Browse Prior Art Database

Open Emitter Follower Detection Enhancement

IP.com Disclosure Number: IPCOM000039885D
Original Publication Date: 1987-Aug-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 40K

Publishing Venue

IBM

Related People

Carter, EL: AUTHOR

Abstract

A common problem associated with current switch / emitter follower circuits is the detection of "open" defects in the emitter follower output stage. An "open" defect in the DC bias path ("pull-down") of the emitter follower will not result in a DC circuit failure. Hence, this defect can not be detected with a standard DC test. Unfortunately, this defect will degrade the circuit's transient performance very significantly. The down-going transition will increase by a factor of typically 10-100. On a very large-scale integrated chip, which contains thousands of emitter followers, it is important to detect these "open" defects. However, it is also desirable to avoid additional AC testing. A method to accomplish these goals is described below. The current technique for detecting "open" emitter followers is shown in Fig. 1.

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Open Emitter Follower Detection Enhancement

A common problem associated with current switch / emitter follower circuits is the detection of "open" defects in the emitter follower output stage. An "open" defect in the DC bias path ("pull-down") of the emitter follower will not result in a DC circuit failure. Hence, this defect can not be detected with a standard DC test. Unfortunately, this defect will degrade the circuit's transient performance very significantly. The down-going transition will increase by a factor of typically 10-100. On a very large-scale integrated chip, which contains thousands of emitter followers, it is important to detect these "open" defects. However, it is also desirable to avoid additional AC testing. A method to accomplish these goals is described below. The current technique for detecting "open" emitter followers is shown in Fig. 1. It consists of adding a large "pull-up" resistor to the output stage. If an "open" defect occurs in the DC bias path of the emitter follower, a DC "stuck-at-one" circuit failure occurs. (Since the "pull-down" path is open, the "pull-up" resistor causes the output to stay at an up level.) Hence, this "open" defect is now detectable via DC testing. Unfortunately, there is a performance penalty

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associated with this technique. Although it is large, the "pull-up" resistor takes some of the available DC bias current away from the emitter follower transistor. This causes the nominal down-going transition to increase. For a high power circuit with a relatively large emitter follower bias current, this degradation is not very significant. But, for a low power circuit with a small emitter follower bias current, the degradation can be quite significant (20-30%, for example). Due to physical design constraints, it is simply not practical to make the "pull-up" resistor large enough to not degrade the low power circuit's performance....