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Constant Current Source for MOSFET Circuits

IP.com Disclosure Number: IPCOM000082678D
Original Publication Date: 1975-Jan-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 21K

Publishing Venue

IBM

Related People

Hoffman, CR: AUTHOR

Abstract

Constant-current sources which are stable over a reasonable range of voltage variations are useful in special circuits, mostly of the analog type, for voltage translation and for establishing bias currents. In large-scale integration (LSI) circuits, such sources should be composed of the same devices as the logic circuits on the chips and should not require externally connected components.

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Constant Current Source for MOSFET Circuits

Constant-current sources which are stable over a reasonable range of voltage variations are useful in special circuits, mostly of the analog type, for voltage translation and for establishing bias currents. In large-scale integration (LSI) circuits, such sources should be composed of the same devices as the logic circuits on the chips and should not require externally connected components.

The circuit diagrammed will provide a uniform current over the normally expected variations in the substrate voltage. In the drawing, 1 is a depletion mode metal-oxide semiconductor field-effect transistor (MOSFET) having its drain connected to a ground level voltage. A second MOSFET 2 is an enhancement mode transistor and has its source connected to 4, the substrate voltage, Vsub, for the chip in which the FET's are deposited. A third enhancement mode FET 3 also has its source connected to the substrate voltage
4. The drain of transistor 3 is available to other circuitry on the chip as a constant-current source. The gates of all transistors 1, 2, and 3 are connected together, along with the drain of transistor 1 and the source of transistor 2.

In operation, assuming that all FET's 1, 2, and 3 are operating at saturation, it can be shown that the drain voltage of FET 2, i.e., Vx, will stay at a constant voltage above Vsub irrespective of variations in Vsub within a normal range. The particular Vx for any circuit is a function of the s...