165-current-mirror-and-current-source-transistor-biasing

Current mirror is electronic scheme given in figure bellow.

current-mirror

Current mirror is used as the current source and it is often used for transistor biasing. How does current mirror works? For transistor biasing in non-discrete (integrated) technology it is important to use as less resistors as possible in order to save substrate surface for other components in the electronics scheme. So we would like to avoid base resistors in transistor biasing. Since both transistors Q1 and Q2 are on the same substrate, they have very similar characteristics. According to upper scheme, base-to-emitter voltages are the same. Base currents and collector currents are the same as well, so according to current Kirchhoff law we have:

current_mirror_eq01

Knowing that

current_mirror_eq02

we have

current_mirror_eq03

i.e.

current_mirror_eq04

For hFE large enough (which is almost always case in practice), particularly for signal transistors, we can say that IC≈I. On the other side,

current_mirror_eq05

It means that collector current of the Q2 transistor is defined only with VCC and resistor R, and not by RC in the wide range of R and RC values. For this reason, we treat current mirror as voltage controlled current source. Current mirror is used for basing transistor differential amplifier, that is the input stage in operational amplifier (op-amp). Better currents matching can be accomplished with Wilson current source, given in figure bellow.

wilson-current-source

Currents matching in Wilson current source is

current_mirror_eq06

For example, if hFE=100 (and that really is not much for signal transistors), difference between input current and IC3 is only 0.2%. Current mirror can also be realized with two PNP transistors:

current-mirror-pnp

or with one PNP transistor and one diode.

current-mirror-diode

Current mirror with one PNP transistor and one diode as well as current mirror with two PNP transistors are often used as current sources for sawtooth voltage generators.

External links:

Current Mirror and Current Source Transistor Biasing on ami.ac.uk
Current Mirror and Current Source Transistor Biasing on radio-electronics
Current Mirror and Current Source Transistor Biasing on Wikipedia

 

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