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SUPPLY INDEPENDENT CURRENT REFERENCE CONFIGURED TO ELIMINATE EARLY VOLTAGE EFFECT

IP.com Disclosure Number: IPCOM000006090D
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2001-Dec-03
Document File: 5 page(s) / 213K

Publishing Venue

Motorola

Related People

Paul T. Bennett: AUTHOR

Abstract

An improved bipolar current reference which is insensitive to power supply variations is explained. The reference offers excellent power supply rejection while operating over a wide voltage range. The performance of the circuit is obtained through a careful design that virtually eliminates the Early voltage effect.

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MO7=OROLA INC. Technical Developments Volume 12 April 1991

SUPPLY INDEPENDENT CURRENT REFhENCE CONFIGURED TO ELIMINATE EARLY VOLTAGE EFFECT

by Paul T. Bennett

ABSTRACT

  An improved bipolar current reference which is insensitive to power supply variations is explained. The reference offers excellent power supply rejection while operating over a wide voltage range. The performance of the circuit is obtained through a careful design that virtually eliminates the Early voltage effect.

CIRCUIT DESCRIPTION

   The circuit in Figure 1 is a common configuration of a current reference and explained by Gray and Meyer'. Whenever the base-emitter voltage, V,, on a pair of matched transistors is held constant the collector current, I,, of each transistor is roughly equivalent?. This is the case in Figure 1 for transistors

Q, and Q,.

The ratio of the satption currents, I&, is equal to the ratio of emitter areas (shown here to be equal to
4). The current referelice is determined therefore by choosing an appropriatk area factor and resistor. The current is relatively siable and virt,ually supply- independent (notice that V,, is not a factor in the equation). This, however, is not the true case.

  The collector characteristics for an npn transistor with finite output resistance is shown in Figure 2. The collector current is tiot determined solely by its saturation current, Is, ,the base-emitter voltage, V,, and the thermal voltage, V,. The collector current also varies with its collectoi voltage, V,, in relation to the Early Voltage, V,, (a finction of the transistor's finite output impedance)'.

I, = I, (exp +) (I + +).

T A

  The collector voltages of Q, and Q, are equal and held to one V, drop below the supply voltage, V,, i.e. almost equal to V,. me collector voltages of Q, and Q, are equal and held ;to one V, drop above ground,
i.e. almost equal to zero. Therefore, whenever the supply voltage varies, the collector voltages of the "matched" transistors no longer provide equal output currents. (Notice that the last term in the equation can be thought of as a n&match factor.) This effect is dramatic and results in poor regulation of the reference current, and insufficient rejection of power supply noise.

SOLUTION

   The circuit depicted in Figure 3 virtually eliminates the effects from the Early Voltage by maintaining equal collector voltages in the "matched" pair of transistors. This is accomplished through an active feedback network, a few resistors and a compensation capacitor.

where,

V, is the base-emitter voltage, k is Boltzman's constant,

T is temperature in degrees Kelvin, q is the charge on an electron,

I, is the saturation current, and I, is the collector current.

  Since I,, = I, for well-matched and equal emitter area transistors, I,, = I,. This condition implies equal currents through Q, and Q,, but not its magnitude.

  The magnitude of the current is set by Q,, Q, and the resistor R,. The difference in junction potential between two junctions ope...