active_loads

active_loads - 6.012 - Electronic Devices and Circuits -...

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1 6.012 - Electronic Devices and Circuits - 11/18/03 Two Active Loads for Differential Amplifiers: The Lee Load and the Current Mirror Load Two Active Loads for Differential Amplifiers: 1. Current mirror load 2. Lee load 1. The current mirror load: The current mirror load provides double-ended to single-ended conversion without suffering the loss of a factor of two in differential-mode gain (the common- mode gain is twice as large also, but still very small) . It comes in a variety of versions (pnp, npn, n-MOS, p-MOS); the examples below use p-channel MOSFETs in the mirror loading an n-channel common-source differential gain stage. We can begin to understand how the current mirror load functions by referring to the two figures below. The active nature of the load doubles the current delivered to the load with differential-mode inputs, and while not sending any current to the load with common-mode inputs. The results of a more detailed analysis are listed below the figures. Difference-mode inputs: Common-mode inputs: + v id /2 - V + Q 4 Q 3 Q 2 Q 1 V - + - v id /2 - + - R L i d ª 2 i d i d i d i d I BIAS r ob v out + v ic - V + Q 4 Q 3 Q 2 Q 1 I BIAS r ob V - + v ic - + R L i c i c i c i c ª 0 v out - A full analysis shows that with differential inputs, ± v
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active_loads - 6.012 - Electronic Devices and Circuits -...

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