Current divider mirror
in [Design]
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From: langwadt on 9 Feb 2010 11:36 On 9 Feb., 01:08, "Tim Williams" <tmoran...(a)charter.net> wrote: > "Fred Bartoli" <" "> wrote in messagenews:4b70a0fd$0$21600$426a74cc(a)news.free.fr... > > > ??? Not even, but the schmidt trigger has wrong feedback (take it from the > > other 3904 collector). > > 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 > > -- posting a schematic here follows a predictable pattern :) all the old guys laugh at how naive and horrible wrong it is, ... old guys find glasses, and agree it might not be so bad.... 20 post later they agree its a great design, they all did exactly the same 40 years ago except they used a few secret tricks that made it even more awesome ;) --Lasse
From: John Larkin on 9 Feb 2010 12:41 On Tue, 9 Feb 2010 08:36:42 -0800 (PST), "langwadt(a)fonz.dk" <langwadt(a)fonz.dk> wrote: >On 9 Feb., 01:08, "Tim Williams" <tmoran...(a)charter.net> wrote: >> "Fred Bartoli" <" "> wrote in messagenews:4b70a0fd$0$21600$426a74cc(a)news.free.fr... >> >> > ??? Not even, but the schmidt trigger has wrong feedback (take it from the >> > other 3904 collector). >> >> 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 >> >> -- > >posting a schematic here follows a predictable pattern :) > >all the old guys laugh at how naive and horrible wrong it is, ... > >old guys find glasses, and agree it might not be so bad.... > >20 post later they agree its a great design, they all did exactly >the same 40 years ago except they used a few secret tricks >that made it even more awesome Except that this is not a great design. It's an interesting all-discrete breadboard that would benefit greatly from a few changes and a little math. It does use the current mirror that I posted previously ftp://jjlarkin.lmi.net/Mirror1.JPG and for some reason mis-read as drawn. Luckily, usenet isn't life. John
From: whit3rd on 9 Feb 2010 18:49
I hate to get back to the original question, but my brain itched last night... There's a common circuit design that fills the original requirement handily: a transconductance multiplier. Instead of a current mirror, Q1 and Q2 with the emitters tied together to a negative supply, add Q3 and Q4, and tie their emitters to that supply instead. base+collector of Q3 is fed a current (call it I_gain), and base + collector of Q4 is fed another current (call this I_ref). Then you use two voltage-follower op amps -- buffer the potential on Q3's collector+base, and connect the emitter of Q1 to that output, and connect the emitter of Q2 to the potential on Q4's collector+base. The balance equation now reads: Vbe_Q1 + Vbe_Q3 = Vbe_Q2 + Vbe_Q4 Multiply both sides by Q_e/kT, exponentiate both sides of the equation, and multiply both sides of the result by I_sat... then recognize the equation to have four terms which are all Ebers-Moll transistor emitter currents, I_Q1 * I_gain / I_ref = I_Q2 Those are emitter currents, so it has another factor of alpha (the collector/emitter current ratio of Q2, which is nearly one). I_output = alpha * I_Q1 * I_gain/I_ref so it's actually a lot like a current mirror with adjustable transistor areas. The transistors all must be at the same temperature (close and on the same heatsink, with small self-heating), and all have the same I_sat, so matched quad transistors are required. Mat-04 or THAT300 are candidates. The voltage-follower op amps hold the gain setting, so they haven't any slew rate requirements to speak of. Usually, this kind of circuit is inconvenient because of the current- in and current-out configuration. |