On 28/01/2016 12:19, Winfield Hill wrote:> 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.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: http://www.eetimes.com/document.asp?doc_id=1277049 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
TO-92 vs sot-23 transistors for power
Started by ●January 27, 2016
Reply by ●January 28, 20162016-01-28
Reply by ●January 28, 20162016-01-28
Tim Williams wrote...> > FWIW, I've got a cutesy test amp here -- > 2N7002 cascode. Built just to see how > well it works. > > http://seventransistorlabs.com/Images/Wideband2N7002.jpgAnd, how well does it work?> I won't bother with a schematic as you can read > the PCB pretty well, I'd guess... 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.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. https://www.dropbox.com/s/f02nqn6w75xvwgx/AMP-70A_fan.jpg The enclosure has more fans to exhaust hot air. -- Thanks, - Win
Reply by ●January 28, 20162016-01-28
John Larkin wrote...> > It looks like it might be difficult to align > those bottomside TO220s and tighten the screws, > especially if there are insulators and grease > involved.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 https://www.dropbox.com/s/clxk0gowedf01ok/TO-220_side.JPG 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. https://www.dropbox.com/s/49npbffqpikjh03/AMP-70A_TO-220.jpg 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 by ●January 28, 20162016-01-28
Jan Panteltje wrote...> Winfield Hill wrote: > >>>> Yes. Here's a schematic of a slowish version. >>>> https://www.dropbox.com/s/xwy4l5audcozylw/AMP-70A-2_sch_.pdf > >>> Could that not be done like the old audio amp a >>> lot simpler? >>> Or is there a reason for all the >>> complicatiatitiated stuff? >> >> Some is to keep the TO-92 dissipation under >> control, other stuff is for thermal matching. >> In the next pass, add protective circuitry. > > 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?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.> And if you are so worried about slew rate why > not decouple it?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 by ●January 28, 20162016-01-28
Chris Jones wrote...> > 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.You have to get a lifetime supply. Some Alibaba sellers, etc., still have some types. -- Thanks, - Win
Reply by ●January 28, 20162016-01-28
In article <n8bdnh02ck1@drn.newsguy.com>, Winfield Hill <hill@rowland.harvard.edu> wrote:> 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.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 by ●January 28, 20162016-01-28
On Thu, 28 Jan 2016 09:49:23 -0500, Ecnerwal <MyNameForward@ReplaceWithMyVices.Com.invalid> wrote:>In article <n8bdnh02ck1@drn.newsguy.com>, > Winfield Hill <hill@rowland.harvard.edu> wrote: > >> 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. > >IMHO 1206 is plenty space saving and fiddly for hand assembly.0805s are starting to look huge to me now. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 28, 20162016-01-28
Den torsdag den 28. januar 2016 kl. 17.02.32 UTC+1 skrev John Larkin:> On Thu, 28 Jan 2016 09:49:23 -0500, Ecnerwal > <MyNameForward@ReplaceWithMyVices.Com.invalid> wrote: > > >In article <n8bdnh02ck1@drn.newsguy.com>, > > Winfield Hill <hill@rowland.harvard.edu> wrote: > > > >> 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. > > > >IMHO 1206 is plenty space saving and fiddly for hand assembly. > > 0805s are starting to look huge to me now.yeh, and 0603 isn't that much smaller -Lasse
Reply by ●January 28, 20162016-01-28
On Thu, 28 Jan 2016 09:49:23 -0500, Ecnerwal wrote:> In article <n8bdnh02ck1@drn.newsguy.com>, > Winfield Hill <hill@rowland.harvard.edu> wrote: > >> 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. > > 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.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 by ●January 28, 20162016-01-28
On Thu, 28 Jan 2016 08:46:47 -0800 (PST), Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>Den torsdag den 28. januar 2016 kl. 17.02.32 UTC+1 skrev John Larkin: >> On Thu, 28 Jan 2016 09:49:23 -0500, Ecnerwal >> <MyNameForward@ReplaceWithMyVices.Com.invalid> wrote: >> >> >In article <n8bdnh02ck1@drn.newsguy.com>, >> > Winfield Hill <hill@rowland.harvard.edu> wrote: >> > >> >> 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. >> > >> >IMHO 1206 is plenty space saving and fiddly for hand assembly. >> >> 0805s are starting to look huge to me now. > >yeh, and 0603 isn't that much smaller > > >-Lasse >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
