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Home Brew Analog Computer System
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GK:
Attached is the complete schematic for my logarithmic amplifier module. Due to the utility of log amps in a analog computer, I've decided to build 10 of these instead of only five. A custom 2U 19" chassis will house 10 of these along with their power supply. Another 2U chassis will house 10 exponential amplifier modules.

The transfer function is simply:

Eout = K * ln (Ein).

K = a fixed scaling factor of 20. The operating input voltage range is 0.01V to 100V, producing an output voltage in the range of +/- 92.1V.
The output amplifier clips at approximately +/-105V, so the dynamic range is a smidge over 4 decades.

The actual log converter based on the SSM2212 is a "fast" design I came up with on my own. Over the 4 decades it is very accurate (overall significantly better than 1% when trimmed).

A conversion to a logarithm of any base is simply a matter of external scaling by the programmer.

SeanB:
Looks nice.
GK:

--- Quote from: SeanB on January 10, 2013, 10:13:36 am ---Looks nice.

--- End quote ---

Thanks!

BTW, if you saved the schematic, please delete it. There was a silly error and I had to take it down. I misplaced the decimal point calculating the resistor values for the offset trim/null range and it was inadequate by a factor of 10.

Attached is the fixed schematic (R8 increased from 10R to 100R and R10 added to restore balance).


EDIT: circuit revised - see reply#288

SeanB:
I do see the difference. You could probably replace D10-14 with 1N4148's as well, mount close to the heatsink inside a shrink tube to keep stray light from them.
GK:
The D10-D14 string used for biasing the output transistors doesn't have to be thermally coupled to the heat sinks, as there is a large total of 132 ohms of emitter degeneration (R30-R33), which makes thermal runaway impossible. Suppose the output transistor heat up delta 50 deg C due to heavy loading. That will cause the emitter-emitter potential difference between Q15 and Q16 to increase by approximately 200mV, returning a quiescent current increase of only 1.5mA.  On +/-110V rails that will increase the quiescent power dissipation of each power output transistor by only ~165mW. This pretty much represents a worst case scenario; a 50 degree C delta heating in both power output transistors could only happen with amplifier being heavily ac driven into current limiting into an Eout short. 

I used 1N4007's instead of 1N4148's for their lower individual voltage drop. Their total voltage drop along with the 132 ohms emitter degeneration result in a reasonably well defined (taking into account component variations) output transistor quiescent current of ~5mA.

I am though open to suggestions on how to best thermally couple the SOIC-8 SSM2212 dual monolithic transistor to the leaded thermistor for optimal temperature compensation of the log converter.
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