Lecture 200 Cascode Op Amps - II (2/18/02) Page 200-1

Transcription

1 Lecture 200 Cascode Op Amps II (2/18/02) Page 2001 LECTURE 200 CASCODE OP AMPS II (READING: GHLM , AH ) Objective The objective of this presentation is: 1.) Develop cascode op amp architectures 2.) Show how to design with the cascode op amps Outline Op amps with cascoding in the first stage Op amps with cascoding in the second stage Folded cascode op amp Summary Lecture 200 Cascode Op Amps II (2/18/02) Page 2002 Input Common Mode Range for Two Types of Differential Amplifier Loads V SG3 V TN V SG3 Input M3 Common Mode Range M1 V SD3 V TN V SD4 V SD3 V SD4 Input M4 Common M3 M4 Mode VBP Range M2 M1 M2 V SS V DS5 V GS1 VBias M5 v icm V SS V SS V DS5 V GS1 VBias V SS M5 v icm Differential amplifier with Differential amplifier with a current mirror load. current source loads. Fig In order to improve the ICMR, it is desirable to use current source (sink) loads without losing half the gain. The resulting solution is the folded cascode op amp.

2 Lecture 200 Cascode Op Amps II (2/18/02) Page 2003 The Folded Cascode Op Amp M14 M4 I 4 M5 I 5 A B R A R B I 1 I 2 v in M1 M2 R 1 M13 M6 R 2 I 6 M7 I7 C L v out VBias I 3 M3 V SS M8 M12 M10 M9 M11 Fig We have examined the small signal performance and the frequency response in an earlier lecture. Lecture 200 Cascode Op Amps II (2/18/02) Page 2004 PSRR of the Folded Cascode Op Amp Consider the following circuit used to model the PSRR: R C gd9 V out r ds9 C M9 gd9 C gd11 V GSG9 C out r V ds11 GS11 M11 R out Vout Fig This model assumes that gate, source and drain of M11 and the gate and source of M9 all vary with V SS. We shall examine V out / rather than PSRR. (Small V out / will lead to large PSRR.) The transfer function of V out / can be found as V out sc gd9 R out sc out R out 1 for C gd9 < C out The approximate PSRR is sketched on the next page.

3 Lecture 200 Cascode Op Amps II (2/18/02) Page 2005 Frequency Response of the PSRR of the Folded Cascode Op Amp db PSRR A vd (ω) 1 C gd9 R out 0dB C gd9 C out Dominant pole frequency GB V out Other sources of injection, i.e. r ds9 log 10 (ω) Fig We see that the PSRR of the cascode op amp is much better than the twostage op amp. Lecture 200 Cascode Op Amps II (2/18/02) Page 2006 Design Approach for the FoldedCascode Op Amp Step Relationship/ Requirement Design Equation/Constraint 1 Slew Rate I 3 = SR C L 2 Bias currents in output cascodes I 4 = I 5 = 1.2I 3 to 1.5I 3 3 Maximum output 8I 5 voltage, v out (max) S 5 = K P V 2, S 7 = 8I 7 SD5 K P V 2 Let S 4 =S 14 =S 5 & SD7 S 13 =S 6 =S 7 4 Minimum output 8I 11 voltage, v out (min) S 11 = K N V 2, S 9 = 8I 9 DS11 K N V 2 Let S 10 =S 11 & DS9 S 8 =S 9 5 Selfbias cascode R 1 = V SD14 (sat)/i 14 and R 2 = V DS8 (sat)/i 6 6 GB = g m1 C L S 1 =S 2 = g m1 2 K N I = GB2 C 2 L 3 K N I 3 2I 3 7 Minimum input CM S 3 = I 3 K N V in (min)v SS K N S V 2 1 T1 2I 4 8 Maximum input CM S 4 = S 5 = K P ( V in (max)v T1 ) 2 9 Differential Voltage Gain v out v = g m1 in 2 g m2 2(1k) R out = 2k 22k g mi R out 10 Power dissipation P diss = ( V SS )(I 3 I 12 I 10 I 11 ) Comments Avoid zero current in cascodes V SD5 (sat) = V SD7 (sat) = 0.5[ V out (min)] V DS9 (sat) = V DS11 (sat) = 0.5(V out (min) V SS ) S 4 and S 5 must meet or exceed the value in step 3

4 Lecture 200 Cascode Op Amps II (2/18/02) Page 2007 Example 3 Design of a FoldedCascode Op Amp Follow the procedure given to design the foldedcascode op amp when the slew rate is 10V/µs, the load capacitor is 10pF, the maximum and minimum output voltages are ±2V for ±2.5V power supplies, the GB is 10MHz, the minimum input common mode voltage is 1.5V and the maximum input common mode voltage is 2.5V. The differential voltage gain should be greater than 5,000V/V and the power dissipation should be less than 5mW. Use channel lengths of 1µm. Solution Following the approach outlined above we obtain the following results. I 3 = SR C L = 10x = 100µA Select I 4 = I 5 = 125µA. Next, we see that the value of 0.5( V out (max)) is 0.5V/2 or 0.25V. Thus, 2 125µA S 4 = S 5 = S 14 = 50µA/V 2 (0.25V) 2 = = 80 and assuming worst case currents in M6 and M7 gives, 2 125µA S 6 = S 7 = S 13 = 50µA/V 2 (0.25V) 2 = = 80 The value of 0.5(V out (min) V SS ) is also 0.25V which gives the value of S 8, S 9, S 10 and S 11 2 I as S 8 = S 9 = S 10 = S 11 = K N V DS8 2 = 110 (0.25) 2 = Lecture 200 Cascode Op Amps II (2/18/02) Page 2008 Example 3 Continued The value of R 1 and R 2 is equal to 0.25V/125µA or 2kΩ. In step 6, the value of GB gives S 1 and S 2 as S 1 = S 2 = GB 2 C L 2 K N I 3 = (20πx10 6)2(1011)2 110x x106 = 35.9 The minimum input common mode voltage defines S 3 as 2I 3 200x106 S 3 = = = 20 I 3 K N V in (min)v SS K N S 1 V 2 T1 110x We need to check that the values of S 4 and S 5 are large enough to satisfy the maximum input common mode voltage. The maximum input common mode voltage of 2.5 requires 2I µA S 4 = S 5 K P [ V in (max)v T1 ] 2 = 50x10 6 µa/v 2 [0.7V] 2 = 10.2 which is much less than 80. In fact, with S 4 = S 5 = 80, the maximum input common mode voltage is 3V. Finally, S 12, is given as S 12 = S 3 = 25 The power dissipation is found to be P diss = 5V(125µA125µA125µA) = 1.875mW

5 Lecture 200 Cascode Op Amps II (2/18/02) Page 2009 Example 3 Continued The smallsignal voltage gain requires the following values to evaluate: S 4, S 5, S 13, S 14 : g m = = 1000µS and g ds = 125x = 6.25µS S 6, S 7 : g m = = 774.6µS and g ds = 75x = 3.75µS S 8, S 9, S 10, S 11 : g m = = 774.6µS and g ds = 75x = 3µS S 1, S 2 : g mi = = 628µS and g ds = 50x106(0.04) = 2µS Thus, 1 1 R II g m9 r ds9 r ds11 = (774.6µS) 3µS 3µS = 86.07MΩ 1 R out 86.07MΩ (774.6µS) 3.75µS 1 2µS6.25µS = 19.40MΩ k = R II(g ds2 g ds4 ) g m7 r = 86.07MΩ(2µS6.25µS)(3.75µS) ds µS = The smallsignal, differentialinput, voltage gain is 2k A vd = 22k g mir out = x x106 = 7,464 V/V The gain is larger than required by the specifications which should be okay. Lecture 200 Cascode Op Amps II (2/18/02) Page Comments on Folded Cascode Op Amps Good PSRR Good ICMR Self compensated Can cascade an output stage to get extremely high gain with lower output resistance (use Miller compensation in this case) Need first stage gain for good noise performance Widely used in telecommunication circuits where large dynamic range is required

6 Lecture 200 Cascode Op Amps II (2/18/02) Page SUMMARY Cascode op amps offer an alternate architecture to the twostage op amp The cascode op amp is typically selfcompensating The cascode op amp generally has better PSRR