Lecture 110 Intro. and Characterization of the Op Amp (1/28/02) Page 110-1

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1 Lecture 110 Intro. and Characterization of the Op Amp (1/28/02) Page 1101 LECTURE 110 INTRODUCTION AND CHARACTERIZATION OF THE OP AMP (READING: GHLM , AH ) Objective The objective of this presentation is: 1.) Introduce and characterize the op amp Outline Static characteristics of the op amp Dynamic characteristics of the op amp Op amp architecture Two stage Foldedcascode Summary ECE 6412 Analog Integrated Circuits and Systems II P.E. Allen 2002 Lecture 110 Intro. and Characterization of the Op Amp (1/28/02) Page 1102 INTRODUCTION AND CHARACTERIZATION OF OP AMPS HighLevel Viewpoint of an Op Amp Block diagram of a general, twostage op amp: Compensation Circuitry Differential Transconductance High Gain v OUT Output Buffer v OUT ' Bias Circuitry Fig Differential transconductance stage: Forms the input and sometimes provides the differentialtosingle ended conversion. High gain stage: Provides the voltage gain required by the op amp together with the input stage. Output buffer: Used if the op amp must drive a low resistance. Compensation: Necessary to keep the op amp stable when resistive negative feedback is applied. ECE 6412 Analog Integrated Circuits and Systems II P.E. Allen 2002

2 Lecture 110 Intro. and Characterization of the Op Amp (1/28/02) Page 1103 Ideal Op Amp Symbol: i 1 V DD i 2 v i V SS v OUT = A v ( ) Fig Null port: If the differential gain of the op amp is large enough then input terminal pair becomes a null port. A null port is a pair of terminals where the voltage is zero and the current is zero. I.e., = v i = 0 and i 1 = 0 and i 2 = 0 Therefore, ideal op amps can be analyzed by assuming the differential input voltage is zero and that no current flows into or out of the differential inputs. ECE 6412 Analog Integrated Circuits and Systems II P.E. Allen 2002 Lecture 110 Intro. and Characterization of the Op Amp (1/28/02) Page 1104 General Configuration of the Op Amp as a Amplifier n p Noniverting voltage amplifier: R 1 R 2 n = 0 = v inp Inverting voltage amplifier: p = 0 = R 2 n R2 vout Fig ECE 6412 Analog Integrated Circuits and Systems II P.E. Allen 2002

3 Lecture 110 Intro. and Characterization of the Op Amp (1/28/02) Page 1105 Example 1 Simplified Analysis of an Op Amp Circuit The circuit shown below is an inverting voltage amplifier using an op amp. Find the voltage transfer function, /. i 1 i 2 R 2 Solution i i v i Virtual Ground Fig If A v, then v i 0 because of the negative feedback path through R 2. (The op amp with fb. makes its input terminal voltages equal.) v i = 0 and i i = 0 Note that the null port becomes the familiar virtual ground if one of the op amp input terminals is on ground. If this is the case, then we can write that i 1 = and i 2 = R 2 Since, i i = 0, then i 1 i 2 = 0 giving the desired result as = R 2. ECE 6412 Analog Integrated Circuits and Systems II P.E. Allen 2002 Lecture 110 Intro. and Characterization of the Op Amp (1/28/02) Page 1106 Linear and Static Characterization of the Op Amp A model for a nonideal op amp that includes some of the linear, static nonidealities: CMRR R icm V OS I B2 i n 2 e n 2 * C id R id R out vout Ideal Op Amp R icm I B1 where R id = differential input resistance C id = differential input capacitance R icm = common mode input resistance V OS = inputoffset voltage I B1 and I B2 = differential inputbias currents I OS = inputoffset current (I OS = I B1 I B2 ) CMRR = commonmode rejection ratio e 2 n = voltagenoise spectral density (meansquare volts/hertz) i 2 n = currentnoise spectral density (meansquare amps/hertz) Fig ECE 6412 Analog Integrated Circuits and Systems II P.E. Allen 2002

4 Lecture 110 Intro. and Characterization of the Op Amp (1/28/02) Page 1107 Linear and Dynamic Characteristics of the Op Amp Differential and commonmode frequency response: V 1 (s)v 2 (s) V out (s) = A v (s)[v 1 (s) V 2 (s)] ± A c (s) 2 Differentialfrequency response: A v0 A v0 p 1 p 2 p 3 A v (s) = s s s = (s p 1 )(s p 2 )(s p 3 ) p 1 1 p 2 1 p 3 1 where p 1, p 2, p 3, are the poles of the differentialfrequency response (ignoring zeros). 20log10(A v0 ) A v (jω) db Asymptotic Magnitude Actual Magnitude 6dB/oct. GB 0dB Fig ω 1 ω 2 ω 3 ω 12dB/oct. 18dB/oct. ECE 6412 Analog Integrated Circuits and Systems II P.E. Allen 2002 Lecture 110 Intro. and Characterization of the Op Amp (1/28/02) Page 1108 Other Characteristics of the Op Amp Power supply rejection ratio (PSRR): PSRR V DD = V A v (s) V o/v in (V dd = 0) = OUT V o /V dd (V in = 0) Input common mode range (ICMR): ICMR = the voltage range over which the input commonmode signal can vary without influence the differential performance Slew rate (SR): SR = output voltage rate limit of the op amp Settling time (T s ): v IN v OUT Final Value ε Final Value Final Value ε v OUT (t) ε ε Upper Tolerance Lower Tolerance Settling Time 0 0 T s t Fig ECE 6412 Analog Integrated Circuits and Systems II P.E. Allen 2002

5 Lecture 110 Intro. and Characterization of the Op Amp (1/28/02) Page 1109 Classification of CMOS Op Amps Categorization of op amps: Conversion Hierarchy to Current Current to Classic Differential Amplifier Differentialtosingle ended Load (Current Mirror) Modified Differential Amplifier Source/Sink Current Loads MOS Diode Load First to Current Current to Transconductance Grounded Gate Class A (Source or Sink Load) Transconductance Grounded Source Class B (PushPull) Current Second Table ECE 6412 Analog Integrated Circuits and Systems II P.E. Allen 2002 Lecture 110 Intro. and Characterization of the Op Amp (1/28/02) Page Two Op Amp Architecture Simple twostage op amp broken into voltagetocurrent and currenttovoltage stages: V DD V CC M3 M4 M6 Q3 Q4 Q6 M1 M2 Q1 Q2 VBias V I M5 M7 V SS I V V I I V VBias V I Q5 Q7 V EE I V V I I V Fig ECE 6412 Analog Integrated Circuits and Systems II P.E. Allen 2002

6 Lecture 110 Intro. and Characterization of the Op Amp (1/28/02) Page FoldedCascode Op Amp Architecture Simple foldedcascode op amp broken into voltagetocurrent and currenttovoltage stages: V DD V CC M3 M10 M11 Q3 Q10 Q11 M1 M2 M6 M7 M8 M9 vout Q1 Q2 Q6 Q7 Q8 Q9 vout M4 M5 V I I I I V V SS V EE Q4 Q5 V I I I I V Fig ECE 6412 Analog Integrated Circuits and Systems II P.E. Allen 2002 Lecture 110 Intro. and Characterization of the Op Amp (1/28/02) Page SUMMARY The op amp is an amplifier whose gain approaches The input is a null port which is useful for analysis Two classical op amp configurations noninverting and inverting amplifier Static characteristics include offsets, bias currents, finite resistance, and noise Dynamic characteristics include frequency, slew rate, transient response, etc. Basic op amp architectures are, Twostage Cascode and foldedcascode ECE 6412 Analog Integrated Circuits and Systems II P.E. Allen 2002