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TO-92 vs sot-23 transistors for power

I was interested to see some ADSL modem transmit amplifiers that don't do this. Provided the load impedance is about right, they simulate the right output impedance with a lower value series resistance than the cable characteristic impedance. I think the circuit is something like a Howland current pump but slightly re-adjusted to give a high (50 Ohm) but not infinite output impedance. In this article it is figure 3:

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This advantage (of not needing the supply voltage to be more than double the output voltage) wouldn't be generally applicable for lab equipment where someone might run it into an open-circuit and might actually want and expect twice the voltage that they would have got with a 50 Ohm load, without clipping.

I am also a bit leery of using ADSL driver chips in a lab function generator, as they are liable to be discontinued when the market for ADSL chips changes. Still that also happened to the Elantec EL2009 and also the MAX038 that I might have also used in that function generator, so maybe the ADSL drivers would not have been the first to go obsolete. I wonder whether it is already too late for the nice fast 300V transistors that used to be used in the video amplifiers on CRT neck boards.

Chris

Reply to
Chris Jones
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And, how well does it work?

Yes, and thanks for the SOT-89 suggestion.

Here's the business end of the amplifier, a CPU server heat-sink and fan bolted to the TO-220 plate, its thermal measures 0.2 C rise per watt.

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The enclosure has more fans to exhaust hot air.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

I worried about that and feared trouble, with 24 screws to mount or demount the assembly.

However I was pleasantly surprised, it comes apart and goes together easily. First bend transistors just beyond the lead-size change

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and insert into the PCB underside. Mount the heatsink plate with 3/8-inch standoffs. Then screw in the TO-220F trannies. This is easy and quick, with no grease, and 3/8-in screws seem to fall right into place. Next solder the TO-220F pins to the PCB. Now the assembly can be unscrewed and manipulated as desired.
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All-in-all it seems surprisingly practical. I was going to change to a clip scheme on my next pass, but this approach seems better.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Just being conservative. There's a PNP mounted on the heat-sink plate to match the PNPs and an NPN to match the NPNs. I'm not happy with the thermal tracking as is, and intend to experiment later with a buffered Sziklai output scheme.

I'm not sure what you're suggesting. The VAS stage is my slew-limiting issue. EFs Q19 and Q25 now "decouple" the 10 output transistors.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

You have to get a lifetime supply. Some Alibaba sellers, etc., still have some types.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

IMHO 1206 is plenty space saving and fiddly for hand assembly. 2512 is a lot nicer to handle and doesn't just vanish when you look the other way for a second, but is 4 times the real estate. AADE won't admit that either is an inductor so no handy numbers there to report (fancier equipment might manage to report something, though I can't imagine there's a lot of inductance to a ceramic chip type resistor.)

I'll self-inflict 0805 caps at times, which are at least thick enough that the size is not utterly maddening (easy to tweeze), but for the sake of my own sanity I try to stick to 1206 or larger resistors. All hand assembly around here.

--
Cats, coffee, chocolate...vices to live by 
Please don't feed the trolls. Killfile and ignore them so they will go away.
Reply to
Ecnerwal

0805s are starting to look huge to me now. 
--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

Den torsdag den 28. januar 2016 kl. 17.02.32 UTC+1 skrev John Larkin:

yeh, and 0603 isn't that much smaller

-Lasse

Reply to
Lasse Langwadt Christensen

For hand assembly with a microscope (generic stereo 10x) I go down to

0603 no problem, and 0402 if I'm feeling motivated to be teeny. The microscope makes far more difference than you'd think possible. 
--
www.wescottdesign.com
Reply to
Tim Wescott

I'm used to 0603s now. 0402 still looks tiny to me, hard to solder. Manufacturing doesn't like them either.

We like to screen reference designators, so going smaller than 0603 doesn't save much room.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

Your P&P machine shouldn't have any trouble with 0402 (or 0201, for that matter). Rework can be an issue but tell them to suck it up. That's life in the fast lane. I have no problems with 0402s, as long as I have my Mantis. Even 4x or 6x OptiVisors work in a pinch.

We have screened reference designators for all components, too. It's not always possible to get them right at the component but it's not too bad. Sometimes there is a list of parts, off to the side of a row of resistors and caps.

Reply to
krw

0402 is our standard size passive. A few designs have used 0201 decoupling caps for large BGAs but they're not qualified my manufacturing or QA so they're not used often.
Reply to
krw

Beware that Andonstar have a 1MP and a 2MP version that look otherwise almost identical. I got the right one, but could have missed it.

I find it a little annoying that tweaking the focus knob makes the mounting rotate slightly, which moves away from the thing you were trying to focus on. Do you (or anyone) have a cure for that Phil?

Clifford Heath.

Reply to
Clifford Heath

I typically use +5 diopter glasses (my prescription, +5 diopters - something with longer working distance would be nice, but spendier), which also make a big difference - but what size I can do and what size is comfortable (and thus quick) are not the same. I have done 0603, and godawful nasty little feedthroughs with contacts on both sides in the middle as well as the ends at either that size or 0805 - but I prefer not to, and don't find any particular joy in miniaturization for miniaturizations's sake.

As my eyes get less flexible I find more need to resort to a microscope for reading part numbers (or part "markings" & good luck finding an actual part number to map to it these days on little parts, if they even are marked.) I don't have good one for working under.

--
Cats, coffee, chocolate...vices to live by 
Please don't feed the trolls. Killfile and ignore them so they will go away.
Reply to
Ecnerwal

Some of the Chinese ones are actually pretty decent for the price. Check out Amscope on the slightly higher end, or Andonstar for a cheapie webcam version:

I used my Andonstar earlier today to help with aligning infrared beams on really small (60 um) photodiodes. Good medicine for $50.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

You can preload it with a strategically-located rubber band. I usually use just the pen part, zap-strapped to a gooseneck salvaged from an Ikea desk lamp. The other end of the gooseneck is mounted to a Newport optical post b ase with JB Weld putty, so it screws into the optical table nicely.

The zap straps let me turn the camera easily after it's in position, so the image is the right way up. This alignment job is pretty hard, but would be impossible without the USB scope.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

18dB gain, -3dB at 50MHz (and -10dB up to 200MHz; it's not a simple gain-bandwidth tradeoff, but as far as I can tell, also not "unlimited speed" as Larkin likes to say).

Input matching needs work. I haven't tried beyond the little inductor seen there.

I get 12.5dBm output, for 2nd harmonic at -20dB to fundamental (and 3rd harmonic about 13dB below that; so, whatever IM3 intercept that might correspond to).

1dB compression, hellifiknow, I don't have a dual channel spec and CBA swapping back and forth to figure it out. ;-)

Guessing, even with those fast output transistors, it's acting fairly class A up there. All that dissipation, it's a feature, this time of year. :-)

Looks great! Only quirk I see is, why the staggered TO-220 pins?

I like models, so I draw 'em like this...

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(or use a larger screw hole for not mounting PCB + device simultaneously)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

What, you never got into staggered pins? (BTW, easy to model.)

--
 Thanks, 
    - Win
Reply to
Winfield Hill

I don't feel like I can trust Alibaba sellers. I see too many reports of outright fraud there. I don't know how well Alibaba protects the buyers, but it looks like it isn't good enough.

--

Rick
Reply to
rickman

Your link didn't work, but typing Amscope into ebay's search line got a bunch of good hits.

I got mine free from a customer who otherwise robbed me blind (with my collusion -- you can't be ripped off for labor unless you allow the account to climb to rip-offable heights). Having one, I think it'd definitely be worth $500 for another one, getting a cheap Chinese one for

1/3 the price is even better.

My one complaint with the thing is neck strain -- if I spend all day populating boards without remembering to put a heat pad on my neck first then I'm very sorry the next day. I wonder if something with a USB camera and a screen that I can look at while sitting in a normal posture wouldn't work better.

I suppose that going too far down that road would just have me building a semi-automatic pick & place machine.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

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