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Why should we use current mirror to make constant current source? Doesn't fixing base current naturally fix the collector current? So why do we use diode connected bjt and then mirror them?

Take voltage divider biasing. Whatever be the value of collector resiatance, the collector current will always be (beta)*(base current). So won't connecting load between Vcc and collector (replacing collector resistance) provide constant dc supply provided bjt is in active mode?

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Doesn't fixing base current naturally fix the collector current?

It does, but the relationship between base current and collector current can vary widely from chip to chip and when the chip temperature changes.

So why do we use diode connected bjt and then mirror them?

This lets us use one device to "probe" what is the appropriate bias point to achieve a certain collector current, and then apply that to the other device (BJT) that will actually sink (assuming an NPN mirror) current from our load.

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Beta (or Hfe to use a more modern term) is highly variable from device to device easily having a 3:1 variation. It also varies with operating conditions.

A current mirror using just the base current would not be usable in practice.

The base current is more of a side effect than the cause of the collector current - a BJT is actually a voltage driven device as shown by the Ebers–Moll model. The base current is just the result of recombination in the base region - in a perfect transistor it would be zero.

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  • \$\begingroup\$ Is the current mirror completely independant of current gain? \$\endgroup\$ – Vishnu Sukumaran Mar 23 '17 at 19:05
  • \$\begingroup\$ No - but it doesn't vary by 3 to 1. A lot of the design variety in current mirrors is to reduce the effect of device parameter and operating condition variations. \$\endgroup\$ – Kevin White Mar 23 '17 at 19:35

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