Current Sources and Voltage References

Multiple current mirrors

Using BJTs to Create Current Sources

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and the output current can be approximated within 1% or 2% by:

As an example, we will create a 13.5-µA current source using a supply voltage of 5

volts DC and an input current of 100 µA. The NPN pair, which forms the first stage,

reduces the 100 µA down to 37 µA at the collector of Q

2

. This current is the input for

Q

3

of the PNP pair, which again reduces the current down, so that Q

4

’s collector out-

puts a constant current of 13.5 µA. We can show this by the following:

and the output will be:

The output is perfectly flat with an input voltage range of between 2 and 10 volts, and

with an input ...