Current divider mirror

On Mon, 08 Feb 2010 13:51:28 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:

On Mon, 8 Feb 2010 15:26:41 -0600, "Tim Williams"
tmoranwms_at_charter.net> wrote:

"John Larkin" <jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote in message
news:3ju0n51fhfrj2pukuet0obtibm2p7lnmu3_at_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

? 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 + 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-(

On Mon, 08 Feb 2010 17:12:19 -0500, Spehro Pefhany
speffSNIP_at_interlogDOTyou.knowwhat> wrote:

On Mon, 08 Feb 2010 13:51:28 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:

On Mon, 8 Feb 2010 15:26:41 -0600, "Tim Williams"
tmoranwms_at_charter.net> wrote:

"John Larkin" <jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote in message
news:3ju0n51fhfrj2pukuet0obtibm2p7lnmu3_at_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

? 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 + 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.

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?

Next problem?

John

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" <jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote in message
news:vf11n5hiervl0ukh1o0qett8gsfut726vk_at_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

On Mon, 08 Feb 2010 14:23:44 -0800, John Larkin
jjlarkin_at_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.

Jim Thompson a écrit :
On Mon, 08 Feb 2010 14:23:44 -0800, John Larkin
jjlarkin_at_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.


??? 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.

And the 100p miller cap is not only
unnecessary but worsens the PSRR without any benefit.

Have no idea why people spray capacitors around

??? Not even, but the schmidt trigger has wrong feedback (take it from the
other 3904 collector).

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
jjlarkin_at_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.

??? 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.

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.

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.

On Mon, 08 Feb 2010 17:12:19 -0500, Spehro Pefhany
speffSNIP_at_interlogDOTyou.knowwhat> wrote:

On Mon, 08 Feb 2010 13:51:28 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:

On Mon, 8 Feb 2010 15:26:41 -0600, "Tim Williams"
tmoranwms_at_charter.net> wrote:

"John Larkin" <jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote in message
news:3ju0n51fhfrj2pukuet0obtibm2p7lnmu3_at_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

? 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 + 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.

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?


Next problem?

John

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

http://analog-innovations.com/SED/SED_Triangle_Posting.pdf

In general, the operating currents are way too high.

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
jjlarkin_at_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.

??? 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.

"Jim Thompson" <To-Email-Use-The-Envelope-Icon_at_My-Web-Site.com> wrote in
message news:ova1n59qgsk0dmokp4js7qk15c4h722vo4_at_4ax.com...
The mirrors are just fine (for a hacker circuit), but I had to patch
up the Schmitt...

http://analog-innovations.com/SED/SED_Triangle_Posting.pdf

In general, the operating currents are way too high.

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

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

Tim

Tim Williams a écrit :
"Jim Thompson" <To-Email-Use-The-Envelope-Icon_at_My-Web-Site.com> wrote in
message news:ova1n59qgsk0dmokp4js7qk15c4h722vo4_at_4ax.com...
The mirrors are just fine (for a hacker circuit), but I had to patch
up the Schmitt...

http://analog-innovations.com/SED/SED_Triangle_Posting.pdf

In general, the operating currents are way too high.

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

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

Tim


Wanting to melt a dab of iron?

Jim Thompson a écrit :
On Mon, 08 Feb 2010 14:23:44 -0800, John Larkin
jjlarkin_at_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.


??? 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.