Looks useful, thanks.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
matching resistor into a 50 ohm load? You can do that with a much smaller resistor and recover a lot of headroom by using a synthetic output impedance. I did that on a design where I needed to drive an output from a single 12 volt rail. I got nearly 8 Vpp from LM8272MM opamps. The series resistor was only 12.1 ohms but looked like 50 ohms because of positive feedback around the resistor. It saves a lot power being wasted in the series resistor as well.
Or do I not understand what you mean by "+-10 behind 50 ohms"?
google://synthetic output impedance
-- Rick C
Right, (my mistake). If the outputs can be flipped over like one of these,
George H.
The people who design switchers and switcher chips have an impressive positive bias. There are few neg-to-neg switchers.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
This might work:
It uses a synchronous switcher chip just to make a stiff 5v p-p square wave. That might be handy for charge pump supplies, too.
I don't have an LT Spice model for that switcher, so maybe I'll breadboard it. Or learn to use Tina. We are seriously messing with the switcher, so the feedback RC will need to be tweaked for loop stability
-- John Larkin Highland Technology, Inc lunatic fringe electronics
oops, needs another resistor, FB to ground.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
PS_Boost.JPG
S_Boost_2.JPG
It's a nice idea
But, the parallel combination of T1 and T2 may be problematic. The ac capac itor feeding them can work to avoid flux walking of an inductor, but when y ou have two in parallel they are not going to be equal so that they need th e same volts second products. One may see different loading or may be unsym metric..
I would add another capacitor to make sure the +14V and -14V are uncoupled
Cheers
Klaus
What a kludge! Why bother with all this when you can open up tons of headroom by changing the amplifier circuit?
Using a 12.1 ohm resistor or similar instead of a 50 ohm resistor, you can add nearly 4 volts of headroom with the same 50 ohm output impedance.
-- Rick C
That's the unbalanced Howland thing John and I were discussing a ways upthr ead. Some problems are that it concentrates the dissipation in the active d evice, is more sensitive to resistor tolerances, and (depending on the amp) allows CMR problems to cause output distortion.
Depending on the amp, it also may not allow it to reach full output unloade d.
Cheers
Phil Hobbs
How would it not reach full output? I didn't see any of the problems you describe and I don't understand this last one.
As to the power dissipation, that is only a problem if you go to extremes. I had a problem getting the range I needed from a single 12 volt supply, so I was aggressive. But to fix the half volt problem a 40 ohm resistor could be used with appropriate feedback. This only adds a small additional power load in the opamp, has virtually no impact on resistor tolerance (who cares when using 1% resistors anyway). It extends the range when loaded. The only way you can't get "full range" is if the PSU won't provide the headroom which was the problem I had.
I just can't see adding all that PSU nonsense rather than adding three resistors per opamp.
-- Rick C
Yeah, each transformer could have its own cap.
The TPS54302 may be a bad idea. It has a low-current pulse-skipping mode that might cause problems. I think I'll just use discrete P and N-fets to make the square wave, from a uP PWM or something. I just need a good non-overlap anti-shoot-through gate driver circuit.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
The generator needs to make +-10 volts into a hi-z load, and needs to be a 50 ohm source. The old product did that with four small, paralleled RRO opamps, but was slow. I want more bandwidth and slew rate on the new version, as some customers have requested.
Opamps tend to slow down when they swing near their rails, RRO types or not. That's not always (or usually) specified. So some supply headroom is prudent.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Last summer I tried a variety of sync bucks in a toplogy a bit like that. I was trying to make two isolated +5 supplies from one +24, using the 'flybu ck' topology and a dual-winding inductor.
That's not bad as long as there's a non-isolated output that's guaranteed t o run at higher current at all times, but in this case there wasn't. I trie s three or four sync bucks, including one whose datasheet promised that it would keep PWMing down to zero current, and none did (the dirty dogs).
So an open-loop solution with maybe an LDO or cap multiplier to smooth out the ripple sounds like the ticket.
Cheers
Phil Hobbs
How about this one?
Using the fets as over-driven followers is softish and has no shoot-through. This one is maybe worth breadboarding.
This makes the supplies for my big opamps, so I'm not very concerned about noise. And I don't want to throw away any of my hard-earned boost voltage on a c-multiplier!
The RB051s are nice... 0.3 volts typ at 1 amp.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
If it's a single output loading the VME supplies and is solely responsible for the droop, you could stick in a local 'cap balance' circuit, similar to those used to equalize battery cells. This should reduce the droop, as both rails become loaded by the unipolar waveform output.
RL
Or if the VME rails are only quasi-regulated, going back into the main supply with higher Vf rectifiers on the main regulated output will give larger voltages on the auxiliaries.
RL
thread. Some problems are that it concentrates the dissipation in the activ e device, is more sensitive to resistor tolerances, and (depending on the a mp) allows CMR problems to cause output distortion.
aded.
or an even faster opamp?
NT
I plan to use a THS6022. 300 MHz, 1900 v/usec, 250 mA. It's a beast, but doesn't swing to its rails. The product's specified bandwidth will be 2.5 MHz.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
upthread. Some problems are that it concentrates the dissipation in the ac tive device, is more sensitive to resistor tolerances, and (depending on th e amp) allows CMR problems to cause output distortion.
nloaded.
AD8397 won't do?
Actually, we have that in stock, as well as the THS part. The AD slew rate is mediocre, but it would solve my supply problems. I need about
140 v/us to avoid slew-rate limiting my output, and the AD part is 53.We have a crazy range of customers for this product, and some want speed. Some want other stuff. So it's hard to settle on final specs. Maybe 53 v/us is enough... who knows?
-- John Larkin Highland Technology, Inc lunatic fringe electronics
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.