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Diagram from Widlar's original patent. A Widlar current source is a modification of the basic two-transistor current mirror that incorporates an emitter degeneration resistor for only the output transistor, enabling the current source to generate low currents using only moderate resistor values. [1] [2] [3]
A current mirror is a circuit designed to copy a current 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.
The mirror circuit of Fig. 5 forces the drain current of M1 to equal the input current and the output configuration assures that the output current equals the drain current of M2. Expanding equation (8) in a two-variable Taylor series about i D 1 {\displaystyle \scriptstyle i_{D1}} and truncating after the first linear term, leads to an ...
Series negative feedback is also used in the two-transistor current mirror with emitter degeneration. Negative feedback is a basic feature in some current mirrors using multiple transistors, such as the Widlar current source and the Wilson current source.
Transistors Q11 and Q10 form a Widlar current mirror, with quiescent current in Q10 i 10 such that ln(i 11 / i 10) = i 10 × 5 kΩ / 28 mV, where 5 kΩ represents the emitter resistor of Q10, and 28 mV is V T, the thermal voltage at room temperature. In this case i 10 ≈ 20 μA.
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Widlar invented the basic building blocks of linear ICs including the Widlar current source, the Widlar bandgap voltage reference [9] and the Widlar output stage. [10] From 1964 to 1970, Widlar, together with David Talbert, created the first mass-produced operational amplifier ICs (μA702, μA709), some of the earliest integrated voltage regulator ICs (LM100 and LM105), the first operational ...