tempco resistors

[gstopp at fibermux.com]

     It's not a stupid question - it took me years to understand this and 
     I'm still not sure I got it right. But I'll try...
     
     The primary use for tempco resistors in analog music circuits is in 
     the exponential converters in VCO's and VCF's.
     
     Exponential converters are used because they allow the VCO's or VCF's 
     to track each other in musical intervals from the same modulation 
     source. Musical notes as generated by vibrating or oscillating things 
     are by nature exponential. Electronic implementation of exponential 
     functions is easy to do with transistors.
     
     Transistors are made out of artificially-made impure mineral crystals. 
     The way that they behave when electricity is passed though them is 
     determined by the actions of the electrons within the crystals. At 
     what is normal room temperature for human beings, the electrons inside 
     of all things are jiggling around constantly like jumping beans 
     because of the heat in the environment. The hotter the surroundings 
     get, the more they jump around. The only time that they are not moving 
     is when the surroundings are at a temperature of absolute zero. We 
     humans hate to play our synthesizers under these conditions, so we 
     have to design them to work at (human) room temperature.
     
     As the room temperature varies from chilly to sweltering (for humans), 
     the operation of transistors will change slightly due to the fact that 
     the electrons in the crystal bounce around faster as the temperature 
     rises. They still work, they just work a little different. The big 
     problem is that a VCO that is built around the transistor will 
     oscillate at a slightly different frequency for slightly different 
     temperatures. This means that it will go out of tune. Since this is a 
     pain in the butt for musicians, it is desireable to eliminate this 
     effect.
     
     The amount of change in an electronic component as related to 
     temperature is called it's "temperature coefficient", or "tempco". It 
     is possible to create resistors that have whatever tempco is desired, 
     simply by changing the materials that they are made from. For example, 
     you can have a resistor that increases its value by three ohms for 
     every ten degrees of increasing temperature change, and you can make 
     another resistor that decreases its value by five ohms for every ten 
     degrees of increasing temperature change.
     
     So what synthesizer designers do to alleviate the temperature-caused 
     tuning problems with analog VCOs, is to put the appropriate tempco 
     resistor in the exponential converter so that the tempco effect in the 
     transistors are cancelled out by the tempco effect in the resistor, so 
     that the VCO does not change its tuning at all when the temperature 
     changes. Even though the individual components are still obeying the 
     laws of physics and changing their behavior slightly with temperature, 
     they have been cleverly arranged in a circuit so that these effects 
     cancel each other out.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: tempco resistors
Author:  ET3185SE at SNYFARVA.CC.FARMINGDALE.EDU at ccrelayout
Date:    4/30/96 11:06 PM


This is a really stupid question, but what are tempco resistors? 
>From what I understand they are temperature compensating resistors. 
but what makes them special, or different from other resistors. 
Thanks
John