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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 , x0.5 ∆ 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
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