Current divider mirror

...

-V'

or=20

plifer.gif

are on=20

I doubt if i still have the schematic. I should be able to do a fresh=20 instance with current parts. In a couple of days as an LTspice file OK. The parts to the original may be unobtainium now.

Hmm. I have some MC1496's laying around, that's halfway there. Or use a TIA. I wouldn't need four quadrants, one would be enough. That would even suggest a JFET. The problem is making the 0.1 to 10 ratio repeatable with an output from 50nA to 50mA (maybe not 50 on the highest range, I could settle for 5mA max.). That means putting the programming current into the tail is tricky (a regular current mirror would do okay), and the output can only be varied from 1 to 0.01 instead of 0.1 to 10 times (well, close enough).

In this circuit,

Thanks, this is something to think about.

Tim

That circuit is so astoundingly bad that it's worth saving.

John

Bad for what?

It has an unusually wide range, as discrete circuits go. On 680pF, I got from 1Hz to 4.5MHz in a single range. Six decades isn't bad for a 2N440x. Smaller RF BJT's would go lower, or you could do range switching or more exotic approaches (leaky photodiode?).

Besides the wide range, what impressed me is the diffamp got the 2N4403 switching in about 20ns, for a balls-out maximum frequency around 35MHz. Faster transistors would also take this arbitrarily high (the PHEMTs you're so fond of would probably make a proper RC oscillator in the GHz). Say, do they even make P type stupidfast transistors, SiGe or otherwise? I remember they don't bother with P type GaAs or InP since they suck for holes.

I don't remember if, when set at ~10Hz or so, the frequency drifted by a decade or so when I touched the leftmost transistor. It ought to. I do remember seeing it change in steps, since I was using a wirewound pot.

Tim

Certainly worth saving. You might learn something from those of us who can _really_ design at the device level, instead of copying from... ;-) ...Jim Thompson

Let's see how many people can each find one problem.

The most obvious one is the the charge/discharge currents depend on the betas of the upper and lower mirror transistors. That's probably why there are two PNPs and three NPNs: they were selected to work.

John

Well, the duty cycle ended up fairly close to 50% just picking random 440x's out of the box. No selection, put it together and it worked. Why, were you expecting 3 PNP's and 2 NPNs if they were selected?

Tim

? The top mirror gives +I, the bottom mirror gives -2*I, where I is the current through the NPN emitter follower.

When the 2N4403 (emitter connected to +8) is "on" you get +I (cause it conducts the -2I away through the diode), when it is off you get a net of -I.

I got a crap mark in Uni for a circuit much like this one, because the TA didn't understand it. 8-(

There are sufficient "errors" in there to suppose drafting goofs. ...Jim Thompson

If you select the transistors carefully, or get lucky.

Putting two or three or seventeen transistors in parallel in a circuit like this doesn't scale the mirror ratios, it just sort of half-assed averages out the betas. If there is a 2:1 ratio, it's because the NPNs have an average beta twice that of the PNPs.

A 2N4403 has a min specified beta of 60 at 1 mA, typ about 250, no specified max.

Or because he did?

Next problem?

John

Hoo Haa! Keep the BS coming, I'm keeping score ;-)

It's not nearly as bad as my first glance suggested.

Snot-nosed comment. Probably voted for Obama ;-)

...Jim Thompson

John Larkin's statement there is SO-DUMBASSED as to warrant special mention ;-) Keep note of that!

The only thing that stands out as defective in the schematic is that the placement of the two 1N914's is wrong to accomplish the desired up/down current steering.

I was using such a saw-tooth scheme (with substantial improvement :-)

Oops, misread the mirror connections. It's not a great mirror setup, but it's not seriously beta dependent.

Plenty of other issues, though.

John

Jim Thompson a écrit :

??? Not even, but the schmidt trigger has wrong feedback (take it from the other 3904 collector). And the 100p miller cap is not only unnecessary but worsens the PSRR without any benefit.

It seems your silicon has one unnecessary pass.

Yep, the real defect is the Schmitt. Swapping phase still doesn't fix it. I can't figure out what was the intent of whoever designed this.

Have no idea why people spray capacitors around ;-)

My "silicon"? Those GenRad supplies used everything off-the-shelf. ...Jim Thompson

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Breadboard, duh ?-)

Tim

Indeed! Then the 4.7k positive feedback is actually positive.

That's ONE error. Come on, can't you so-called professionals do better than this? ;-D

Tim

Hmmm, look harder...

I meant that "virtual one" when you said: >>> The only thing that stands out as defective in the schematic is that >>> the placement of the two 1N914's is wrong to accomplish the desired >>> up/down current steering.

The mirrors are just fine (for a hacker circuit), but I had to patch up the Schmitt...

In general, the operating currents are way too high. ...Jim Thompson