Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!umix!b-tech!umich!itivax!dhw
From: dhw@itivax.UUCP (David H. West)
Newsgroups: comp.ai.neural-nets
Subject: Re: Sigmoid transfer function
Message-ID: <207@itivax.UUCP>
Date: 25 Aug 88 16:27:39 GMT
References: <1982@aecom.YU.EDU> <917@buengc.BU.EDU>
Organization: Industrial Technology Institute
Lines: 27


Using ideal transistors with transfer function Ic = I0 * exp(Vbe/Vt)
the 'long-tailed pair':

            |          |
            | I1       | I2
            |          |
          |/            \|
     V1 --|              |-- V2
          |\            /
            Y__________Y
                  |
                  O
                  O I3
                  |

(sorry about those emitter symbols!)
has transfer function

	delta_I = I3 * tanh (delta_V / Vt)

where delta_I = I1 - I2
      delta_V = V1 - V2
      Vt = kT/e (some tens of mV at any reasonable temperature) 
            

-David West            dhw%iti@umix.cc.umich.edu