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I think a quick way would be to use the characteristics of a FET. In the linear region they act like voltage-controlled resistors. In the saturation region they are voltage controlled current sources. The voltage-controlled resistor characteristic of a FET is usually exploited in building gain controlled amplifiers. For a certain gate input voltage, you get a drain current. The relationship between the two in the linear region mimics a resistor.
For example, you can put a FET in the feedback path of an opamp to control the gain. If you assume a standard inverting configuration then Gain = Vout/Vin = Rfeedback / R1 = Rfet / R1. Where R1 is the resistor that is not in the feedback path.
I'm not too sure if this helps, but a few fets you might be able to use:
3SK38A, 3N170, CD3600, 2N4351, SD210.
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