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



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

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