Browse Prior Art Database

LOW COST CURRENT BIAS CIRCUIT GENERATOR

IP.com Disclosure Number: IPCOM000009791D
Original Publication Date: 2000-May-01
Included in the Prior Art Database: 2002-Sep-19
Document File: 2 page(s) / 70K

Publishing Venue

Motorola

Related People

Jules Campbell: AUTHOR [+3]

Abstract

This bias circuit is a low cost bias circuit applicable for any situation where a reasonably accurate current will suffice. The current accuracy across process, voltage, and temperamre varies approximately 25%, which is dominated by the resistor variation.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 58% of the total text.

MOTOROLA

Technical Developments

LOW COST CURRENT BIAS CIRCUIT GENERATOR

by Jules Campbell, Jeff Ganger and Kelvin McCollough

BACKGROUND

Vdd

Rb ~ I Mpl Mp2 N:I

Mnl Mn2

Fig. 1 lllustrates a simple current bias generator.

I is developed from a L\ V GS imposed across the resistor Rb. Through negative feedback devices MPl, MP2, MNl, and MN2 force the current through MP2 and MPl to be equal. Ignoring the body and channel length modulation effects on the P-channel mosfets, the following relationships can be derived:

2 [lii;, ~ Jdpl = kNS2(Vsgpl- Vtp) or Vsgpl = kNS + Vtp pZ

(EQ I)

2 = ~IdP2 + Idp2 = kSp2( V sgp2- V tp) or V sgp2 rs- Vrp p2

(EQ2)

Idpl = Idp2 = Id

(EQ3)

AV b = V sgp2 - V sgpl

(EQ4)

Motorola, Inc. 2000

tiVb = ~- ~

47- ~7

tiVb= RkS b p2

(EQ5)

(EQ6)

I

~=lM Rb (R2kS ) b pl

(EQ7)

Due to the voltage drop across Rb, MPl has body effect causing an increase in the variation of I and a decrease in the allowable working voltage of Vdd. With a simple adaptation of the topology, the body effect on MPl can be eliminated reducing the variation of I and increasing the working VDD range.

IMPROVED CURRENT BIAS GENERATOR

Fignre 2 illustrates the improved topology which eliminates MP1's body effect due to the voltage developed across Rb.

Vdd

Mpl

+

Vgs Rb ~I

Mp2 N:l

Mnl

Mn2

Fig.2

91

May 2000

MOTOROLA

Technical Developments

This bias circuit varies less over voltage, process, and temperature than the bias circuit shown in Figure I. It occupies less area than a more expensive current bias ...