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# Complementary Driven Current Source Interface Circuit

IP.com Disclosure Number: IPCOM000083374D
Original Publication Date: 1975-May-01
Included in the Prior Art Database: 2005-Mar-01
Document File: 2 page(s) / 34K

IBM

## Related People

Chin, W: AUTHOR [+4]

## Abstract

In general, many methods can be used to downshift a small swing logic signal by using Vbe shifts and resistor divider networks. Variations in Vbe power supply tolerances and resistor tolerances can render simple designs inoperable.

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Complementary Driven Current Source Interface Circuit

In general, many methods can be used to downshift a small swing logic signal by using Vbe shifts and resistor divider networks. Variations in Vbe power supply tolerances and resistor tolerances can render simple designs inoperable.

This design provides a means of translating a small swing logic signal into a larger swing signal (based about a more negative level), which is relatively independent of Vbe variation, temperature effects and resistor tolerances. Instead of using a direct level shift, a "complementary" current source is used to drive the base of the collector drive output stage which does the final voltage translation.

When node 1 of the illustrated circuit is at a "0" logic level, node 2 is at a corresponding "1" logic level. The current I, is given by: I(1) approx.= [V(1) - Vbe(T1) - Vbe(T3) - (-V)]/R(5).

The circuit configuration (with matched devices T3 and T4) requires that I(cT4) approx.= I(cT3). At the same time, the logical 1 level at node 2 allows T2 to supply a higher current than T1. This causes the voltage at node 6 to rise, until device T5 turns on and clamps node 6 to V(6) approx.= -V + Vbe T5. The difference between I3 and I(cT4) (approx.=I1) is available as base drive o T5.

For the opposite logic conditions, (i.e. node 4 at 1 level and node 2 at a 0 level), the current in T1 is as previously determined. Since node 1 is higher, the current I1 is at a higher value.

The condition IcT4 a...