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Current Sources Disclosure Number: IPCOM000074906D
Original Publication Date: 1971-Jun-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 2 page(s) / 37K

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Cole, TG: AUTHOR [+1]


High-density integrated circuits often require many current sources to be fabricated on one chip.

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Current Sources

High-density integrated circuits often require many current sources to be fabricated on one chip.

Fig. 1 shows a circuit configuration with good current definition which makes economical use of space on the chip. The circuit uses similar transistors operating in both normal and inverted modes, the transistors typically having a forward current gain of 40 to 160 and an inverted current gain of 4 to 16.

Devices 'a' and 0 form a Darlington pair. Since device 'a' is operated in the normal mode, the base current of 'a' is negligible compared with I. Hence I(e0) = I over alpha(i) = inverted alpha and V(be0 ) alpha I over alpha (i) Also V(bea) alpha I over beta(i) alpha I over alpha (i) Hence the voltage on the base of device a/b is proportional to I and alpha (i). Assuming: (i) V(be) tracks between devices,
(ii) alpha(i) tracks for devices, and (iii) Emitter area of device b is increased proportionally to fan out, the current sources will track exactly with the defined current.

By using the circuit arrangement shown in Fig. 1, multiple current sources can be provided in a space significantly smaller than previous layouts by using a common base diffusion and a common collector region for the inverted devices, with the added advantage of considerably simpler wiring. Also, the definition of the current is made independent of beta and the number of current sources being driven.

When distributing the current sources around a chip the resistance of the base d...