Mosfet current Buffer - Help Needed! - Page 1
Forum section: electronics / rf-microwave

author: JhonStan
author_rank: Contributor
timestamp: October 30, 2022, 01:13:36 pm
content: Hello!I designed this current buffer because I should use it as a final stage to drive low impedance loads with an almost completely linear phase (10 ° phase shift at 100Mhz). The problem is that from the measurements made in simulation, this circuit needs +/- 200mA to be driven correctly but the stage to which I would like to connect it can only generate 10mA so the output gain would be much lower than 2. If possible the question is the following: How can I supply the necessary input current (+/- 200mA) if the stage to which it will be connected can only supply +/- 10mA? Because I had an idea of the current buffer that would allow to deliver more current by driving it with a lower quantity but obviously I was wrong. Thank's!

author: boB
author_rank: Frequent Contributor
timestamp: October 30, 2022, 04:55:51 pm
content: Driving this at 3V peak, you will be luck to get much out of it, if anything...The driving voltage must first of all, exceed the Gate-Source threshold of ~around~ 3V. Yes, you have diodes to biasbut I suggest using negative feedback to take care of non-linearity as well as crossover dead spot. SOME bias is good but at 100 Hz, the feedback should be able to take care of it all.The op-amp can be powered by the +/- 12V too but you may only get around +/- 9V output swing.boBPS, you might just DC couple it instead of using so many capacitors ?

author: fourfathom
author_rank: Super Contributor
timestamp: October 30, 2022, 05:56:23 pm
content: Quote from: boB on October 30, 2022, 04:55:51 pmThat's 100MHz.

author: JhonStan
author_rank: Contributor
timestamp: October 30, 2022, 06:18:58 pm
content: Quote from: boB on October 30, 2022, 04:55:51 pmHello, so by using an OpAmp for negative feedback? I think is possible but i'm asking myself the possibility to do it with discrete componente instead OpAmp.Quote from: fourfathom on October 30, 2022, 05:56:23 pmI hope isn't a problem doing in simulation.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: October 30, 2022, 06:22:27 pm
content: M4 is wrong way around, swap source and drain.Diodes wont bias on a mosfets you need about 3-4 volts to bias on a mosfet.Probably best done with a Vbe multiplier.

author: JhonStan
author_rank: Contributor
timestamp: October 30, 2022, 07:08:03 pm
content: Quote from: nigelwright7557 on October 30, 2022, 06:22:27 pmIf i swap source and drain the circuit doesn't work. I attached the file ,if you use LTspice you can try it.Download Buffer.asc

author: boB
author_rank: Frequent Contributor
timestamp: October 31, 2022, 01:47:34 am
content: Quote from: JhonStan on October 30, 2022, 06:18:58 pmYeah, sorry. I thought it was 100 Hz. I saw what I wanted to see to make it work !This is an RF amplifier issue then.More bias and possibly some passive L-C filteringboB

author: Odysseus
author_rank: Regular Contributor
timestamp: October 31, 2022, 02:20:52 am
content: 1) It is actually M3 which is backwards. As drawn, M3 is just acting like a diode.2) After fixing M3, you'll need to replace D1 and D2 with a ~10 V zener or VBE multipler to forward bias M3 and M4.3) The IRF510 has ~100 pF gate-to-drain capacitance when forward biased, and the IRF9510 is probably similar. So there is 200 pF to ground at the input of your amplifier, which is only 8 Ohms impedance at 100 MHz, hence the large input current. Basically, these devices are almost unusable for broadband use up to 100MHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 31, 2022, 03:56:10 am
content: Yeah M4 is obviously wrong; did you even check bias current, it must be some amperes!Simulations can be tricky to get into. You need to be careful to mind the intended limits of your circuit, including what seem like fairly obvious things, like biasing.A real circuit will smoke and burn if you wire it wrong. SPICE happily continues on its way, feeding you -- not bullshit numbers, but simply what the model says it would be -- it's a computer, not a circuit, it doesn't know anything about real circuits beyond what you've put into the model. GIGO.Check a good selection of things to be reasonably sure it's doing what you expect it to. Ever had a real op-amp generate 10kV? Or a battery generate 10kA? Probably not! But certain sim models gladly will. This isn't even ludicrous, necessarily: it's just that you need to use models appropriate for the purpose. Such models (3-terminal "general" or "linear" op-amps, or reasonably ideal voltage sources), are useful in certain contexts -- they're very simple and can be used to quickly capture a certain idealization of a circuit being modeled -- but can't be used to represent the totality of behavior of a real device!It's a learning curve, and this here is a good example of the differences between real and virtual worlds.Quote from: JhonStan on October 30, 2022, 07:08:03 pmIt won't workwithout other changesbecause the bias voltage isn't enough!Stack up more diodes, or better yet use a Vbe multiplier, as indicated.Vbe mult. is effectively an adjustable diode: take a BJT, emitter as cathode, collector as anode, resistor divider from C to B to E.You will need to adjust the Vbe ratio in practice because Vgs(th) has a wide manufacturing spread (+/- a volt or so). In the sim, just trim one resistor until bias current looks right. In practice, use a trimmer resistor for the bottom one.The other glaring problem here: the low impedance source. Of course it shows near unit gain and low phase shift -- the 0.1 ohm source is able to brute-force its way through all that capacitance. You aren't amplifying anything, but you will indeed get a low output impedance from this. Not lower than the source, mind; or, more specifically, not with any current gain.Normally we model RF circuits out of 50 ohms, as that's convenient for transmission lines (50 ohms into BNC, SMA, etc.) and test equipment (most general electronics lab equipment is 50 ohms; video stuff, mostly 75).So, the source should have 50 ohms in series, and the load might also want to be tested with 50 ohms, or maybe with a range of impedances.It might also be worth checking it in reverse, i.e. see how much power reflects off the output, or couples back to the input (isolation).In general, we can model an amplifier as a two-port, a matrix of 4 coefficients (input reflectance; forward gain; output reflectance; reverse gain or isolation). These correspond to input and output resistance and impedance, and gain and feedback, but generalize it so that we don't need to construct an equivalent circuit, just measure the values and do a little matrix math to work with any system we need.That said, we can make some observations:- IRF510 is essentially dead weight at 100MHz.This might not be well modeled, actually. Most MOSFETs have a diffusion characteristic, which is to say, as the gate voltage spreads out over the die (say from a step change in terminal voltage), it kind of "soaks" into the structure, going through distributed resistances and capacitances until it finally covers all corners of the die.Older designs like IRF510 have a pronounced diffusion characteristic, meaning gain goes as ~1/sqrt(F) above cutoff.This also means, not just the gate circuit, but the drain circuit as well (and the relatively significant overlap between both!), tends to be rather lossy. So, not only is the gain dropping, but you're losing a lot of signal in the chip itself, in the process.They can still be effective in the SW band (up to low 10s MHz), but by there, capacitance is so dominant, and gain dropping off noticeably, that it's not economical to push higher.It might still have gain at 100MHz, say, but it's not that you want to actually use it up there: the impedance will be so low, the gain tiny (<10dB), available maximum power small (<10% of bias?), and the package strays (lead inductance) almost impossible to deal with (it might be impossible to keep from oscillating!).Proper RF transistors have compact designs with low-resistance connections, so the gate voltage goes from terminal to active MOS elements with little loss or excess capacitance. In particular, Cgd is lower, which further helps with isolation factor and stability.Newer power transistors tend to have a more RC (single pole) dominant sort of response, I think usually a matter of fractal connections (i.e. a wide interconnect spans the width of the die, then branches into many thinner interconnects, that branch further into myriad strips of actual active MOS cells). They're still unsuitable for RF purposes (capacitance is highly nonlinear, and still dominant so that signal bandwidth is only some 10s MHz), but this all contributes to somewhat higher bandwidth as an RF amp, and improved efficiency at lower frequencies (particularly the switching application they're designed for).So, among common parts (and especially among anything that can reasonably be described as "complementary"*), there really aren't a lot of things you can use here, anyway; something like 2N7002/BSS84 is too small (low power dissipation), and already too high capacitance (drops off in the 10s MHz again).*BJTs are much more complementary than MOSFETs, incidentally. MOSFETs are directly limited by electron/hole mobility, which is about 2.5 times worse for P-channel in silicon. Normally, the P-ch complement is made twice the size of the N-ch, so its capacitance is double the N-ch's, and Rds(on) only a little higher, giving an acceptable compromise. So, this is especially annoying for RF purposes where we'd love to have complementary parts, but it just can't happen.RF MOSFETs comparable to jellybean BJTs are long since obsolete, and what counts as "RF" these days is up in the GHz (and you won't ever find P-ch among them). Even RF BJTs are getting rarer and rarer; the old standby 2N3866/2N5160, despite the antique metal can and boutique pricing, are still surprisingly promising for this sort of application. MMBTH10 and 81 are still, mmmh, mildly available I guess, but also quite low power. BFT92 are now obsolete.- Strays can be modeled, at least roughly, by building a circuit, then estimating the node capacitances and stray inductances of it. Figure every mm of wire is about a nH.Note that even minimal lead length TO-220 incurs about 5nH to begin with! SMT can be better (stray inductance rank: TO-247 > TO-220 > D2PAK > DPAK > DFN) but you'll still be screwed on the capacitance of that part.- Input isn't 50 ohm matched, making this useless for practical purposes.To be more specific, if its impedance were unmatched, butvery high, that would also be okay, it can then be used as a probe -- see JFET probes for example. But IRF510 will most definitely have too low an impedance. And it's not even that it can be resonated out, due to the high losses mentioned above.Tim

author: profdc9
author_rank: Frequent Contributor
timestamp: November 03, 2022, 04:55:12 am
content: This is a RF amplifier and requires a RF transistor. Some new, easier ones to work with are the MRF101 and MRF300. These can go up to 250 MHz.You can look on page 25 for how to build an amplifier between 87.5 to 108 MHz:https://www.nxp.com/docs/en/data-sheet/MRF101AN.pdfhttps://www.nxp.com/docs/en/data-sheet/MRF300AN.pdfThe output impedance of the MRF101 example is about 13 Ohms. The MRF300 example is about 4.5 ohms. The drain output impedance is going to be approximately R = V^2 / P where V is the drain voltage and P is the output power. You could use an output matching network to transform the impedance to the necessary value. Similarly, you can feed the amplifier with a relatively high 50 ohms impedance and use a matching network to transform it down to 4 ohms, which is about the input impedance of the gate.Note that with the TO220/TO247 package, you will absolutely have to minimize source lead inductance for the common source configuration. In the RF package, the drain tab or body tab is the source, and so you can ground the heatsink and connect the transistor to it without an insulator, for example, a copper plated aluminum block that you can solder the package to. You can study the layouts in the data sheet and try to copy them for best results.