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Unformatted text preview: Device Mismatch in Diff. Amps. (7/1/00) Page 1 ECE 4430 - Analog Integrated Circuits and Systems P.E. Allen, 2000 3.6 - DEVICE MISMATCH IN DIFFERENTIAL AMPLIFIERS INTRODUCTION Objective The objective of this presentation is: 1.) Characterize the dependence of bias circuits on the power supply 2.) Introduce circuits that have various degrees of power supply independence Outline • Characterization of power supply dependence • Simple bias circuits • Bootstrapped bias circuits • Temperature characterization of bias circuits Objective The objective of this presentation is: 1.) Illustrate the method of analyzing mismatches 2.) Analyze the input current and voltage offsets for differential amplifiers Outline • The general approach to analyzing mismatches • Input voltage and current offsets of BJT differential amplifiers • Input voltage offsets of MOS differential amplifiers • Summary Device Mismatch in Diff. Amps. (7/1/00) Page 2 ECE 4430 - Analog Integrated Circuits and Systems P.E. Allen, 2000 MISMATCH ANALYSIS METHODS General Method Suppose that two performances, p 1 and p 2 , can be written can be written as p 1 = f 1 ( x 1 , y 1 , z 1 ,...) and p 2 = f 2 ( x 2 , y 2 , z 2 ,...) Ideally, y 1 should be equal to y 2 , but in practice their difference could be expressed as Error = e ( p 1 , p 2 ) = f( x 1 , y 1 , z 1 ,..., x 2 , y 2 , z 2 ,...) Now assume that x 1 , y 1 , z 1 ,... and x 2 , y 2 , z 2 ,...) can be expressed in terms of their difference and average values. We illustrate only for x 1 and x 2 , ∆ x = x 1- x 2 and x = x 1 + x 2 2 We can solve for x 1 and x 2 in terms of ∆ x and x as follows, x 1 = x + 0.5 ∆ x and x 2 = x- 0.5 ∆ x Now the error can be express as e ( p 1 , p 2 ) = f( x +0.5 ∆ x , x-0.5 ∆ x ) This expressing can generally be simplified by assuming that ∆ x << x and using the following approximations, 1 1- ε ≈ 1+ ε or 1 1+ ε ≈ 1- ε and neglecting higher power values of ε , i.e. ε 2 Device Mismatch in Diff. Amps. (7/1/00) Page 3 ECE 4430 - Analog Integrated Circuits and Systems P.E. Allen, 2000 INPUT VOLTAGE AND CURRENT OFFSETS OF THE BJT DIFFERENTIAL AMPLIFIER Model for Input Offset Voltage and Current Circuit with no mismatches v in V CC V EE Q1 Q2 I EE DM01 +- v OD R C R C V CC V EE Q1 Q2 I EE +- V OS +- v OD R C R C v in +- I OS 2 +- v BE 1 +- v BE 2 where V OS = V BE 1- V BE 2 = V t ln I C 1 I s 1- V t ln I C 2 I s 2 = V t ln I C 1 I C 2 I s 2 I s 1 How does I s depend upon the semiconductor parameters? I s 1 = qn i 2 D f8e5 n N A W B 1 ( V CB ) A 1 = qn i 2 D f8e5 n Q B 1 ( V CB ) A 1 and I s 2 = qn i 2 D f8e5 n N A W B 2 ( V CB ) A 2 = qn i 2 D f8e5 n Q B 2 ( V CB ) A 2 where W B ( V CB ) is the base width as a function of V CB , N A is the acceptor density in the base and A is the emitter area....
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This note was uploaded on 10/23/2011 for the course SDASD 102 taught by Professor Dsfas during the Spring '11 term at Baptist Bible PA.
- Spring '11