# I don't get transistors

**URL:** https://boards.straightdope.com/t/i-dont-get-transistors/829041
**Category:** Factual Questions
**Created:** [February 6, 2019, 1:17am UTC](https://boards.straightdope.com/t/i-dont-get-transistors/829041 "2019-02-06T01:17:29Z")
**Posts on this page:** 1
**Page:** 2

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### Author: ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)
#### Post date: [February 7, 2019, 5:12am UTC](https://boards.straightdope.com/t/i-dont-get-transistors/829041/21 "2019-02-07T05:12:21Z")

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In an electric circuit, voltage is controlled by current. Current is controlled by voltage. What controls what?

A BJT has an exponential (diode) response to voltage on the Base/Emitter junction. It is difficult to control any of the currents, because you have to generate a log function of the input signal to go into the exponential object to get the output you want. If you just want a switch that doesn’t matter.

The Emitter / Collector current is roughly proportional to the base current, so you can get the output you want by controlling the base current as you want.

But a FET isn’t called a voltage controlled device because it has a nice linear response. It’s called a voltage-controlled device because it requires very little current to control it. Language is funny like that sometimes.

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