Berkeley ELENG 105 - Current Mirrors (3 pages)

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Current Mirrors



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Current Mirrors

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Lecture Notes


Pages:
3
School:
University of California, Berkeley
Course:
Eleng 105 - Microelectronic Devices and Circuits
Microelectronic Devices and Circuits Documents

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Discussion 8 Current Mirrors 1 Motivation Typical biasing schemes introduced in Chapters 4 and 5 fail to establish a constant collector current if the supply voltage or temperature is subject to change Although highly undesirable these are unavoidable effects in transistor operating conditions So how could we supply a constant current while avoiding supply and temperature dependence The answer is using current mirrors 2 Bipolar Current Mirror Assume that there is a golden current IREF that is essentially independent of external conditions Our goal is to produce Icopy that has same characteristics as IREF In short the black box could be realized as shown below Neglecting the base currents for now note that QREF diode connected and Q1 have equal VBE Therefore V IREF IS REF exp V VT Icopy IS 1 exp and hence Icopy VT IS IS REF I REF If QREF and Q1 are identical IS REF and IS 1 Icopy IREF Note that this holds true even though VT and IS vary with temperature Example Explain why the following circuits are bad current mirrors Example Find the expression for Icopy in terms of IREF 3 Effect of Base Current Thus far we have neglected the base current at node X by all transistors an effect leading to a significant error as the number of copies increases This error is due to a fraction of IREF that through the base rather than the collector of QREF Let us analyze the error with the following diagram For a large and moderate n the second term in the denominator is negligible and Icopy nIREF However as the number of copied current n increases so does the error in Icopy Let us try to suppress the error by introducing an emitter follower QF that is interposed between the collector of QREF and node X Note that we have assumed IC F IE F Now we have reduced the effect of the base current by a factor of In other words emitter follower allows room for more copied currents Example HW 7 Problem 1 Find the expression for Icopy in terms of IREF



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