Browse Prior Art Database

Current Threshold Circuit

IP.com Disclosure Number: IPCOM000082163D
Original Publication Date: 1974-Aug-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 27K

Publishing Venue

IBM

Related People

Taub, DM: AUTHOR

Abstract

In the detection of signals occurring on transmission-lines particularly those carrying data, it is often necessary to discriminate between currents above and below a defined threshold value.

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Current Threshold Circuit

In the detection of signals occurring on transmission-lines particularly those carrying data, it is often necessary to discriminate between currents above and below a defined threshold value.

In the circuit shown i(in) represents the input current, I(t) a threshold current and i(out) the output current. T1 and T2 are a matched pair of transistors connected to form a current-mirror circuit; so also are T3 and T4. Silicon semiconductors are assumed throughout.

The circuit has the property that when i(in) < I(t), I(out) = 0 and when i(in) > I(t), i(out) = i(in) - I(t). V and R1 are so proportioned that the collector current of T1 equals the required threshold current I(t). The collector current of T2 will be substantially the same due to the current-mirror action between these two transistors. R2 has a somewhat lower value than R1 so that the current it carries is not less than I(t).

As long as i(in) < I(t) D3 must remain conducting, and so the collector voltage of T2 will be about 0.7 volt. This is too low to allow any current to flow through D4 and T3, and so i(out) will remain at zero. When i(in) exceeds I(t) the collector voltage of T2 rises, cutting off the current in D3 and allowing the excess current i(in) - I(t) to flow through D4. The current-mirror action between T3 and T4 causes a substantially equal current to flow in the output circuit.

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