I turn the bias up okay that comes on but even when I turn that the bias off and turn the current all the way all through that diet I'm still getting some current going through that diode or assuming some signal going through that diode because the capacitance is a little bit too large now if I simply replace this diode with was little switching diode to me yank this guy out of here okay now they've got a switching diode sitting down here on the table and let's get the polarity right and kind of put him in in the circuit okay so now with him in the circuit with the bias turned off I see no signal at that eight point eight three megahertz and if I turn the bias up on the diode now there it is now I can go see that so so for these higher frequency applications you want to use a diode that has got a lower off state capacitance like the little switching diodes okay but all of this has been for these small signal applications we're going to talk in a moment about how to get around that and use pin diode switching for larger signals but first let's take a quick look at some configurations for switches we've been doing this very it was kind of a full of series switch where the switching element is in series with our RF path you can also do a shunt switch we're essentially turning the diode on will short out the signal itself okay now some of these may have better isolation and different applications can also do a compound switch we have our series element at a shunt element and depending on you know whether you want the path to be on or off you can actually maybe get more isolation between the input and output make this make it a better switch now part of the problem like we said is that all this is really for kind of small signal applications where the RF current is smaller and because we've got to kind of adhere to what's going on here we don't want to you know have that art that RF so it'll be large enough to actually turn it off but there's another thing that we start considering when it comes down to RF applications and that's something called carrier lifetime or reverse recovery time any time that a diode is turned on there's some charge it gets stored in the junction and in order to turn that diode off that charge has to be removed okay so and so even if you instantly turn the bias off in the diode there's still some charge that has to come out before the diode actually gets turned off so that will affect the ability or the operation of the circuit but also has some some interesting implications where we start talking about a different type of diode many times when you talk about RF pin or RF diode switches you talk about something called pin diodes okay a P I n n diode actually stands for kind of the structure of the semiconductor censoring it because normally a junction diode is the PN Junction kind of together okay but a PN a P I n diode has a an intrinsic or very very lightly doped region in between the p-type and n-type materials that actually has some advantages for us and takes away some of the limitations of some of the other PN junctions when we use this as switches so this intrinsic region will actually store a lot of charge okay and obviously the again we the diode won't get turned off until all that charge is removed and that charge did remove that charge takes a while so the carrier lifetime in this type of diode is actually quite long meaning it has a long reverse recovery time so what that means is that this type of a diode a pin diode is really a bad rectifier right because it won't turn off very quick okay but as a switch it might turn off very quick because you think about switches that they don't have to operate as fast as a as a rectifier but there would be a lousy rectifier but but having this intrinsic region gives us some advantages here because if you don't give it enough time to pull all that charge out the switch doesn't turn off so at high frequencies there isn't enough time to remove all that charge so the really the significant thing here that makes this really useful is that we can use this as a switch for RF currents that are much greater than the bias current I mean real common application might be a pin diode that you bias on with say 50 milliamps and that could be used to switch RF currents of an amp or more okay so that certainly greatly exceeds what we saw on that IV curve but it's all because of the store charge the pind out so or in the API end structure so that makes it really useful for RF switches even for RF power switching but it's only typically useful for VHF and above because the carrier lifetimes while being slow are not slow enough to work down at the lower HF frequencies so you I mean it's kind of tough to make a PN diode that would work at lower HF but get up upwards of 30 50 megahertz or above pin diode switching has actually worked really well even for power applications okay now there are there are diodes you can get that work for HF and there are amplifiers that use them and it's just that there are pretty expensive okay so the other thing that's kind of nice is that the pic the intrinsic region and the doping that's applied to the p and n are designed in such a way to make the resistance okay remember we talked about this this being kind of a current controlled resistor the current controlled resistance is actually quite linear in the P I M so in some sense this looks like a current controlled resistor so we can get by controlling how much current we pass through the P I enter we can control essentially the resistance that's seen by the RF path going through it so not only can we turn the diode on or off to make a switch we can also use it as a variable resistor which means we could do things like make attenuators modulators phase shifters things like that by simply adjusting the bias through a pin diode to change the resistance and then use that change in resistance in the circuit to create these various types of functions so anyway I hope this kind of helped you to understand how diodes can be used as switches whether they're a small signal RF switches like we looked at here and some of those schematics from the Q R P transceivers and then where this whole thing called pin diodes fits into the picture I wish I had one here to play with but I don't so but we're these P I end I uh got some advantages of being able to switch higher power levels and having this kind of linear resistance versus bias current characteristic that can be really useful in RF circuits so thanks again comments and questions are always welcome thank you

Channel: w2aew

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Keywords: W2AEW, Tek, Tektronix, RF, diode, diodes, PIN, PIN diode, switch, attenuator, modulator, radio, QRP, Heathkit, MFJ, transceiver, transmit, receive, tutorial, basics, bias, RF choke, choke, current, voltage, Attenuator (electronics)

Id: YBNIq_d56sA

Length: 15min 3sec (903 seconds)

Published: Wed Feb 27 2013