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From: C Egernet on 6 Jan 2010 10:38
> The nonlinear emitter impedance and temperature drift of the common-base
> device that you need between the resistor and the emitters of the noise
> canceller's diff pair.

Okay, so it is the action of the common-base transistor I don't
understand:

The TIA will convert the comparison photodiode current into a voltage
which is shared among the 43 noise cancellers, right? The large-ish
resistors will then convert that voltage into currents for each noise
canceller, right? So what does the common-base BJT do? Act as a
current-follower, pinning the voltage of the emitters of the
differential pair?

Sorry for asking elementary questions, but I'd very much like to
understand this clearly.

> Lots of airflow is going to produce a certain amount of low frequency
> noise, from microphonics and temperature variations due to turbulence.
> I'd want to put a bit of insulation around the transistors--a 1 mK
> fluctuation will give you 2 uV of drift, which is ~100 ppm in collector
> current--not particularly subtle.

It's good that you mention it. It seems that in physical science, it
is always easy to make a thermometer. The hard part is making
something that is either _not_ a thermometer or _only_ a thermometer.

> Please keep me posted on how it's going--that's the largest noise
> canceller instrument I've heard of.

So far I am only toying with the concept so I am increasingly
convinced that this is the way to go.


Best regards,

Chris Egernet
From: Phil Hobbs on 6 Jan 2010 15:32
On 1/6/2010 10:38 AM, C Egernet wrote:
>> The nonlinear emitter impedance and temperature drift of the common-base
>> device that you need between the resistor and the emitters of the noise
>> canceller's diff pair.
>
> Okay, so it is the action of the common-base transistor I don't
> understand:

You need to give the diff pair of the canceller a good quality current,
because otherwise the cancellation won't work as well--changing Vbe will
change the total current as well as the splitting ratio, which is Bad News.

The easiest way to do that is to stick the current into the emitter of a
transistor, bias the base someplace reasonable--somewhere _very_ quiet,
1 nV/sqrt(Hz) or less--and come out the collector...i.e. a common base
buffer.
>
> The TIA will convert the comparison photodiode current into a voltage
> which is shared among the 43 noise cancellers, right? The large-ish
> resistors will then convert that voltage into currents for each noise
> canceller, right?

Yup.

> So what does the common-base BJT do? Act as a
> current-follower, pinning the voltage of the emitters of the
> differential pair?
>
No, it's the other way up: low impedance in, high impedance out--the
collector goes to the diff pair emitters. You want the current not to
depend on the voltage at the emitters.

> Sorry for asking elementary questions, but I'd very much like to
> understand this clearly.
>
>> Lots of airflow is going to produce a certain amount of low frequency
>> noise, from microphonics and temperature variations due to turbulence.
>> I'd want to put a bit of insulation around the transistors--a 1 mK
>> fluctuation will give you 2 uV of drift, which is ~100 ppm in collector
>> current--not particularly subtle.
>
> It's good that you mention it. It seems that in physical science, it
> is always easy to make a thermometer. The hard part is making
> something that is either _not_ a thermometer or _only_ a thermometer.
>
Yup.

>> Please keep me posted on how it's going--that's the largest noise
>> canceller instrument I've heard of.
>
> So far I am only toying with the concept so I am increasingly
> convinced that this is the way to go.
>
Good luck!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: Jon Kirwan on 6 Jan 2010 15:51
On Wed, 06 Jan 2010 15:32:34 -0500, Phil Hobbs
<pcdhSpamMeSenseless(a)electrooptical.net> wrote:

><snip>
>The easiest way to do that is to stick the current into the emitter of a
>transistor, bias the base someplace reasonable--somewhere _very_ quiet,
>1 nV/sqrt(Hz) or less--and come out the collector...i.e. a common base
>buffer.

><snip>

>> So what does the common-base BJT do? Act as a
>> current-follower, pinning the voltage of the emitters of the
>> differential pair?
>>
>No, it's the other way up: low impedance in, high impedance out--the
>collector goes to the diff pair emitters. You want the current not to
>depend on the voltage at the emitters.

><snip>

What about Early effect here? With the common base arranged
collector feeding the diff pair emitters that are moving
around? That would seem to move the collector and thus some
difference due to the Early effect, to me. What am I
missing?

Jon
From: Phil Hobbs on 6 Jan 2010 16:33
On 1/6/2010 3:51 PM, Jon Kirwan wrote:
> On Wed, 06 Jan 2010 15:32:34 -0500, Phil Hobbs
> <pcdhSpamMeSenseless(a)electrooptical.net> wrote:
>
>> <snip>
>> The easiest way to do that is to stick the current into the emitter of a
>> transistor, bias the base someplace reasonable--somewhere _very_ quiet,
>> 1 nV/sqrt(Hz) or less--and come out the collector...i.e. a common base
>> buffer.
>
>> <snip>
>
>>> So what does the common-base BJT do? Act as a
>>> current-follower, pinning the voltage of the emitters of the
>>> differential pair?
>>>
>> No, it's the other way up: low impedance in, high impedance out--the
>> collector goes to the diff pair emitters. You want the current not to
>> depend on the voltage at the emitters.
>
>> <snip>
>
> What about Early effect here? With the common base arranged
> collector feeding the diff pair emitters that are moving
> around? That would seem to move the collector and thus some
> difference due to the Early effect, to me. What am I
> missing?
>
> Jon

The magnitude of the change. In general purpose NPNs, the Early voltage
is ~100V, whereas the emitters only move 50 mV at most, and the resistor
current sources won't be dropping anywhere near 100V. The OP can
probably use BF862 JFETs just as well, but I wouldn't lose too much
sleep over it, at least at the 60 dB cancellation level. If he needs
more than that, it'll start to matter, but so will a lot of other things.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: Jon Kirwan on 6 Jan 2010 17:47
On Wed, 06 Jan 2010 16:33:13 -0500, Phil Hobbs
<pcdhSpamMeSenseless(a)electrooptical.net> wrote:

>On 1/6/2010 3:51 PM, Jon Kirwan wrote:
>> On Wed, 06 Jan 2010 15:32:34 -0500, Phil Hobbs
>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote:
>>
>>> <snip>
>>> The easiest way to do that is to stick the current into the emitter of a
>>> transistor, bias the base someplace reasonable--somewhere _very_ quiet,
>>> 1 nV/sqrt(Hz) or less--and come out the collector...i.e. a common base
>>> buffer.
>>
>>> <snip>
>>
>>>> So what does the common-base BJT do? Act as a
>>>> current-follower, pinning the voltage of the emitters of the
>>>> differential pair?
>>>>
>>> No, it's the other way up: low impedance in, high impedance out--the
>>> collector goes to the diff pair emitters. You want the current not to
>>> depend on the voltage at the emitters.
>>
>>> <snip>
>>
>> What about Early effect here? With the common base arranged
>> collector feeding the diff pair emitters that are moving
>> around? That would seem to move the collector and thus some
>> difference due to the Early effect, to me. What am I
>> missing?
>>
>> Jon
>
>The magnitude of the change. In general purpose NPNs, the Early voltage
>is ~100V, whereas the emitters only move 50 mV at most, and the resistor
>current sources won't be dropping anywhere near 100V. The OP can
>probably use BF862 JFETs just as well, but I wouldn't lose too much
>sleep over it, at least at the 60 dB cancellation level. If he needs
>more than that, it'll start to matter, but so will a lot of other things.
>
>Cheers
>
>Phil Hobbs

Got it. The diff pair bases can't be driven around that much
so their shared emitters won't move much, etc.

By the way, 100V seems to be kind of a "blind default" case
(unmeasured, it seems to me, but estimated by some gross
sweep of the hand) for modeling small signal NPNs like the
2N3904 or 2N2222. But I don't have much experience actually
measuring the parameter -- something I should remedy. The
specific NPNs the OP might use for the common base design
might have significantly smaller magnitude Early voltage,
though. Regardless, your point stands. I can't recall ever
seeing a model (as I said, I've not measured so I'm lacking
experience there) with an Early voltage less than 20V. And
even there movement in the tens of millivolts shouldn't be a
serious problem. (Maybe before I say that, I should see what
50mV movement does on a VAF=20V device against the 60dB
level, though. At my level of understanding, this is kind of
interesting to think about.)

Thanks,
Jon
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