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From: Bitrex on 11 May 2010 00:18
Jim Thompson wrote:

> OK, so the problem statement is "Output current doubles for each one
> volt increase in input... temperature compensated" ??
>
> ...Jim Thompson

More succinctly, yes that's about it. The first part is not difficult,
but the second part was/is apparently a bigger challenge, as there are
plenty of old synthesizers that could never stay well in tune.
From: Robert Baer on 11 May 2010 03:23
Bitrex wrote:
> John Larkin wrote:
>> On Sat, 08 May 2010 20:19:15 -0400, Bitrex
>> <bitrex(a)de.lete.earthlink.net> wrote:
>>
>>>
>>> If you put a reference current into the collector of one half of a
>>> differential pair,
>>
>> You've lost me already. What do you mean by putting a current into the
>> collector of a diff pair? Normally one stuffs a current into the
>> paralleled emitters.
>>
>> Can you post a schematic?
>>
>> John
>>
>>
>
> Sorry for not being more clear - in the meantime I was able to find a
> website that has an example of the circuit configuration I am talking
> about here, figure 3:
>
> http://www.uni-bonn.de/~uzs159/
* HUH??? _no_ figures; where shall one look???

>
> It appears from the text the most common way to compensate the 1/T
> temperature dependence of the circuit is to use a resistor with a
> positive temperature coefficient in the circuit that provides the
> control voltage. To make it work well I guess you would want to bind
> the differential pair to the tempco resistor with some kind of thermally
> conductive adhesive.
>
>
>
>>
>> and then take an output current from the collector of
>>> the other transistor of a differential pair, the output current as a
>>> fraction of the reference current will be compensated against
>>> variations of Is. However, because of the dependence of VT on
>>> temperature Vbe will now have a positive tempco, and the output
>>> current as a fraction of the reference current will decrease with
>>> increasing temperature of the pair. I'm wondering what strategies the
>>> analog designers here have used to compensate for this second-order
>>> temperature dependence? TIA!
>>
From: whit3rd on 11 May 2010 04:29
On May 10, 9:18 pm, Bitrex <bit...(a)de.lete.earthlink.net> wrote:
> Jim Thompson wrote:
> > OK, so the problem statement is "Output current doubles for each one
> > volt increase in input... temperature compensated" ??

> More succinctly, yes that's about it.

So, use a THAT2181 as your gain stage, and attenuate the gain-control
signal with a two-resistor attenuator. R1 is a straight resistor, say
40k ohms,
and R2 is a 1kohm +3300ppm/C PTC thermistor. The thermistor
has to be thermally coupled to the amplifier IC, of course.
The 'Ec' program pins were made for this kind of thing.
From: Bitrex on 11 May 2010 04:42
miso(a)sushi.com wrote:
> On May 9, 9:44 pm, Bitrex <bit...(a)de.lete.earthlink.net> wrote:
>> Michael Robinson wrote:
>>> "Bitrex" <bit...(a)de.lete.earthlink.net> wrote in message
>>> news:5Yednf5fg-xt1nvWnZ2dnUVZ_rydnZ2d(a)earthlink.com...
>>>> m...(a)sushi.com wrote:
>>>>> On May 8, 8:01 pm, Bitrex <bit...(a)de.lete.earthlink.net> wrote:
>>>>>> John Larkin wrote:
>>>>>>> On Sat, 08 May 2010 20:19:15 -0400, Bitrex
>>>>>>> <bit...(a)de.lete.earthlink.net> wrote:
>>>>>>>> If you put a reference current into the collector of one half of a
>>>>>>>> differential pair,
>>>>>>> You've lost me already. What do you mean by putting a current into the
>>>>>>> collector of a diff pair? Normally one stuffs a current into the
>>>>>>> paralleled emitters.
>>>>>>> Can you post a schematic?
>>>>>>> John
>>>>>> Sorry for not being more clear - in the meantime I was able to find a
>>>>>> website that has an example of the circuit configuration I am talking
>>>>>> about here, figure 3:
>>>>>> http://www.uni-bonn.de/~uzs159/
>>>>>> It appears from the text the most common way to compensate the 1/T
>>>>>> temperature dependence of the circuit is to use a resistor with a
>>>>>> positive temperature coefficient in the circuit that provides the
>>>>>> control voltage. To make it work well I guess you would want to bind
>>>>>> the differential pair to the tempco resistor with some kind of thermally
>>>>>> conductive adhesive.
>>>>>>> and then take an output current from the collector of
>>>>>>>> the other transistor of a differential pair, the output current as a
>>>>>>>> fraction of the reference current will be compensated against
>>>>>>>> variations
>>>>>>>> of Is. However, because of the dependence of VT on temperature Vbe
>>>>>>>> will
>>>>>>>> now have a positive tempco, and the output current as a fraction of
>>>>>>>> the reference current will decrease with increasing temperature of the
>>>>>>>> pair. I'm wondering what strategies the analog designers here have
>>>>>>>> used
>>>>>>>> to compensate for this second-order temperature dependence? TIA!
>>>>> Looks like frames did you a disservice. It this the page in question?
>>>>> http://www.uni-bonn.de/~uzs159/expo_tutorial/index.html
>>>>> If not, then right click on the link in question and open it in
>>>>> another window to get the correct URL.
>>>> Thanks, that's the correct page.
>>> That guy is such a bad writer. He never gives you the most basic
>>> information: what's the circuit supposed to do, what kind of input signal
>>> goes in, just the most basic stuff that should be the first thing he tells
>>> you. Instead he starts out from nowhere and goes into excructiating detail
>>> about a whole bunch of stuff that has no context. It looks like he's
>>> describing an attempt to build a transconductance amplifier with an
>>> exponential transfer function. And there's a sentence buried deep in the
>>> text, something about in input of one volt per octave, and he wants the
>>> output current to double for a change of one volt in the input signal. I
>>> haven't figured out whether the circuit he shows is fit for that purpose.
>>> How many octaves (volts) is the input going to vary? How big of a signal
>>> can you put into the circuit he drew? You could spend forever getting lost
>>> in the details. Better to ask first: what's the right approach?
>> It does make more sense when you get to the page above through the
>> main page:http://www.uni-bonn.de/~uzs159/index.html, the material then
>> has some context!
>
> I thinks frames messed you up again. I have no idea where you are
> trying to send us. However, I don't think that website is particularly
> useful. Not only is the electronics questionable, but the page refers
> to figures that are not even labeled.
>
> SSM and Curtis Electro Music used to make chips for analog
> synthesizers. ADI absorbed SSM. Doug Curtis died a few years ago, and
> he had already folded that company into his design service company.
> THAT makes many of the same analog circuits. I would try to find
> designs from those companies rather than mess with that questionable
> website.

I've actually built a few circuits around the SSM2044 VCF - I bought
several off Ebay years ago before I understood how large the industry of
IC fakes is. The ones I've used appear to function as intended within
the bounds of what tests I'm able to perform, so who knows, maybe
they're _good_ fakes? :)

>
> Don't get me wrong. I would like to encourage you to do the analog
> design since nowadays people think engineering is programming a damn
> pic. Just don't put too much faith into one website. This stuff really
> isn't rocket science, but analog can be difficult to understand if it
> is not explained well.

Thanks for the encouragement - I do some microcontroller programming too
but it's the analog aspect of circuit design that really gets my mind going.
From: Jim Thompson on 11 May 2010 12:56
On Tue, 11 May 2010 00:18:21 -0400, Bitrex
<bitrex(a)de.lete.earthlink.net> wrote:

>Jim Thompson wrote:
>
>> OK, so the problem statement is "Output current doubles for each one
>> volt increase in input... temperature compensated" ??
>>
>> ...Jim Thompson
>
>More succinctly, yes that's about it. The first part is not difficult,
>but the second part was/is apparently a bigger challenge, as there are
>plenty of old synthesizers that could never stay well in tune.

OTA-based sine-wave oscillator?

Or would square wave suffice?

...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 |

The only thing bipartisan in this country is hypocrisy
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