Practical uses for logarithmic forward voltage

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cmartinez

Joined Jan 17, 2007
8,218
Yes.

The PN junction logarithmic V-I relation figures prominently in much, if not all, of analog circuit design. Aside from describing the relationship of forward voltage to current in a diode or in a BJT's base-emitter junction, the logarithmic V-I relationship is used explicitly in analog multipliers/dividers, balanced modulators/demodulators, voltage-controlled variable-gain amplifiers, circuits for generating square/square root and cube/cube root of analog signals, and log/antilog circuits. Many of these uses have embraced digital signal processing over traditional analog processing in recent years, but there are still appropriate applications.

Some reading:

http://www.ti.com/ww/en/bobpease/assets/AN-31.pdf National Semiconductor Application Note AN-31, Op Amp Circuit Collection, pp. 30-32

http://www.ti.com/lit/ds/snls385a/snls385a.pdf National Semiconductor LM194/394 datasheet, Supermatched Pair

http://users.ece.gatech.edu/~lanterma/sdiy/datasheets/transistors/AN-222.pdf National Semiconductor Application Note AN-222, Super Matched Bipolar Transistor Pair Sets New Standards for Drift and Noise, pp. 10-12

and

http://www.thatcorp.com/Design_Notes.shtml THAT Corporation design notes for their line of Blackmer Cell VCVGA ICs.
Thanks! I had already read AN-31 before... but this Supermatched Pair thing is entirely new to me... very interesting reading... thanks again!
 
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