Current divider mirror
in [Design]
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From: Jim Thompson on 8 Feb 2010 16:29 On Mon, 08 Feb 2010 12:58:04 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Mon, 8 Feb 2010 13:19:43 -0600, "Tim Williams" ><tmoranwms(a)charter.net> wrote: > >>"JosephKK" <quiettechblue(a)yahoo.com> wrote in message >>news:8q7vm5hrekhi5rq86ffrbflv655tjg45nm(a)4ax.com... >>> For fully continuous i would look at a application of a Gilbert cell >>> multiplier. >>> There are some relatively exposed Gilbert cell ICs available. >> >>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, >>http://webpages.charter.net/dawill/tmoranwms/Circuits_2008/Triangle.gif > > >That circuit is so astoundingly bad that it's worth saving. > >John > 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 -- | 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.
From: John Larkin on 8 Feb 2010 16:51 On Mon, 8 Feb 2010 15:26:41 -0600, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >news:3ju0n51fhfrj2pukuet0obtibm2p7lnmu3(a)4ax.com... >>>http://webpages.charter.net/dawill/tmoranwms/Circuits_2008/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 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 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
From: Tim Williams on 8 Feb 2010 16:53 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:vf11n5hiervl0ukh1o0qett8gsfut726vk(a)4ax.com... > 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. 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 -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Jim Thompson on 8 Feb 2010 17:12 On Mon, 08 Feb 2010 13:51:28 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Mon, 8 Feb 2010 15:26:41 -0600, "Tim Williams" ><tmoranwms(a)charter.net> wrote: > >>"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >>news:3ju0n51fhfrj2pukuet0obtibm2p7lnmu3(a)4ax.com... >>>>http://webpages.charter.net/dawill/tmoranwms/Circuits_2008/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 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 > > >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 There are sufficient "errors" in there to suppose drafting goofs. ...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.
From: Jim Thompson on 8 Feb 2010 18:02
On Mon, 08 Feb 2010 14:23:44 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: [snip] > >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. > [snip] > > >Next problem? > >John 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 :-) 30+ years ago in GenRad switchers, which continue to work just fine to this day (built around LM339's). ...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. |