Problem with current mirrors

[snip]

You are quite welcome!

And Thank YOU for posting a circuit problem! ...Jim Thompson

-- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

I like that emitter-to-emitter thing. It's not often you get to use it.

John

that's because it doesn't offer you any current gain. In fact, you'll lose a little.

But the go-up current and the go-down current will match to 1% or better. If the betas are equal, the currents match perfectly.

Use mosfets and the currents *will* match. Hmmm, that would be a really interesting double servo.

John

Yes, one DOES lose a little current gain, bit methinks that is not the reason one sees little use of common base circuitry.

Not MUCH! alpha = beta/(beta+1), so, if beta=100, you have less than

1% "lost".

That NPN-PNP configuration used to be as common as dirt in bipolar I/C days. With the advent of CMOS encroaching analog, I last used the NPN-PNP thingy in 2002 (qrk's sonar stuff ;-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

Bah, common base is great for saving parts. :)

The error amp / comparator 2N4403 has to be noninverting.

Too bad about the 1k base voltage divider, it would be more efficient if it had complementary output, which could drive a 2N7000 FET instead of the 4401 switch.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Hey, I can do that too!

ftp://jjlarkin.lmi.net/Inverter.jpg

A neat logic level shifter is TTL-resistor-PNPemitter with the PNP base grounded, collector going to somewhere negative.

John

Every cascode pair is a 'use of common base', and lots of differential amps are, technically, a follower and a common-base transistor.

Others will point this out, no doubt, but ... unless the gate resistors are high precision, a percent of the current lost is NOT going to do anything bad to this circuit. It'd be rather hard to find a small- signal transistor that would lose two percent in common-base connection.

Pffbt... mine has fewer parts, and way fewer total transistors. ;-)

Size? Ohh, about the same I guess, since both can be made from SMT parts, with output limited by their power dissipation. The FET is more efficient of course.

If CC_Buck were actually "CV_Boost", it would look more like:

which is almost exactly what you've done. Invert all the polarities and it is exactly what you've done. But I like to save on components by putting a wire jumper around the inductor.

For best results, the error amp (grounded base strikes again!) ought to be replaced with a bandgap reference.

It's a shame they don't make noninverting TL431's. As is, the anode carries output current, and the offset is -2.5V, not as nice. Compensation would be a curious affair though...

LT did it to test their LT1016 (pg.7 of the datasheet). Although they used a beefy NPN instead.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Yabbut.. in small production quantities, an inductor is a heck of a lot more attractive than a transformer -- especially if it's not an off-the-shelf transformer.

Not exactly. I'm using a Schmitt RC oscillator that sets a default startup duty cycle, and then using a feedback loop to back off the duty cycle when the output hits -12.

But I like to save on components by putting a

YIKES! The charging current into C1 is beta limited! And a little overshoot on the output could fry somewhere between one and three transistors.

But I confess I do like blocking oscillators. But custom magnetics are a pain, so I tend to avoid them in favor of more complex circuits that use standard parts.

I'm using a +5 supply that I have already. It kind of soft starts, even nicer.

John

[snip]

Sort of like I was doing _36_years_ago_...

except that the inductor was constant charge, with rate backing off when set-point was reached.

[blow-hard-ego-trip-stuff snipped] ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

=20

Offhand, i would guess they are there to say within the Vce and Vgs = ratings of the transistors.