RF power amp that can withstand infinite VSWR
RF power amp that can withstand infinite VSWR
(OP)
Hey all. I got roped into building an RF power amplifier for a physics professor friend and I wanted some second opinions on which way to go with the design. The amplifier needs to deliver up to 300Vpp and 100W across the frequency range of 0.1 to 1 MHz into a purely capacitive load (~200-1000pF), preferably without tuning...
Much as I'd like to, I don't believe I can get away with whipping up a quick-and-dirty push-pull Class B out of some spare SMPS parts. It seems like I've only got two choices here, but I'd love to hear otherwise:
1) Single-ended Class A (with either a choke or a cascode current source feeding the drain to at least get 25% eff.)
2) high-speed current feedback op-amp driving a FET voltage gain stage then a FET current gain stage (ala EDN's Design Ideas from April 26, 2001).
The distortion requirement isn't too critical, so that's a plus, at least. Any ideas or comments would be most welcome.
-Jeff
Much as I'd like to, I don't believe I can get away with whipping up a quick-and-dirty push-pull Class B out of some spare SMPS parts. It seems like I've only got two choices here, but I'd love to hear otherwise:
1) Single-ended Class A (with either a choke or a cascode current source feeding the drain to at least get 25% eff.)
2) high-speed current feedback op-amp driving a FET voltage gain stage then a FET current gain stage (ala EDN's Design Ideas from April 26, 2001).
The distortion requirement isn't too critical, so that's a plus, at least. Any ideas or comments would be most welcome.
-Jeff





RE: RF power amp that can withstand infinite VSWR
RE: RF power amp that can withstand infinite VSWR
RE: RF power amp that can withstand infinite VSWR
Power MOSFETs can get to the speed, voltage and power you need. Look at the offerings from IXYS.
http://www.ixys.com/powerproducts.html
I think what you need to build is a higher frequency audio amplifier without needing to consider crossover distortion. I have designed a couple for linear servo amplifier applications with a p-p amplitude of 100V and a 1 MHz bandwidth.
RE: RF power amp that can withstand infinite VSWR
RE: RF power amp that can withstand infinite VSWR
RE: RF power amp that can withstand infinite VSWR
RE: RF power amp that can withstand infinite VSWR
RE: RF power amp that can withstand infinite VSWR
I believe a BTL configuration would be ideal, but a sanity check would be appreciated. The load I'm trying to drive is an "ion guide", which is basically a stack of plates where each is driven 180 deg. out of phase to the next. It seems, then, that it would be logical to drive all of the even plates, say, with the non-inverting half of a BTL amp while all of the odd plates are driven by the inverting half. Sound reasonable?
Keeping in mind, though, that the amp will need a slew rate of ~2kV/uS to be able to deliver 300Vpp at 1MHz... Hence why I was thinking of using CFB op-amps in the feedback loop.
RE: RF power amp that can withstand infinite VSWR
RE: RF power amp that can withstand infinite VSWR
That opens up a few more possibilities for you. They may be long out of fashion, but vacuum tubes can still be useful sometimes for some unusual applications. They make quite good high frequency current sources with few worries about voltage transients destroying things.
RE: RF power amp that can withstand infinite VSWR
Warpspeed - This was precisely my initial plan: whip up a simple broadband transformer coupled push-pull amplifier with cheap off-the-shelf SMPS mosfets. Unfortunately, along the way I read that Class B and C biased amplifiers hate driving pure reactance loads because all of the output power is reflected back to the active devices where it results in either 2x or 4x the normal dissipation (depending on the circuit, etc.).
The one thing that really has me stumped here is that I can't determine at what frequency/power range one should abandon an "audio amplifier" style circuit and go with an "RF" one. Clearly, an audio amplifier could care less if it is asked to develop maximum output voltage with no load attached whereas this is not often true of the RF amp, yet, one need not be concerned with slew rate in an RF amp as long as the proper impedance matching network is used at the input and output... It's enough to drive a man to "drink the kool-aid", if you know what I mean.
IF the solution to using a Class B RF design is to simply up-size the heatsinking and double (quadruple) the voltage and/or current margins for the active devices then I'd consider such excellent trade-offs compared to either requiring 4x the power supply wattage or the vastly more complicated circuitry required to achieve 2kV/uS with an audio amplifier style circuit.
Once again, comments appreciated.
RE: RF power amp that can withstand infinite VSWR
The input power is the supply voltage times the average current. The average current is 1/pi x Ipeak.
Iave = 1/1.57 x 1 = 0.637 Amps
Pin = 150 VDC x 0.637 = 95.5 Watts
This is split between the two output transistors so each has to dissipate 48 Watts.
The peak power dissipationis is also high at about 200 Watts for each transistor but at 0.1 to 1 MHz the transistors will probably only care about the continous power.
RE: RF power amp that can withstand infinite VSWR
One application that has just come to mind is the driving of large electrostatic loudspeakers to several hundred volts peak to peak. While your drive current will be much greater, the concept is probably not that much different.
In the past this has sometimes been done by connecting the speaker directly to the anodes of a tube type push pull amplifier, where the existing center tapped output transformer was terminated in a nominal high power load resistor. Negative feedback from the transformer secondary kept distortion reasonably low, and the load resistor damped the transformer inductance and any resonance. It may not have been ideal, but it was practical.
RE: RF power amp that can withstand infinite VSWR
* a short
* an open
* a pure capacitance
* a pure inductance
- and this mismatch results in all of the forward power being reflected back to the active devices (less losses in cables, etc...) where it results in twice as much dissipation for a Class B amplifier but no change in dissipation for a Class A amplifier (according to vendors of amplifiers for EMC compliance testing)
Hence, why there is much pulling of hairs and gnashing of teeths around here...
Warpspeed - you are truly a master of understatement... ;)
Designing the feedback loop for an amplifier that is unconditionally stable at up to 1MHz while pushing significant amounts of power around will not be trivial, hence the near-resignation to just live with wasting 3W of power for every 1W delivered to (then immediately returned by) the load...
The unbelievably dedicated people/nutcases at DIYAudio.com attempt to design an amplifier capable of directly driving EL speakers every so often but my last search of their archives didn't turn up anything promising. Seems they all end up resorting to a transformer coupled output - which is fine, mind you - but even then their designs tend to blow up with distressing frequency (probably from not being compensated at all...).
But hey, if this were an easy problem to solve I wouldn't be begging for help on forums such as this one!?! :)
RE: RF power amp that can withstand infinite VSWR
A whole bunch of high voltage mosfets might work, to physically distribute the horrific power dissipation. Your idea of a cascode (common gate) output circuit, would certainly help with the Miller feedback problem.
RE: RF power amp that can withstand infinite VSWR
1) A Class A amplifier should be rejected due to power dissipation in the drive transistor.
2) A Class C amplifier only works with a tuned circuit at one frequency.
3) You are left with a "Push-Pull" class B amplifier as the only viable alternative.
4) Driving a capacitive load may have some unique problems;
a) Heat dissipation; you have recognized this and are
dealing with the problem.
b) Instability; caused by too much phase shift from
feedback at the amplifier output. If this is a
problem it can probably be solve with a small
resistor in series with the output.
I've taken this week off work. If you send me your email address to stevelreid-at-aol-dot-com I'll try to draw up a rough schematic of what I think will work.
RE: RF power amp that can withstand infinite VSWR
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: RF power amp that can withstand infinite VSWR
This will be a high frequency current source. What the voltage across the load does is of no real interest beyond any required peak voltage clamping. A current driver like this should be able to work equally well into a dead short.
RE: RF power amp that can withstand infinite VSWR
RE: RF power amp that can withstand infinite VSWR
[IMG]http://i1.tinypic.com/6jqel9i.jpg[/IMG]
RE: RF power amp that can withstand infinite VSWR
The opamp provides the gain for the amplifier. R1 sets the bandwidth; make it bigger until you get the bandwith you want. C1 makes the gain (20, 1k input res and 20k feedback res) exact.
The PNP level shifts the signal to the negative power rail.
The NPN level shifts the signal to the Gate drivers.
The key to this circuit is the +/-15VDC that floats on the amplifier output. It is a DC bootstrap and also supplies power to the HA5005 power buffer gate drivers. The zener sets the gate voltages to just below conducting.
RE: RF power amp that can withstand infinite VSWR
[IMG]http://i14
A video speed amplifier capable of a couple of volts output is ac coupled to a 1:1:1 pulse transformer. This amplifier has total dc feedback, and has the inverting input ac tied to a current shunt in series with the capacitive load.
At each end of this, is a voltage to current converter consisting of a bipolar transistor and suitable emitter resistor. This will be held approximately in class AB by the biasing diodes in the series string. This series string could also provide power to the op amp.
Both these voltage to current converters drive one or more common gate mosfets in cascode, providing a constant current high impedance drain output current to drive the load.
Warpspeed is not feeling his best right now, but offers this as a Christmas suggestion, but will probably regret it later, once completely recovered from the after effects of Christmas.
RE: RF power amp that can withstand infinite VSWR
sreid - Your design requires a conventional (i.e. - voltage feedback) op-amp such as the EL5100, to be stable, correct? Also, directly driving the base of the PNP (no base resistor with speedup capacitor, Baker clamp, etc.) as well as it being in common collector mode seems to be about as slow as is electronically possible... Finally, I want to confirm that the collector of the NPN connects to the floating 15V supply.
Warpspeed - I really like the use of a pulse transformer here, but I'm not sure if the phase lag from it will cancel out any of the phase lead from driving a capacitive load when the feedback signal is the load current. This is otherwise fairly similar to the op-amp based design I was contemplating.
RE: RF power amp that can withstand infinite VSWR
The trick to stability is to minimize phase shifts everywhere. The PNP is actually pretty fast since it has a gain of 1/2 which means there is little Miller capacitance slow down. The NPN level shifter has a bigger problem since small changes in collector current cause big voltage changes. So the NPN needs to be fast. This part of the circuit may have to be changed to a cascode.
The collector of the NPN is tied to the zener as shown.
There are tens of thousands of this circuit in the field working at +/-50V, +/-10A at 300 kHz driving an LR load (servo motor).
RE: RF power amp that can withstand infinite VSWR
Ok - after thinking about it I see my mistake concerning the PNP - it's a voltage controlled current source. E.g - let's say the op-amp output is +10.6V. This will cause 1mA to flow through the PNP's collector which will develop 5V across its 5k load resistor. It is at this point though that my analysis falters as there seems to be no DC pathway from the +15F supply to the -150v supply that the NPN emitter is referred to unless the lower (p-ch) FET is turned on. Mind you, I do feel like I might be missing something obvious here, so take all the joyful glee you deserve in pointing it out if I have ;)
I agree that a Cascode instead of the lone NPN is likely necessary for 1MHz of bandwidth. What is the Ccb on the NPN you are using for the 100Vpp/300kHz driver now?
RE: RF power amp that can withstand infinite VSWR
On further reflection, My main concerns with the above circuit would be the bipolar transistors, and stabilizing the dc bias operating point. It is really just an initial concept circuit and definitely needs some further detailed development. But at least, that is how I would probably begin.
RE: RF power amp that can withstand infinite VSWR
I do a lot of high power (>500W) SMPS design and at these power levels one rarely exceeds a switching frequency of 300kHz because of parasitics and strays. Hence, my unfamiliarity/trepidation with attempting to deliver even a modest amount of power at up to 1MHz. Frankly, this is not my strong point (I guess if it were I wouldn't be asking for help here, huh?). So, thanks again for your comments and patience (even though I hope this provides some entertainment value as well, if for nothing else than the 'chuckle factor'... :)
RE: RF power amp that can withstand infinite VSWR
A brick type RF power amp module could drive a broadband transformer, and run the whole thing open loop without any direct feedback at all. Just use some sort of AGC system around the whole thing to control the output amplitude.
Using a grounded gate cascode output stage vastly reduces the Miller problem, and the required drive power. You just need a suitable voltage to current source in the mosfet source. Maybe even another mosfet? Or how about a whole bunch of small jfets in parallel? They would need to carry around an amp, but the voltage is low. Jfets make excellent high frequency current sources, and they are readily available in both N and P sexes. Finding suitable bipolars with enough high frequency gain to work well, may be problematic.
The op amp can obviously be powered separately, but I do not see high resistor power dissipation as a disadvantage. Those gold metal clad resistors are excellent for this type of application. The output mosfets are going to get pretty hot anyway, a few extra watts of uselessly wasted power will not be noticed.
RE: RF power amp that can withstand infinite VSWR
The basic push-pull (Class A or B) RF power amp design was what I figured I could get away with in the first place. The amplitude does need to be held fairly constant: <1% would be ideal; 5% would probably be fine. That said, they are currently driving the funnel open loop with a general purpose function generator and a very pricey ENI AP400B 400W laboratory wideband amplifier. The single-ended output is converted into push-pull with a custom transformer I whipped up out of some junk box parts (it replaced one *they* had whipped up out of junk box parts, except they used the wrong core material, poor winding technique, had no concept of skin effect, etc.., and therefore their transformer always started smoking within 30s of being powered up).
At any rate, the op-amp based circuits are easy to control amplitude with; for the more basic push-pull RF amp topology do you all think that AGC or a smidgen of negative feedback (e.g. - a shunt R-L-C network from drain to gate) would be easier to implement? I keep reading that negative feedback is tricky to use in RF power amplifiers, yet I've seen it in plenty of circuits designed by amateur radio folks (usually over the 2-30MHz range).
I dream of getting the best of both the RF and audio worlds bere by using a high output current op-amp as the input stage of a basic transformer-coupled push-pull amplifier. Unfortunately for me, I suspect, this setup may be nigh impossible to stabilize (feedback after two transformers in series and a capacitive output load... sounds more like a power *oscillator* than an *amplifier*, doesn't it?)
RE: RF power amp that can withstand infinite VSWR
It is difficult to see the current path for the floating gate drive circuit. The return path is the source connection of the output FETs. It may be easier to visulize if one biases the FETs class AB and connect the output to a load resistor.
A note on the output FET gate drive current requirement. Both FETs are used as Source Followers operated in the linear region, the primary drive current requirement is charging / discharging the gate capacitance. The P channel FET has an input capacitance of 10,000 pF. At 1 MHz this is 16 Ohms. But the gate drive excursion is perhaps
2 V p-p. So
Irms = Vrms/R = 0.7/16 = 50 ma (RMS)
a current that the HA5005 power buffers can easily supply.
RE: RF power amp that can withstand infinite VSWR
The power output stage absolutely must be a very high impedance current source to charge/discharge a capacitor. Any voltage feedback would lower the output impedance and be self defeating. It is the current passing through the load that needs to be controlled, driven, measured, and fed back. This should be possible over the required 10:1 frequency range with very low phase error and distortion if the voltage to current conversion is carried out properly.
Now for another completely different line of thinking.....
Would it suit your application to tune the load to resonance with a stepper motor driven roller inductor, and perhaps some switched lumped inductance? That would solve a whole raft of problems and be far more power efficient.
Automatic antenna tuners do precisely this, and have been successfully home built by many radio amateurs. It is just a case of bringing the voltage and current into exact phase and perhaps also to a required impedance, by having a simple microcontroller do the appropriate things.
RE: RF power amp that can withstand infinite VSWR
I also agree that a lot of design problems could be sidestepped if any sort of "antenna matching" were employed. I'm saving that for if/when all else fails, though, as it either greatly increases the complexity of the overall design or requires manual intervention on the part of the operator (e.g. - the grad students running the experiments...).
sreid - I believe I understand now how your circuit works but SPICE (TINA DesignSuite v7) doesn't seem to. The output is a distorted sine wave with a -100V DC offset. Of course, it is crap like this that reminds me why I almost never simulate a circuit in SPICE...
RE: RF power amp that can withstand infinite VSWR
The amplitude control problem is really a case of adjusting the drive level to the power stage to reach the desired output voltage. Direct true negative voltage feedback is not really feasible, and not really necessary. It probably will not control the output amplitude very well either, if amplitudes are drifting or uncertain elsewhere in the system.
Read the actual peak output voltage with a fast peak reading rectifier, and then use a gain controlled amplifier stage to adjust and correct the amplitude. It is sure to produce the most dependable final result, independent of time, temperature or operating frequency.
RE: RF power amp that can withstand infinite VSWR
sreid - your circuit does appear to be one of those that SPICE says will never work but which in real life works just fine (it's certainly simple enough to go ahead an build one regardless). This, anyway, is preferable to the kind of circuit SPICE says will work just fine but ends up as a total disaster when actually built.
RE: RF power amp that can withstand infinite VSWR
* - Manual SPICE; i.e. - quite screwin' around with a mouse and breadboard the damn thing!
RE: RF power amp that can withstand infinite VSWR
Do you think the driver stage with a source impedance of around 10K will be up to the job?
RE: RF power amp that can withstand infinite VSWR
That said, the NPN driver is, as expected, the Achilles Heel here (as evidenced by the op-amp output starting to go rectangular around 230kHz). But there is no evidence of instability all the way up to 1MHz so it looks like only straightforward tweaking will be needed to get the speed up (e.g. - changing the collector and emitter resistors for the NPN to 3k and 1.5k, respectively, boosted the upper limit to ~ 400kHz).
Warpspeed - I'd like you to revisit your circuit now that you have (presumably) recovered from the holiday excesses. Attached is a picture showing the schematic as I re-drew it with what I felt were reasonable component values. Your circuit as drawn only produces a 90V DC output within 5uS of startup.
The output impedance of the NPN driver in sreid's circuit is 3.33k, not 10k, and the max. freq. I need to deliver is 1MHz, but I think I get your point. That said, I don't see the problem as long as there is good (read: local) bypassing of the buffer supply pins...
RE: RF power amp that can withstand infinite VSWR
This one outputs a signal riding on top of a 90Vdc offset and has serious convergence issues when attempting transient analysis. Once again, I am not a big fan of SPICE so I don't use it that often (and has already been illustrated, I am not immune to 'operator error').
RE: RF power amp that can withstand infinite VSWR
Ha-ha, all through this I have been thinking 1Mhz to 10 Mhz frequency range. I should have gone back and read your first post more carefully.
Your main objection to my design seems to be the 1:1:1 coupling transformer. If this was just a "normal" Faraday type voltage transformer I would most certainly agree with that objection.
Correctly terminated broadband transmission line transformers are fundamentally very different in operation. If you are completely unfamiliar with these things, a little reading may open up a whole lot of new possibilities.
Usually the windings of broadband transmission line transformers all operate at a common dc voltage, but there is no practical reason why they must. They can offer high dc isolation without losing any of their excellent high frequency (and phase) broad band characteristics.
The two great pioneers in this field were Guanella and Ruthroff, and there is an excellent book available containing a wealth of practical hands on design information.
www.hi
www.frenning.dk/OZ1PIF_HOMEPAGE/AN749.pdf
ww
Google will turn up vastly more.
RE: RF power amp that can withstand infinite VSWR
I am familiar with transmission line transformers (let's call them TLTs for short), though if anything qualifies as black magic in electronics they are it.
From what I understand, the "windings" in TLTs must have a characteristic impedance that is the geometric mean between the primary and secondary. So, all I need to do here is figure out the output impedance of the op-amp and the input impedance of the MOSFET then either select the twisted pair or coaxial cable with the right impedance... oh boy, that doesn't sound like a productive way to spend one's time. Probably best to just go with a conventional transformer.
Since I design a lot of switchers, I tend to have lots of ferrite cores laying about the place, and it was one of these that I intended to co-opt for this little project until I got all worried from reading how Class B push-pull power amplifiers hate infinite VSWR loads (hence, this thread). That said, I still haven't found a single definite explanation as to what exactly occurs at the transistor's terminals when an amplifier it is part of attempts to drive a purely reactive load*... And yes, I've googled quite a bit as well as read through all the books I have even remotely relevant to the subject.
My intuitive analysis is that at worst either the voltage or the current will double through the C-E or D-S, but I can't confirm this and so my biggest worry is that the figure is actually much higher.
RE: RF power amp that can withstand infinite VSWR
With a switching supply, the switch is either open or closed most of the time, and the very violent drama of actual switching is a fleeting effect, perhaps lasting only hundreds of nanoseconds each switch cycle. While total switching losses can still rise to some fearsome numbers, the massive dissipation regime of simultaneous high voltage and high current combined can hopefully be kept to fairly short duration each switch cycle.
The problem with class B is that while it is in conduction, it is never entirely on, or entirely off, but stuck permanently in the act of "switching". There will be many simultaneous volts and amps together across the switch to burn things up.
Any effect in the switch such as current crowding, or second breakdown will be pretty lethal to the output devices, so mosfets are probably about the only feasible choice for something like this.
What you are building is more like a constantly varying dissipative series regulator than a switching supply.