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There are three main specifications that characterize a current mirror. The first is the transfer ratio (in the case of a current amplifier) or the output current magnitude (in the case of a constant current source CCS). The second is its AC output resistance, which determines how much the output current varies with the voltage applied to the ...
In a standard two-transistor current mirror, the output impedance would be the dynamic early resistance of the output transistor, the equivalent of which in this case is . The Wilson current mirror has an output impedance that is higher by the factor β 2 {\displaystyle \scriptstyle {\frac {\beta }{2}}} , on the order of 50 times.
Figure 1: A version of the Widlar current source using bipolar transistors. Figure 1 is an example Widlar current source using bipolar transistors, where the emitter resistance R 2 is connected to the output transistor Q 2, and has the effect of reducing the current in Q 2 relative to Q 1.
Thus, the load current is constant (neglecting the output resistance of the transistor due to the Early effect) and the circuit operates as a constant current source. As long as the temperature remains constant (or doesn't vary much), the load current will be independent of the supply voltage, R1 and the transistor's gain.
Most commonly the active load is the output part of a current mirror [1] and is represented in an idealized manner as a current source. Usually, it is only a constant-current resistor that is a part of the whole current source including a constant voltage source as well (the power supply V CC on the figures below).
in parallel with the collector–emitter junction of the transistor. This resistor can thus account for the finite output resistance of a simple current mirror or an actively loaded common-emitter amplifier. In keeping with the model used in SPICE and as discussed above using the resistance becomes:
Figure 3 shows a bipolar current mirror with emitter resistors to increase its output resistance. [nb 1] Transistor Q 1 is diode connected, which is to say its collector-base voltage is zero. Figure 4 shows the small-signal circuit equivalent to Figure 3. Transistor Q 1 is represented by its emitter resistance r E:
So in more sophisticated designs, an element with high differential (dynamic) resistance approximating a constant current source/sink (the bottom of Fig. 3) is substituted for the “long tail”. It is usually implemented by a current mirror because of its high compliance voltage (small voltage drop across the output transistor).