Current mirror
From Free net encyclopedia
A current mirror is a circuit designed to copy a current flowing through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being 'copied' can be, and sometimes is, a varying signal current.
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BJT Current Mirror
Operation
Transistor Q1 is connected such that it has a constant current flowing through it (due to R1 and Vs) and behaves as a forward-biased diode, and the current is determined mainly by the resistance R1. It is important to have Q1 in the circuit, instead of a regular diode, because the two transistors can be matched, and thus the two branches of the circuit will have similar characteristics. The voltage at the base of Q1 will necessarily be the exact voltage that sustains the collector current. The second transistor, Q2, which then has the same base voltage, changes its own collector current so that the total effective resistance in the second branch of the circuit is the same as the total resistance in the first branch, regardless of the load resistor, R2. Since the total resistance in each branch is the same, and they are connected to the same supply, VS+, the amount of current in each branch is the same.
The constant current flowing through R1 can be varied by altering the value of R1 to change the amount of current going through R2. Since R2 can change dynamically, and the current through it will stay the same, the current mirror is a current regulator. It can also be thought of as a constant current source, due to the way it is used in integrated circuits. Note that the current mirror is not a "true constant current source (CCS)" because its output current is not independent of the supply voltage. In order for the current mirror to deliver constant current (i.e., act as a CCS), it must be supplied by a constant voltage source. The term "true CCS" here does not denote 'ideal' but denotes a CCS which gives a reasonably constant current output for varying supply voltage, load and (optionally) temperature. Temperature variations can be ignored for non-critical applications or where accuracy is of secondary concern. There are limits to how much the current, load and temperature can vary for a CCS to operate within specification.
Additional matched transistors can be connected to the same base and will supply the same collector current. In other words, the right half of the circuit can be duplicated several times with differing values of R2 on each.
Circuit analysis
The current through R1 is given by:
<math>I_{r1} = I_{c1} + I_{b1} + I_{b2}</math>
Where <math>I_{c1}</math> is the collector current of Q1, <math>I_{b1}</math> is the base current of Q1, <math>I_{b2}</math> is the base current of Q2.
The collector current of Q1 is given by:
<math>I_{c1} = \beta I_{b1}</math>
Where <math>\beta</math> is the DC current gain of Q1. If Q1 and Q2 are perfectly matched, <math>\beta</math> of Q2 will be that same as that of Q1. Similarly, the base currents of Q1 and Q2 will be the same. (Note: <math>\beta</math> is also commonly denoted as hFE)
After substituting and rewriting, one finds that the collector current of Q2 is given by:
<math>I_{c2} = \frac{\beta I_{r1}}{\beta + 2}</math>
If <math>\beta >> 1</math>, then
<math>I_{c2} \approx I_{r1}</math>
Typical values of <math>\beta</math> will yield a current match of 1% or better. Even better matching can be achieved with more sophisticated current mirrors, such as the Widlar current source, Cascoded current sources and Wilson current source.
MOSFET Current Mirror
Operation
Transistor T1 is operating in the saturation region, and so is T2. In this setup, the output current Iout will be directly related to Iref. Template:Electro-stub