Re: BJT Pair for Push-Pull, 1W, 33V, 200mA, 1MHz (& maybe no heatsink)?

Eeyore wrote:
tomg@xxxxxxxxxxx wrote:

Eeyore wrote:
tomg@xxxxxxxxxxx wrote:

John Jardine. wrote:
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John Jardine. wrote:
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[...]

Seems like the power output stages of every Function generator ever
made,
use the 2N2219-2N2905 pair.
john

John,

Good call!

I had looked at those, first, after no TO-92-cased models seemed to be
quite up to the task. And just this morning I tested 2N2219A and
2N2905A (metal can TO-39 cases) in the circuit and.... it looks like
no heat sink will be necessary (well, maybe for the 2905A; but only a
small one. I'd probably stick one on both, anyway, if there's room.).

I'm guessing that maybe they will also operate down to lower currents
than larger devices. Or is that way off?

Thanks!

Tom Gootee


You should have little problem controlling signals down to the prevailing
noise. Courtesy of available loop gain and offset voltage of that input
opamp.
Reading the words I get a gut feeling that the circuit has somehow run out
of gain.
It's awkward (I'm idle!) to mentally reconstruct your circuit description.
Any chance you could put the relevant part of the drawing up somewhere, then
maybe all the lads can have a perusal?.
john

John,

Well, now that you mention it, there is one small problem that I
noticed in Spice simulations: For the maximum step size, the largest
step doesn't QUITE make it to its intended value. When it should reach
15v or 150 mA, it only gets to about 14.9V or 149mA.

I had to play with the 806 Ohm split bias resistors' values, to get the
max output as high as possible (something like 14.5v out for 15v in),
and then had to add the "boost" capacitors from the output to the bias
resistors' junctions, to get it to go to 14.9v for 15v in.

If I lowered the emitter resistors' values some more, from, say, 1 Ohm
to 0.1 Ohm, then the output would reach 15 v, for 15v in. I'm not sure
that I really need them, actually, but didn't want to take the chance
of not having them. And 1 Ohm already seemed a little low.

I could also change the opamp's input and feedback resistor ratios. I
forget why I didn't want to do that, though.

I will post the schematic at:

http://www.fullnet.com/u/tomg/vccs.jpg (95 kbytes)

What's the design load impedance ? 50 ohms ?

There's a number of comments I could make. Are you able to make any serious
changes at this time ?

By the way, I remember Motorola's old 1 watt 'extended TO-92' devices that would
be a fit for this application too aand probably quite inexpensive.

It's the MPSW variants of the TO-92 MPSA range.
http://www.onsemi.com/PowerSolutions/search.do?searchString=mpsw&searchType=others&tabbed=Y&clearFilters=Y

Graham

Graham,

Thanks, much, for the replies.

YES, I CAN still make serious changes. All ideas are *most* welcome.

As far as the load impedance, it could be virtually anything; whatever
the user of the curve tracer decides to connect to the output terminal.
But, typically, this (staircase) output would be used to drive the
base or gate of a BJT or FET, the other two leads of which would be
connected to the curve tracer's sweep-signal terminals (with anywhere
from 1v p-p to 30v p-p sawtooth, triangle, or sine driving them). So
I've been simulating this with everything from a short circuit for the
load (typically in current-source mode) to an open-circuit load
(typically in voltage-source mode), as well as various transistors
(with the additional sweep circuitry then also used).

I take it you can't access binaries groups ? I'd post something there otherwise.

One thing puzzles me. If this is either current or voltage output, how is the output level and impedance
configured ?

Graham

Graham,

Thanks again.

That's correct. I haven't found any free way to access the abse group.
I'll check as to whether or not my ISP will agree to subscribe to it.

But, you could email something to me. My regular email is still
"hosed". So I'm currently reachable at: tomgootee at yahoo dot com. I
could also then post it on a webpage, if you'd like.

Regarding the output level and impedance configuration: With the
Howland topology, at least, the 100 Ohm current-sense resistor (in
series, between the output and the load), and the 100 Ohm resistor that
could be manually switched-in, to ground, from just before the
current-sense resistor, gave excellent output-levels' accuracies, FOR
the intended load conditions, i.e., when using current-source mode
(i.e. without the 100 Ohms to ground) for low-impedance (e.g. BJT
bases) and short-circuit loads, and when using voltage-source mode
(i.e. WITH the 100 Ohms to ground) for high-impedance (e.g. FET gates)
and open-circuit loads.

That may be a wholly-wrong approach. I hope that it was implicit, in
my even asking about all of this, that I don't (yet) consider myself to
be very experienced, or even good at, analog circuit design and
analysis, and pretty-much continuously wonder (and would like to be
told) if I'm making any serious blunders, maybe-especially regarding
the very basic things, such as my overall approaches to design
solutions. (I do hate having to possibly expose my ignorance in public.
But, if I must, I must.)

And, in case I haven't mentioned it lately, and just so everyone here
knows that they might need to "dumb-down" (somewhat, at least), their
responses to me: I DO have a BSEE degree (very near the top of my
class, actually, at Purdue, 1978), and even took quite a few
graduate-level courses in EE and math. *HOWEVER*, after only three or
four years of doing (cooool) guidance and control work on
fighter-aircraft programs (And I still like to brag that I made a major
contribution to an air-to-air automatic-weapon-delivery system; Great
fun!), I spent most of the rest of my life writing software, letting
much of my EE knowledge rust or evaporate, and have only been
really-seriously trying to learn and gain experience at circuit design
for the last several years. I HAVE been working quite diligently at it,
but still accutely sense what feels like a gaping hole where a
career's-worth of experience would be so extremely useful to have, now.
This group (and its archive at http://groups.google.com) has been
invaluable to me.

Thanks again.

Humble regards,

Tom

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