Hmm. A SOT-23 with a big collector tab would be nice here.
Look at their SOT-89 parts. They list them as 1W dissipation, and 18.3 C/ W junction to leads -- that's about 1/10th the thermal resistance to copper as their SOT-23 parts, and a SOT-89 shouldn't take up much more space than a TO-92.
My earlier comment about getting heat out of the circuit board itself still applies.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
The push-pull configuration yields twice the slew rate at a given quiescent-current level. I'm ultimately aiming for about 3kV/us slewing. fmax full-swing = S/2pi A. If A (amplitude) = 40V, then 3kV/us yields full-power to 12 MHz.
Some is to keep the TO-92 dissipation under control, other stuff is for thermal matching. In the next pass, add protective circuitry.
--
Thanks,
- Win
It appears from the labels on the I/O that this is a one-channel amplifier -- is that true? Are you then using 5 each input transistors in push-pull? Why not two bigger ones?
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Yes, of course.
After the complementary input EF, there are two each common-emitter transistor pairs, biased up with current sources + resistors.
A pair is used to lower the dissipation, so the Vbe's won't thermally get out of hand. I haven't been able to find slightly larger parts, e.g., 2x or 3x larger. We're always looking to keep the Cob / Pd ratio low.
--
Thanks,
- Win
What you didn't mention, and what so far nobody asked, is why you want to go SMD at all. I don't see potential for a significant reduction of PCB footprint if that's what you're after.
robert
It's usually done to allow using a pick-and-place/reflow process for assembly.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I decided that I could dissipate 3 watts from one of those Supertex SOT89 depletion fets.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
FETs, yes. Some sot-23 MOSFETs do well, rated over 1 watt.
--
Thanks,
- Win
SMD resistors take far less space than 1/4-watt axial-lead parts. But mixing 0805 in among TO-92 makes hand assembly a pain. Solder the 0805s first, yes. But when coming back to change the value, it's hard to get to with the iron tip.
--
Thanks,
- Win
Bipolar chips are too damned small. Which is why c is low.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I sorta lost the signal path in the middle, OK I found the feedback. But the output! I was thinking the 3.3 ohm balancing R's were big, and then I saw 50 ohms on the output. Doesn't that suck up all your voltage? What's the load? Driving a coil? (is there a big capacitance path I'm missing?)
Re thermal stuff, I was going to suggest little brass or Al standoffs from the pcb "thermal" plane to the heat sink below. You always end up needing electrical isolation... how much pcb overlap do you need between power and ground to have one thermal ohm. (a thermal ohm is is one degree K/W.) It's ok I can work out the answer.
George H.
Usual TO-92 construction has a bent C tab, to which the die mounts; that's lots worse than a power package, but it does mean the C lead wire carries significant heat, and can be sinked. It also means that the flat face of the TO-92 is less effective at removing heat than the curved back (the C tab bends away from the flat face). I've seen wrap-around TO-92 heatsinks, as well as epoxy-into-an-aluminum-block (but that was for a log amp matching issue)
One could line up SOT transistors, put a strip of silicone pad atop 'em, and clamp an aluminum L-member, one leg to a heatsink and the other leg atop the transistors. Heat path through the epoxy is probably better (shorter path) than TO-92.
FETs are so damned large, C is high.
--
Thanks,
- Win
I bend the 1/4 W TH R's into a hair pins and stick 'em on a 0.1" grid. That gets it tight... . Yeah your low inductance R's want to sit flat and straight.
I must admit I feel pretty silly telling you how I do circuit layout.
For the transistors in the air, one thing to think about is where they are in the thermal path, if they are in contact w/ cool moving air.. good. other wise maybe it's best to get them in touch with something in contact with the case.
It's cool what legg said about the to-93 cases! I've got a to-93 that gets a little hot and noisier under maximum load*, and I've often thought about one of those heat sink top hats.
Oh and thanks for the circuit diagram! George H.
*it's in a cap multiplier and max load is also max thermal load.
Traditionally signal generators have 50-ohm outputs to match the coax they'll be driving. They have a 50-ohm source resistor to insure proper coax-transmission-cable driving.
{Please see AoE III, 12.10.1, pages 858-874, and Appendix H, pages 1116 to 1130.
Most signal generators are limited to 10 or 20V to drive their 50-ohm output resistors. This voltage is cut in half at the load, if there's the expected nominal 50-ohm load.
In addition to the traditional 50-ohm output the AMP-70 has a low-Z output, 1-ohm, meant to drive some RF 4x step-up transformers that create higher voltages, RIS-
For the 1-ohm output, with up to 5A load, the drop across each resistor could be 3.3V, so the resistor value seems a bit high. It was picked to avoid excessive quiescent current, whatever the output transistor's temp might be.
Very interesting, I'll think about that. One point: wrt this amplifier, the only candidate transistors I've been able to find are TO-220F types, with thick plastic insulation.
--
Thanks,
- Win
Legg, I really appreciate your remarks in this case.
--
Thanks,
- Win
Hah, I've been reading that section, I know about sticking 50 ohms in the source line for matching coax... I didn't know about the "echo" in the source end shown in fig 12.115.
I guess I was confused by the
5A/ 45 V and 50 ohms.George H.
I don't do professional work but...
It looks I see light coming through your PCB. SMD devices will be severely de-rated if there's no copper plane behind them.
Power SMDs would be a good place to get real numbers for various board designs. TI refers to document SPRA953 for using their flip chip regulators. Lumileds and CREE have thermal calculations in their application design notes.
-- I will not see posts from astraweb, theremailer, dizum, or google because they host Usenet flooders.
On a sunny day (27 Jan 2016 11:46:59 -0800) it happened Winfield Hill wrote in :
I understand your quest for fast slew rate, but take for example the bias generator Q20 and Q24 in series. If is just mirroring right? ONE transistor with different resistors as zener would do just as well? And if you are so worried about slew rate why not decouple it? The rest is the same carbon copy mirror joke excuse my opinion.
FWIW, I've got a cutesy test amp here -- 2N7002 cascode. Built just to see how well it works.
I won't bother with a schematic as you can read the PCB pretty well, I'd guess... anyway, you can easily see the pad size for Q1 and Q2 (they are as you see, no vias or buried stuff). At 100mA (about 4Vds per transistor, so 400mW), they get rather toasty, enough that I don't think I would approve of putting this amplifier inside a hot box with other stuff.
So for safe purposes, with minimal footprints, I think I'd want to limit SOT-23s to 200mW or so, with the option for higher, but only with serious consideration of the thermal environment (i.e., how much trace/pour area can you *really* spare; how about vias?).
As others have mentioned, SOT-89s are pretty darn good for small power SMT.
Tim
-- Seven Transistor Labs, LLC Electrical Eng>
[ snip ]I mentioned before my worry about replacing a set of TO-92 transistors with sot-23 parts. Not only do the sot-23 parts usually have a lower Pdiss rating, they dissipate their heat into the PCB (unlike TO-92 into air**), so a set of them in close proximity would raise the PCB ambient, further reducing their real power-dissipation handling capability.
Specifically I've been using Zetex' ZTX450 and '550 E-line complementary transistors,
Their corresponding sot-23 types might be FMMT619, '624, or '722, all rated at 625mW.
The datasheets have a note: For a device surface mounted on 25mm X 25mm FR4 PCB with high coverage of single sided 1 oz copper.
Can anyone give me some guidance on further derating for closely-spaced sot-23 parts?
BTW, going to packages like sot-223 could mean an even larger PCB footprint.
** I know that TO-92 transistors dissipate some heat into the PCB, so I'm limiting their dissipation to under 500mW.-- Thanks, - Win
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