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High Performance Analog Circuit Buffer for Logic Modeling

IP.com Disclosure Number: IPCOM000109803D
Original Publication Date: 1992-Sep-01
Included in the Prior Art Database: 2005-Mar-24
Document File: 3 page(s) / 68K

Publishing Venue

IBM

Related People

Ditlow, G: AUTHOR [+6]

Abstract

Logic circuits are simulated using an analog circuit simulator (ASTAP or SPICE) during circuit defect detection analysis. The inputs of each circuit under test are the outputs of input buffers, and the outputs of each circuit under test are inputted into output buffers (Fig. 1). The purpose of the buffers is to simulate a real chip environment.

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High Performance Analog Circuit Buffer for Logic Modeling

       Logic circuits are simulated using an analog circuit
simulator (ASTAP or SPICE) during circuit defect detection analysis.
The inputs of each circuit under test are the outputs of input
buffers, and the outputs of each circuit under test are inputted into
output buffers (Fig. 1).  The purpose of the buffers is to simulate a
real chip environment.

      Traditionally, a six-transistor OR circuit (Fig. 2) was used
for both the input and output buffers.  This circuit requires 204
ASTAP equations per input/output (2 X 17 branches X 6 transistors).
Input and output buffers were designed such that each consists of
only one transistor.  ASTAP generates only 34 equations per
input/output (2 X 17 branches X 1 transistor) for each buffer.

      The input buffer (Fig. 3) is an emitter follower transistor
that is configured so the output voltage level follows the
controllable input voltage minus the base to emitter voltage.  The
transistor acts like a current amplifier and represents a real chip
environment for the circuit under test.

      The output buffer (Fig. 4) consists of an input transistor.
The transistor turns on when its input is high and turns off when its
input is low, so the bias current is set higher than the turn-on
stage so that the transistor is on constantly.  This guarantees the
desired full output voltage swing.  The buffer simulates the average
loading effects due to fanout t...