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Scott K

Member Since: May 23, 2007

Country: United States

  • Pete, contrary to your claim, current mirrors/sources/sinks can be constructed easily using discrete, unmatched components, and are done so all the time. You’ll find them in virtually any discrete audio amplifier. A current mirror is simply a current amplifier with a gain of… one. The output of a class A/B amplifier is often biased with a constant current source to reduce crossover distortion. The constant current LED driver you said was impractical is actually easy. I’ve put several of them into production.

    If your goal is to produce only a constant current, rather than a mirrored current, you need only one transistor. You’ll find such a circuit in figure 5 on the Wikipedia page for “current source”… http://en.wikipedia.org/wiki/Current_source On that page you’ll see several methods for setting the current and reducing its temperature dependence, none of which require matched transistors.

    If you really want a mirror, and there are applications that need them, you can get good performance from discrete, unmatched transistors using emitter degeneration. You’ll find more about that here… http://en.wikipedia.org/wiki/Two-port_network#Example:_bipolar_current_mirror_with_emitter_degeneration

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