Lecture 20a. Circuit Topologies and Techniques: Opamps

Transcription

1 Lecture a Circuit Tplgies and Techniques: Opamps In this lecture yu will learn: Sme circuit tplgies and techniques Intrductin t peratinal amplifiers Differential mplifier IBIS1 I BIS M VI1 vi1 Vi vi I BIS3 What if ne wants larger differential gain? Large gain, small gain bandwidth (Miller effect), large input resistance, large utput resistance 1

2 Differential mplifier Cascade I BIS4 IBIS1 I BIS M3 M4 M VI1 vi1 Vi vi VO v I BIS3 Very large gain, small gain bandwidth (Miller effect in bth stages f the cascade), large input resistance, large utput resistance, high CCMR Telescpic Cascde Tplgy: Nn-Ideal Current Surce rc ~ r1, r I D I BIS VOUT v ut vut va g g r r m mb c V BIS V BIS1 v in M I D1 V v a v a 1 rc gm1 r1 1 v in gm gmb r gm1 rc ~ 1 gm gmb r ~ Large gain, large input resistance, large utput resistance, utput swing culd be limited, large gain bandwidth (n Miller effect)

3 Telescpic Cascde Tplgy: Mre Ideal Current Surce r r, r c 1 I D I BIS VOUT v ut vut va g g r r m mb c V BIS M I D1 V v a V BIS1 v in v a 1 rc gm1 r1 1 v in gm gmb r Very large gain, large input resistance, large utput resistance, utput swing culd be limited, small gain bandwidth (Miller effect frm CS stage) Flded Cascde Tplgy r c1 I BIS M V v a V BIS I D1 V BIS1 v in r c I D I BIS VOUT v ut Easier t set input bias vltages than in the telescpic cascade, large utput swing, mre pwer dissipatin than in the telescpic cascde 3

4 Telescpic Cascde Differential mplifier with a Cascde Current Mirrr M3 V BIS M4 M VO v R g g r r g g r r ut mn mnb n n mp mpb p p g g R vd mn mnb ut VI1 vi1 Vi vi I BIS Very large gain, very large utput resistance, utput swing culd be limited, small gain bandwidth (Miller effect) Flded Cascde Differential mplifier with a Current Mirrr I BIS I BIS M3 V M4 DD VBIS1 V BIS 1 I1 i1 M V V v V v O v i i I BIS1 Large gain, large gain bandwidth, large utput swing, large gain bandwidth (n Miller effect) 4

5 Flded Cascde Differential mplifier with a Current Mirrr I BIS I BIS M3 M4 VBIS1 V BIS 1 i i M V v VO v V v I1 i1 I BIS1 Flded Cascde Differential mplifier with a Cascde Current Mirrr I BIS I BIS M3 V M4 DD VBIS1 V BIS 1 I1 i1 M V v V v i i VO v I BIS1 Very large gain, very large utput resistance, decent utput swing, small gain bandwidth (Miller effect) 5

6 Flded Cascde Differential mplifier with a Cascde Current Mirrr V BIS M3 V M4 DD VBIS1 V BIS 1 I1 i1 M V v V v i i V BIS3 VO v V BIS4 Cmplete Op-mp ( lmst) M5 M6 M7 M9 V v I1 i1 M ID1 I D V v I i C VO v I BIS M8 M3 M4 Frequency cmpensatin capacitr Culd be any ne f the single-ended utput differential stages we have discussed CS gain stage Lw utput resistance, CD utput stage 6

7 Lecture b Negative Feedback, Stability, Gain Margins, Phase Margins In this lecture yu will learn: Negative Feedback and Stability High Frequency Behavir f mplifier Circuits Gain Margin, Phase Margin, and Stability Frequency Cmpensatin Cmplete Op-mp Stability prblems: The utput will be sensitive t the temperature and/r the pwer supply vltage. s the temperature r the pwer supply fluctuates, the utput is ging t fluctuate. 7

8 Open lp system: Negative Feedback and Stability v v in ut v in Open lp gain: Stability prblems: suppse the pen lp gain is sensitive t the temperature r the pwer supply vltage. s the temperature r the pwer supply fluctuates, the utput is ging t fluctuate. Clsed lp system with negative feedback: v in + - Clsed lp gain: K K << 1 v in Kvut vut vut vin 1 K 1 K v ut 1 K v in Negative Feedback and Stability Clsed lp system with negative feedback: v in + - K K << 1 v ut 1 K Clsed lp gain: vin 1 K If fr small frequencies, assume () >>>> 1 and K() >> 1 : Clsed lp gain: ~ 1 1 K ~ K Stability prblems reslved: Nw as the temperature r the pwer supply fluctuates, the utput is ging t be much mre stable (because it is almst independent f ()!! Negative feedback imprves stability at the expense f gain psitive feedback can lead t instability and/r scillatins! 8

9 Differential mplifiers, Negative Feedback, and Stability high-gain differential amplifier is almst always perated using a negative feedback: vut v in v in R 1 R R R vin v ut v v R R 1 K 1 1 K R 1 R 1 If ( ) >>>> 1, then: R v ut v in Negative feedback imprves stability at the expense f gain psitive feedback can lead t instability and/r scillatins! Cnsider a differential amplifier: mplifier Gain: Frequency Respnse v v in ut v in The amplifier gain can be expressed (mst generally) as: 1 jt11 jt 1 jt j 1 j 1 j Multiple zers Multiple ples Suppse, fr simplicity, the amplifier gain can be expressed as: 1 j11 j 1 j 3... Multiple ples 9

10 Magnitude and Phase f Respnse Functins: Single Ple Case 1lg 1 j 1lg 1 - db/dec j j 1-7 Magnitude and Phase f Respnse Functins: Multiple Ple Case 1lg 1 j 1 j 1 j lg 1 - db/dec -4 db/dec -6 db/dec j 1 j j

11 Negative Feedback and Psitive Feedback Clsed lp system with negative feedback: v in + - K v ut 1 K Clsed lp gain: vin 1 K 1 j 1 j j t frequencies between 1/ and 1 3, is 18-degrees => The feedback is psitive, nt negative!!! Phase Respnse and mplifier Stability Cnsider a differential amplifier perated using a negative feedback: vut v in v in R 1 R 1 R R - + v in v ut v v R R 1 K 1 1 K R 1 R 1 psitive feedback can happen at high frequencies when: 18 Denminatr can becme very small r zer! 11

12 Phase Respnse and mplifier Stability Clsed lp system with feedback: v in + - v ut 1 K v in K Hw t avid unwanted utput scillatins at the frequency at which?? vut vin vin 1 K 1 K Output will be nn-zer, even if the input is zer, if: 1K 1 1 K Slutin: T avid this psitive feedback frm causing scillatins, the magnitude gain must get much less than unity befre f the 1lg 1 1lg 1 Phase Respnse and mplifier Stability 1 j 1 j j db/dec -4 db/dec -3 db -6 db/dec

13 1lg 1 1lg 1 Gain Margin - db/dec -4 db/dec 1 j 1 j j db db/dec Gain margin = 3 db Hw small is the gain cmpared t unity ( db) when the phase f the respnse becmes lg 1 1lg 1 Phase Margin - db/dec 1 j 1 j j db/dec db/dec Phase margin = 5 Hw small is the phase cmpared t -18 when the amplitude f the respnse becmes unity ( db) -157 Phase margin =

14 Frequency Cmpensatin Cnsider a differential amplifier perated using a negative feedback: vin R 1 Very ften, almst always in fact, when yu are dne designing the amplifier yu figure ut that yu dn t have enugh gain and phase margins!! - + R v ut Prblem! Hw t slve this prblem? Frequency Cmpensatin: dd a lw frequency ple inside () (by adding extra capacitrs, fr example), sacrifice bandwidth, but regain stability (PTO ) 6 1lg 1 Frequency Cmpensatin 1lg 1 - db/dec - db/dec dded ple -4 db/dec -4 db/dec db/dec -6 db/dec

15 Frequency Cmpensatin The frequency cmpensatin capacitr is generally placed in the Miller psitin Texas Instruments LM 741 Operatinal mplifier Frequency Cmpensatin capacitr 15

16 Texas Instruments LM 741 Operatinal mplifier Frequency cmpensatin capacitr It is weren t in the Miller psitin, it wuld need t be much larger!! 16