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ADDING VOLTAGE WITHOUT USING AMPLIFIERS IN A FEEDBACK LOOP

IP.com Disclosure Number: IPCOM000004724D
Original Publication Date: 2001-Apr-23
Included in the Prior Art Database: 2001-Apr-23
Document File: 2 page(s) / 8K

Publishing Venue

Motorola

Related People

Ricardo Erckert: AUTHOR

Abstract

In analogue circuits adding two voltages is an often-needed operation. Usually this is done using operational amplifiers and resistors. Operation amplifiers are expensive because they need a significant chip area for the frequency compensation capacitor. The circuit described offers a solution to the problem of adding voltages without the need of an operational amplifier with its large capacitor.

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ADDING VOLTAGE WITHOUT USING AMPLIFIERS IN A FEEDBACK LOOP

by Ricardo Erckert

INTRODUCTION

In analogue circuits adding two voltages is an often-needed operation. Usually this is done using operational amplifiers and resistors. Operation amplifiers are expensive because they need a significant chip area for the frequency compensation capacitor. The circuit described offers a solution to the problem of adding voltages without the need of an operational amplifier with its large capacitor.

PROBLEM TO BE SOLVED

Adding an offset to a reference voltage requires a very precise reproduction of the reference voltage. The offset added to this reference voltage usually needs not to be of such a high precision. A typical example would be a comparator with hysteresis. In such an application one trip point must be highly accurate thus using the reference voltage given in the system.

The second trip point defining the hysteresis may have a reduced accuracy as long as it is guaranteed there is an offset between those two trip points. Besides that the offset added in most cases is about one magnitude smaller than the reference voltage. So an error of even l0% of the offset reproduction will be reduced to a few percent of the total voltage (Vref Voffset) in such applications.

Using an operational amplifier for adding voltages is expensive because operational amplifiers in this application must be frequency compensated for a gain of two. In modern technologies the compensation capacitor can get larger than the operational amplifier itself. Even compensated amplifiers in feedback loops are sensitive to noise that can excite ringing of the output signal. So for cost and for stability reasons a circuit not needing an operational amplifier in a feedback loop is needed.

SOLUTION

This article describes a circuit adding two voltages. One of them is not amplified or converted into other representations. So this first voltage is reproduced at lowest possible errors. The second voltage is converted into a current. With the help of matching resistors this current is converted back into a voltage that can be added to the first voltage.

In Figure 1Vin1 is converted into a current with a voltage controlled current source. This current is copied twice using two current controlled current sources.

One of the copied current is used to add a voltage proportional to Vin1 to Vin2 using a resistor. The second copy of the current is used to cancel the current on the other side of the resistor. This current cancellation minimizes the impact on Vin2 due to the source resistance of Vin2.

Note that Vin2 has never been converted into an intermediate representation. This way Vin2 is reproduced as part of Vout with the lowest possible error. (In the ideal case of Vin2 being provided by theoretical voltage source with impedance+0O there would be no reproduction error of Vin2 at all.)

Vout can be calculated as follows:

Vout S* Vin1*K*R+ Vin2

With S being the transconductance of the voltage controlled current so...