Current divider mirror

"Spehro Pefhany" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Didn't. It was a bit different because it was a linear IC design so
> the emitter resistors were not required to gobble up potential Vbe
> mismatch, and I shut off the |2*I| current source with a saturated
> transistor across the Vbe junction. It worked fine when built on a
> breadboard with monolithic transistor arrays.

I did that in an even earlier drawing (which I don't think I ever
breadboarded, actually),
http://webpages.charter.net/dawill/t..._Generator.gif
but turning off transistors slows things down. Nice part about diode gates,
they are basically instantaneous.

Tim

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

On Mon, 08 Feb 2010 17:12:19 -0500, Spehro Pefhany <(E-Mail Removed)> wrote:

>On Mon, 08 Feb 2010 13:51:28 -0800, John Larkin
><(E-Mail Removed)> wrote:
>
>>On Mon, 8 Feb 2010 15:26:41 -0600, "Tim Williams"
>><(E-Mail Removed)> wrote:
>>
>>>"John Larkin" <(E-Mail Removed)> wrote in message
>>>news:(E-Mail Removed) ...
>>>>>http://webpages.charter.net/dawill/t...8/Triangle.gif
>>>>
>>>> That circuit is so astoundingly bad that it's worth saving.
>>>
>>>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 bya
>>>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
>>
>>
>>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
>
>? 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-(
>

Protest time.