Analog IC Design 2011

Transcription

1 Analog IC Design 2011 Lecture 11 - Fully differential Opamps Markus Törmänen Markus.Tormanen@eit.lth.se All images are taken from Gray, Hurst, Lewis, Meyer, 5th ed., unless noted otherwise Markus Törmänen 1

2 Contents Fully differential operational amplifiers: Amplifier properties Amplifier structures Common-mode feedback Markus Törmänen 2

3 Amplifier properties Better signal-to-noise ratio: SNR = max. output signal power / output noise power v on,diff 2 = 2(R 32 /R 12 )4kTR 1 f S out,diff = 0.5V sig,peak 2 = 4S out,se (Eq. 12.3) SNR diff = S out,diff /v on,diff 2 = 2SNR SE Less susceptible to common-mode disturbances Even-order nonlinearities are cancelled at the output Markus Törmänen 3

4 Amplifier properties Markus Törmänen 4

5 Amplifier properties Small-signal half circuits: CM input: DM input: DM output: CM output: Markus Törmänen 5

6 Amplifier properties Ideal small-signal model: Infinite input impedance and zero output impedance Differential mode gain: v od = v o1 -v o2 = a dm v id Common mode gain: v oc = (v o1 +v o2 )/2 = a cm v ic (Eq , 12.29) Markus Törmänen 6

7 Amplifier properties A basic one-stage FD opamp: Differential mode gain: a dm = -g m1 (r o1 r o3 ) Common mode control gain: a cmc = v oc /v cmc -g m5h (r o1 g m1 r o5h r o3 ) Common mode gain: a cm = v oc /v ic -g m1 (r o1 g m1 r o5h r o3 ) /(1+g m1 r o5h ) (Eq ) Markus Törmänen 7

8 Amplifier properties A two-stage FD opamp: Common-mode signals! Sense the CM output voltage Control I 5 through V cmc Markus Törmänen 8

9 Amplifier properties Half circuits: Differential mode gain: a dm0 = -g m2 (r o2 r o4 )g m6 (r o6 r o7 ) Common mode control gain: a cmc0 g m5h (r o2 g m2 r o5h r o4 ) x g m6 (r o6 r o7 ) CM loop gain T cmtot = a cmc0 a cms0 (Eq ) Markus Törmänen 9

10 Amplifier structures A folded-cascode two-stage FD opamp: Markus Törmänen 10

11 Amplifier structures What about the input impedance? Markus Törmänen 11

12 Amplifier structures Opamp implementation with two input pairs: Common-mode feedback by controlling I 1 or I Markus Törmänen 12

13 2 minute question! Discuss in groups of 2 or 3 the following question: For the implementation of the opamp with two input pairs, what is the impact on the chip area of the integrated circuit compared to a single-ended opamp? Be prepared to give a short comment based on your group discussion Markus Törmänen 13

14 CMFB Concept of common-mode feedback: CM sensing: V cms = a cms (V oc -V cm ) + V csbias If V oc =V CM then V cmc = V CSBIAS Markus Törmänen 14

15 CMFB Possible implementation of CM feedback loop: Disadvantage: resistive loading Markus Törmänen 15

16 CMFB Buffers at the output: Markus Törmänen 16

17 CMFB Capacitive loading of the output Small size Sensitive for v o1 and v o2 of the opamp Markus Törmänen 17

18 CMFB Implementation: a fully differential opamp with CMFB Markus Törmänen 18

19 Extra Capacitive neutralisation: The Miller effect is used to reduce the effective input capacitance: C idh = C gs1 + C gd1 (1-a dm1 ) + C n (1+a dm1 ) (where a dm1 = v d1 /0.5v id ) Markus Törmänen 19

20 Information Reminder - Lab reports: Lab 3 to be handed in this week on Tuesday / Wednesday! Exam Wednesday Oct. 19th, 08:00-13:00, Vic:2D Markus Törmänen 20