Design Procedure With Specified Currents
To design the mirror, the output current must be related to the two resistor values R1 and R2. A basic observation is that the output transistor is in active mode only so long as its collector-base voltage is non-zero. Thus, the simplest bias condition for design of the mirror sets the applied voltage VA to equal the base voltage VB. This minimum useful value of VA is called the compliance voltage of the current source. With that bias condition, the Early effect plays no role in the design.
These considerations suggest the following design procedure:
- Select the desired output current, IO = IC2.
- Select the reference current, IR1, assumed to be larger than the output current, probably considerably larger (that is the purpose of the circuit).
- Determine the input collector current of Q1, IC1:
- Determine the base voltage VBE1 using the Shockley diode law
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- where IS is a device parameter sometimes called the scale current.
- The value of base voltage also sets the compliance voltage VA = VBE1. This voltage is the lowest voltage for which the mirror works properly.
- Determine R1:
* Determine the emitter leg resistance R2 using Eq. 1 (to reduce clutter, the scale currents are chosen equal):
Read more about this topic: Widlar Current Source
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