Current divider mirror

"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 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

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

"John Larkin" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> 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

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 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

? 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-(

Jim Thompson a écrit :
> On Mon, 08 Feb 2010 14:23:44 -0800, John Larkin
> <(E-Mail Removed)> 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.
>

??? 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.


--
Thanks,
Fred.

"Jim Thompson" <To-Email-Use-The-Envelope-(E-Mail Removed)> wrote in
message news:(E-Mail Removed)...
>>And the 100p miller cap is not only
>>unnecessary but worsens the PSRR without any benefit.
>
> Have no idea why people spray capacitors around ;-)

Breadboard, duh ?-)

Tim

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

"Fred Bartoli" <" "> wrote in message
news:4b70a0fd$0$21600$(E-Mail Removed)...
> ??? 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

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

Jim Thompson a écrit :
> On Tue, 09 Feb 2010 00:40:45 +0100, Fred Bartoli <" "> wrote:
>
>> Jim Thompson a écrit :
>>> On Mon, 08 Feb 2010 14:23:44 -0800, John Larkin
>>> <(E-Mail Removed)> 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.
>>>
>> ??? Not even, but the schmidt trigger has wrong feedback (take it from
>> the other 3904 collector).
>
> 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.
>

Hmmm, look harder...


>> And the 100p miller cap is not only
>> unnecessary but worsens the PSRR without any benefit.
>
> Have no idea why people spray capacitors around ;-)
>
>> It seems your silicon has one unnecessary pass.
>
> My "silicon"? Those GenRad supplies used everything off-the-shelf.
>

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.



--
Thanks,
Fred.

"Jim Thompson" <To-Email-Use-The-Envelope-(E-Mail Removed)> wrote in
message news(E-Mail Removed)...
> The mirrors are just fine (for a hacker circuit), but I had to patch
> up the Schmitt...
>
> http://analog-innovations.com/SED/SE...le_Posting.pdf
>
> In general, the operating currents are way too high.

Augh! Now it'll hardly break 5MHz. :-(

I say crank 'dat **** up to 50mA and make the 4401's squeal.

Tim

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

Tim Williams a écrit :
> "Jim Thompson" <To-Email-Use-The-Envelope-(E-Mail Removed)> wrote in
> message news(E-Mail Removed)...
>> The mirrors are just fine (for a hacker circuit), but I had to patch
>> up the Schmitt...
>>
>> http://analog-innovations.com/SED/SE...le_Posting.pdf
>>
>> In general, the operating currents are way too high.
>
> Augh! Now it'll hardly break 5MHz. :-(
>
> I say crank 'dat **** up to 50mA and make the 4401's squeal.
>
> Tim
>

Wanting to melt a dab of iron? :-)

--
Thanks,
Fred.

On Mon, 08 Feb 2010 14:23:44 -0800, the renowned John Larkin
<(E-Mail Removed)> wrote:

>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) m...
>>>>>>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 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
>>
>>? 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.
>>
>
>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.

Beta only enters this as an error due to the base currents, which is
of the order of 1/beta. In the case of the -2*I current sink there are
3 base currents effectively subtracted from the input current, so for
a beta of 250 there will be about 1.2% error = 3/250 in the output
current.

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

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.

>Next problem?
>
>John


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
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