ACWWong
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yes precision requirement is all important... but options include:
1) if you already have flat bandgap voltage over temperature, you can use a simple V to I converter. 2) add PTAT current to CTAT current in the right proportions PTAT current is usually delta-vbe, and CTAT current is vbe with V to I converter
both cases are made easy and flat temp over temperature if you have a zero temp coefficent resistor available to you.... if you don't then it comes back to the question of precision requirement.... often the requirement may not just be vs temp.... bear in mind you can get good zero temp coefficient current, but absolute accuracy over process and mismatch etc. can vary alot and will need more techniques or an external resistor.
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