Analog Integrated Circuits. Lecture 7: OpampDesign

Transcription

1 Analog Integrated Circuits Lecture 7: OpampDesign ELC 601 Fall 2013 Dr. Ahmed Nader Dr. Mohamed M. Aboudina Department of Electronics and Communications Engineering Faculty of Engineering Cairo University

2 2 Contents Specifications Single Stage Opamp Simple Telescopic Folded Cascode 12/7/2013 Ahmed Nader, 2013

3 3 Introduction to Opamps Specifications DC-gain (G m and R out ): The open-loop gain of an opamp determines the precision of the feedback system employing the opamp. Small-Signal Bandwidth. Large-Signal Bandwidth: Slew Rate =min (, ) 1 I 1 I 2 C 1 1 C 2 Output Swing: The range of outputs where the opampis still performing in the linear region. ( Linearity) 12/7/2013 Ahmed Nader, 2013

4 4 Introduction to Opamps Specifications Noise and Offset: Noise: Minimum signal level that can be processed with reasonable quality. Offset: Indicates the amount of mismatches between the two halves. Supply Rejection: A measure of how good the differential circuit can reject common-mode variations on supply lines. Output Current Driving Capability CMFB loop in fully differential OpAmp 12/7/2013 Ahmed Nader, 2013

5 5 Specifications Gain (Precision) R v = 2 v o = β v 1 R + R 1 2 R β = 2 R + R 1 2 β is called feedback factor 12/7/2013 o =, v out Ahmed Nader, 2013 = Av = A (v v) = A 0 id 0 in 1 0 = Gain Error is given by (v in GE= (ideal gain)-(actual gain) For non-inverting amplifier, 1 A 1 GE= = β 1+ Aβ β(1+ Aβ) βv out ) Gain error is also expressed as a fractional or percentage error. 1 A β Aβ 1 FGE= 1+ = 1 1+ Aβ β 1 Aβ

6 6 Specifications GBW (Linear Settling) = = Gain-Bandwidth Product: =. Closed-loop bandwidth.. 12/7/2013 Ahmed Nader, 2013

7 7 Specifications Slew Rate (Non-linear Settling) Differential input is high enough to cause the current source I SS to steer in one direction and charge the output capacitor. Maximum current in capacitor C L is: ±. Slew Rate = To avoid slewing for V out =A.sin(w o t) SR > A.w o 12/7/2013 Ahmed Nader, 2013

8 8 Simple Opamp Architecture DC-gain is too low: A =, ( ) Output swing: OK 12/7/2013 _ = _ = _ =, + + _ Speed: = Ahmed Nader, 2013 ( ) =, Stability (2 poles + 1 zero) - Dominant at the output - Non-dominant at the diode connected node - 1 zero due to multi-path =180 +

9 9 Simple Opamp CMFB 12/7/2013 Ahmed Nader, 2013

10 10 Simple Opamp CMFB (Stability) 12/7/2013 Ahmed Nader, 2013

11 11 Telescopic Opamp Architecture Single-Ended High-Swing CM Fully Differential 12/7/2013 Ahmed Nader, 2013

12 12 Telescopic Opamp Single-Ended A (Find exact) =, Note: We still have a zero due to the multi-path from input to output. What if the input and output of the telescopic opampare tied together, what about the input and output common range? Assignment 4C: Show that _ _ < 12/7/2013 Ahmed Nader, 2013

13 13 Telescopic Opamp Differential Topology C L 2 2 C L =, (, (1+,, ), (1 +,, )) Output Swing: Much smaller What are the conditions on the voltages V b1, V b2 for proper operation (all transistors in saturation region)? What are these values for maximum output swing (Optimum Biasing)? _ =,, _ = +, _ =, +, + _ =, Maximum current in capacitor C L is: ± SR= Intersection between input and output available ranges is very limited. Noise 12/7/2013 Ahmed Nader, 2013

14 Opamps Folded Cascode Opamps 14 M5 M3 M1 M7 Input and output common ranges intersect in larger ranges. The input can go now to VDD (for PMOS it can go to ground) Slew Rate = Drawbacks: Higher noise Extra capacitance at folding node. Lower gain Current Consumption. M9 12/7/2013 Ahmed Nader, 2013

15 15 Comparison with simple opamp Simple opamp Telescopic opamp Folded cascode DC-Gain Low High High GBW Similar Similar Similar Output Swing Larger Smaller Smaller Miller Effect Worse Better Better Input and output tied together LargeAvailable Swing Small Swing (intersection) Large Available Swing Noise Low Low Medium 12/7/2013 Ahmed Nader, 2013

16 Opamps What if more Gain is needed? 16 How to increase the gain even more? Multi-level Cascodes Boosted Cascode Multi-Stage Opamps A Combination of them 12/7/2013 Ahmed Nader, 2013

17 17 Multi-Level Cascodes cannot be boosted more than the transistor allows. Increasing is the only way to boost the DC gain. Main Drawback: Headroom What s the limit of cascoding? Resistive loads. Leakage (parasitic) resistance at the output. DC gain ~ (Check) 12/7/2013 Ahmed Nader, 2013

18 18 Boosted (Active or Super) Cascode Opamps = ~ ~ Similar to Multi-Level Cascode 12/7/2013 Ahmed Nader, 2013

19 19 Boosted (Active or Super) Cascode Opamps 12/7/2013 Ahmed Nader, 2013

20 20 Gain Boosting Applied at Both Signal and Load Paths 12/7/2013 Ahmed Nader, 2013

21 21 Opamps Noise and Offset Similarities Input referred Noise and Offset are very similar. Similar trade-offs Cascode devices: exhibit low noise/low offestwhen referred to the input (except at high frequencies) The difference is: Offset is random and doesn t have a deterministic value, but is a DC signal. Noise is a random signal in time that spreads in frequency and a Noise Bandwidth must be defined for that. 12/7/2013 Ahmed Nader, 2013

22 Opamps Fully-Differential: Input referred offset 22 Analysis similar to noise analysis = ( // ) = ( // ) Output Referred Offset: = + Input Referred Offset: = + To reduce effect reduce / ratio. For PMOS current sources working as active load, to improve offset, use large L and small ", hence obtaining large trade-off with Swing. L is still limited due to speed limitations. Δ = = is a technology given parameter. 12/7/2013 Ahmed Nader, 2013

23 Opamps Fully-Differential: Input referred offset Cascode devices 23 What about the effect of the offset for cascode devices? Gain from Gate of cascodedevice to Output is much smaller than the opampdifferential gain. Input referred offset due to is negligible. (Check with equations) 12/7/2013 Ahmed Nader, 2013

24 Opamps Fully-Differential - Example: Folded Cascode Opamp 24 Which offset voltage has almost no effect on input referred offset? and : Cascodedevices Total input referred offset: = + + Use Larger L for current sources and minimum L for cascode devices. 12/7/2013 Ahmed Nader, 2013