Browse Prior Art Database

SPIKE INSENSITIVE INPUT STAGE FOR DIFFERENTIAL AMPLIFIER

IP.com Disclosure Number: IPCOM000009127D
Original Publication Date: 1999-Jun-01
Included in the Prior Art Database: 2002-Aug-08
Document File: 1 page(s) / 53K

Publishing Venue

Motorola

Related People

Vladimir Koifman: AUTHOR [+3]

Abstract

The proposal relates to the input stage of an operational amplifier (op amp). Figure 1 illustrates a single-ended op amp hav- ing current-source powered transistors Ml and M2 (e.g., P-PETS), output state Al, feedback resistor Rl , and parasitic capacity Cl (dashed) between the common transistor sources (Ml, M2) and ground. The op amp receives differential input signals "In-" and "In+" (at Ml, M2) and provides output signal Out.

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MOTOROLA Technical Developments

SPIKE INSENSITIVE INPUT STAGE FOR DIFFERENTIAL AMPLIFIER

by Vladimir Koifman, Yachin Afek and Israel Kashat

INTRODUCTION

The proposal relates to the input stage of an operational amplifier (op amp).

PROBLEM

Figure 1 illustrates a single-ended op amp hav- ing current-source powered transistors Ml and M2
(e.g., P-PETS), output state Al, feedback resistor Rl , and parasitic capacity Cl (dashed) between the common transistor sources (Ml, M2) and ground. The op amp receives differential input signals "In-" and "In+" (at Ml, M2) and provides output signal Out.

  If the bulks of Ml and M2 are tied to their sources, the high-frequency noise from ground rail or from the substrate goes through Cl to transistors Ml and M2. At high frequencies, capacity Cl at low impedance causes that the drain current of Ml supplied to stage Al is larger than the drain current of M2. This nonlinearity appears at the output recti- fied as an unwanted DC offset. The problem is not limited to single-ended op amps, but also occurs in differential op amps and differential voltage-to-cur- rent converters. Further, noise can be translated into unwanted low frequency signals.

SOLUTION

  By adding transistors'M3 and M4 receiving the same input signal In+ and, In-, respectively, but sink- ing currents to ground, the signal path (with Ml, M2) is separated from the capacitor charging path Parasitic capacity Cl substantially appears at M3 and M4 so that charging effects f...