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Low-Level Output Tracking Circuit

IP.com Disclosure Number: IPCOM000102120D
Original Publication Date: 1990-Oct-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 2 page(s) / 59K

Publishing Venue

IBM

Related People

Strom, JD: AUTHOR [+2]

Abstract

An opamp shown in the figure controls the tracking circuit comprised of R1, Q1, R3, Q2, R2, RX2 and RX1 by adjusting the voltage at node N1. The negative opamp input is connected to a reference voltage, and the opamp forces node NX1 to the same reference by forcing N1 to the appropriate voltage. R1 and Q1, which is biased by R2, drive N3. R3 and emitter follower Q2 bias external node NX2 and external resistor RX2 sets the current in Q2 so that the collector current of Q2 pulled through external resistor RX1 biases node NX1 at the same voltage as the reference voltage that is connected to the negative input to the opamp.

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Low-Level Output Tracking Circuit

       An opamp shown in the figure controls the tracking
circuit comprised of R1, Q1, R3, Q2, R2, RX2 and RX1 by adjusting the
voltage at node N1.  The negative opamp input is connected to a
reference voltage, and the opamp forces node NX1 to the same
reference by forcing N1 to the appropriate voltage.  R1 and Q1, which
is biased by R2, drive N3.  R3 and emitter follower Q2 bias external
node NX2 and external resistor RX2 sets the current in Q2 so that the
collector current of Q2 pulled through external resistor RX1 biases
node NX1 at the same voltage as the reference voltage that is
connected to the negative input to the opamp.

      Predrive circuitry biases Q10 and Q14 on, while turning Q9 and
Q13 off.  R8 and Q11, which is biased by the base current in Q15,
Q17, Q18 and Q19, level shift the voltage at node N1 to node N15 by
one Vbe drop.  The current in Q10 through R9 bias N14 below N13 and
Q12, which is biased Q14, drives N16 to a voltage such that Q15 turns
on and Q16 turns off.

      The transistor sizes of Q1, Q2, Q11 and Q15 and values of
resistors R1, R2, R3, RX1, RX2, R9, R10, RX6 and RX8 are chosen so
the voltage drop across the resistors and current densities in
transistors between the tracking circuit and output drivers are the
same to provide optimum tracking. Thus, the low level at node NX6 is
equal to the reference voltage.

      When the predrive circuitry switches and turns Q9 and Q13 on
and Q10...