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1 Lecture 17 Date: Feedback and Properties, Types of Feedback Amplifier Stability Gain and Phase Margin Modification

2 Elements of Feedback System: (a) The feed forward amplifier [H(s)] ; (b) A means of sensing the output; (c) The feedback network [G(s)] ; (d) A means of generating the feedback error [X(s) G(s)Y(s)] Indraprastha Institute of Feedback General Considerations: Negative Feedback System Feedforward Network Feedback Network Open Loop Transfer Function Y ( s) H ( s) X ( s) G( s) Y ( s) Error Signal or Compensation Signal Subtraction at this node makes this system negative feedback Y ( s) H ( s) X ( s) 1 G( s) H ( s) Closed Loop Transfer Function Open-Loop Transfer Function In our discussion: H(s) represents an amplifier and G(s) is a frequencyindependent quantity representing the feedback network

3 Properties of Feedback Circuits 1. Gain Desensitization Node generating error term The closed-loop gain is much less sensitive to device parameters V out m1 o1 V g r in V V out in C2 1 g r C g r m1 o1 1 m1 o1 Feedback makes gain of this CS stage independent of process and temperature Dependent on process parameters, temperature, bias etc. For large g m1 r o1 V V out in C C 1 2

4 Properties of Feedback Circuits (contd.) Y X Y X A A 1 1 A A The impact of variations in A on the closed loop gain is insignificant The quantity βa (loop gain) is critical for any feedback system higher the βa, less sensitive is the closed loop gain to the variations of A Accuracy of the closed loop gain improves by maximizing either A or β β increases closed loop gain decreases means a trade-off exist between precision and the closed loop gain

5 Properties of Feedback Circuits (contd.) Loop Gain Calculation Set the main input to zero Break the loop at some point Inject a test signal while maintaining the direction and polarity Follow the signal around the loop and obtain the expression/value V ( 1) A V t F V V F =-βa t Example: Calculate Loop Gain: V F C =- 2 V C C t 1 2 g r m1 o1

6 Properties of Feedback Circuits (contd.) 2. Input Impedance Modification Lets check input impedance with and without feedback CG stage (M 1 ) capacitive divider senses V out and applies it to gate of current source (M 2 ) M 2 returns a current feedback signal to the input of M 1 R in, open g 1 g m1 mb1 Assumption: no channellength modulation present

7 Properties of Feedback Circuits (contd.) VX 1 1 Rin, closed I X gm 1 gmb 11 X C1 gm2rd X C1 X C2 R R in, closed in, open 1 1 X g R C1 m2 D X C1 X C2 Loop Gain Feedback reduces the input impedance in this instance quite useful circuit topology Four Elements of Feedback: feed-forward amplifier consists of M 1 and R D, the output is sensed by C 1 and C 2, the feedback network comprise of C 1, C 2, and M 2, subtraction occurs in current domain at the input

8 Properties of Feedback Circuits (contd.) 3. Output Impedance Modification R out, open R D R out, closed 1 RD gm2rs gm 1 gmb 1 RD X C1 g g R X X m1 mb1 S C1 C 2 Loop Gain Can you identify if this is a positive feedback or negative feedback circuit? Why?

9 Properties of Feedback Circuits (contd.) 4. Bandwidth Modification One pole transfer function: One pole transfer function of a closed-loop system: A0 As () s 1 0 A0 s 1 Y 0 () s X A0 1 s 1 0 Y X () s Decreased Gain 1 A0 1 A s 1 A Constant GBW High pole frequency Increased Bandwidth

10 Properties of Feedback Circuits (contd.) 4. Bandwidth Modification GBW of a single pole system doesn t change with feedback. But how to improve the speed of the system with high gain? Suppose we need to amplify a 20 MHz square wave by a factor of 100 and maximum bandwidth but we only have single pole amplifier with an open-loop gain of 100 and 3-dB bandwidth of 10 MHz. Apply feedback in such a way that the gain and bandwidth are modified to 10 and 100 MHz. Then use two stage amplification to achieve the desired.

11 High Impedance Indraprastha Institute of Types of Amplifiers Type: Based on the type of parameters (current or voltage) they sense at the input and the type of parameters (current or voltage) they produce at the output Amplifier sensing voltage at the input: exhibit high input impedance (as a voltmeter) Amplifier sensing current at the input: exhibit low input impedance (as an ammeter) Amplifier sensing voltage at the output: exhibit low output impedance (as a voltage source) Amplifier sensing current at the output: exhibit high output impedance (as a current source) Voltage Amplifier Low Impedance CS Amplifier Possess Relatively High Output Impedance Buffer / CD Stage CS Amplifier with a buffer

12 High Impedance Low Impedance Indraprastha Institute of Types of Amplifiers (contd.) Transimpedance Amplifier Buffer / CD Stage Low Impedance Possess Relatively High Output Impedance CG Amplifier CG Amplifier with a buffer Transconductance Amplifier High Impedance CS Stage Less control on input impedance CS Stage with CS Amplifier at the input

13 Low Impedance Indraprastha Institute of Types of Amplifiers (contd.) Current Amplifier High Impedance CG Stage Less control on input impedance CG Stage with CG Amplifier at the input Sense and Return Mechanism Placing a circuit in the feedback requires sensing the output signal and then returning a fraction to the input Voltage and Current as input and output quantities provide 4 different possibilities for feedback circuit (sense and return circuit) Voltage-Voltage: both the input and output of the feedback circuit is voltage Voltage-Current: input of feedback is voltage and output is current Current-Voltage: input of feedback is current and output is voltage Current-Current: both the input and output of feedback circuit is current

14 Sense and Return Mechanism (contd.) To sense a voltage: High Input Impedance Low Input Impedance To sense a current: The addition/subtraction at the input can be done in current or voltage domain: (a) currents are added by placing them in parallel; (b) voltages are added by placing them in series The sense and return mechanism ideally do not affect the operation of feed-forward amplifier in practical circuits they do introduce loading effects

15 Sense and Return Mechanism (contd.) Practical Implementations of Sensing: Voltage Sensing Current Sensing Current Sensing

16 Sense and Return Mechanism (contd.) Practical Implementations of Voltage Subtraction: Provides the amplified version of difference between V in and the portion of V out This CS stage provides output in terms of voltage difference V in - V F This CG stage provides output in terms of voltage difference V in - V F

17 Sense and Return Mechanism (contd.) Practical Implementations of Current Subtraction: Feed-forward Amplifier Important: voltage subtraction happens when they are applied to two distinct nodes whereas current subtraction happens when they are applied to a single node a precursor to feedback topologies

18 Feedback Topologies Voltage-Voltage Feedback (also called Shunt-Series Feedback): both the input and output of the feedback circuit is voltage Voltage-Current Feedback (also called Shunt-Shunt Feedback): input of feedback is voltage and output is current Current-Voltage (also called Series-Series Feedback): input of feedback is current and output is voltage Current-Current (also called Series-Shunt Feedback): both the input and output of feedback circuit is current Sensed Quantity is Voltage Voltage Voltage Feedback Returned Quantity is Voltage Voltmeter Type Characteristics Should be added/subtracted in series

19 Voltage-Voltage Feedback Increased Input Impedance Subtracted in series Voltmeter Type Connection Parallel Sensing Reduced Output Impedance This high impedance is in parallel to the feedforward amplifier V V V e in F V A V A V V out 0 e 0 in out V F V out V A out 0 Vin 1 A Closed loop gain 0 modified gain smaller!!!

20 Voltage-Voltage Feedback (contd.) Example: Voltage-Voltage Feedback For voltage sensing parallel to the output node of this differential input but single ended output amplifier Ve VF VX The voltage signal from feedback network is fed to the other input node of the differential amplifier V A V A V M 0 e 0 X Open-Loop Output Impedance Output Impedance: V F V X VX Rout R out, closed I Reduced Closed-Loop 1 A Output Impedance X 0 I X V ( A V ) X R out 0 X

21 Voltage-Voltage Feedback (contd.) V V V e X F V I R e X in A0 I X Rin Input Impedance: Open-Loop Input Impedance V A I R F 0 X in V V V V A I R I R e X F X 0 X in X in V R R 1 A X in, closed in 0 I X Increased Input Impedance Example: calculate gain and output impedance of this circuit:

22 Voltage-Voltage Feedback (contd.) Step-1: determine open-loop voltage gain Grounding ensures there is no voltage feedback Open-loop gain is: A g r r 0 m1 o2 o4 Step-2: determine the loop gain CS Stage Vt Drain Current C V V g r r 1 F t m1 o2 o4 C1 C2 Therefore, C1 A g r r C C 0 m1 o2 o4 1 2 Output impedance

23 Voltage-Voltage Feedback (contd.) A 0 Aclosed 1 A0 1 C gm 1 ro 2 ro 4 C1 g r r C 1 2 m1 o2 o4 For βa 0 >>1, A The closed-loop output impedance, closed g r r C 1 C g C r r C m1 o2 o m1 o2 o4 1 C2 R out, closed Rout, open ro2 ro4 1 A C g r r C C 1 2 m1 o2 o4 For βa 0 >>1, 2 R out, closed C 1 C 1 g 1 m1 Relatively Smaller Value

24 Stability Issues in Feedback Amplifiers The generic closed-loop transfer function: A closed ( j) A () s 1 A ( s) ( s) 0 It is assumed that both the open-loop gain 0 and the feedback gain is frequency dependent At low frequencies: β(s) is assumed as a constant value and A 0 (s) is also assumed as a constant value the loop gain becomes constant obviously this happens for any direct-coupled amplifier with poles and zeros present at high frequency the loop gain (Aβ) should be positive value for negative feedback At high frequencies: A closed Therefore it is apparent that the loop gain is: A0 ( j) ( j) 1 A ( j) ( j) 0 j ( ) L( j) A ( j) ( j) A ( j) ( j) e 0 0 Magnitude Phase Angle It is real with negative sign at the frequency when ϕ(ω) is 180 О If for ω=ω 1, the loop gain is less than unity What happens to stability?

25 Stability Issues in Feedback Amplifiers (contd.) At high frequencies: If at ω=ω 1, the loop gain (L) is equal to unity with negative sign Closed-loop gain will be infinite even for zero input there will be some output an oscillation condition!!! For no input: (just excited by some noise) X H ( j) X o i X H ( j) X X f i i A H ( j) Provides a positive feedback which can sustain oscillation Summary: The phase angle of the loop gain equals -180 О The magnitude of the loop gain is either unity or greater than unity Both exist together to cause oscillation Barkhausen Criterion

26 Stability Issues in Feedback Amplifiers (contd.) Boundary Condition: H( j ) 1 1 H( j ) Notice that the total phase shift around the loop at ω 1 is 360 О because negative feedback itself introduces 180 О 360 О phase shift is necessary for oscillation since the feedback signal must add in phase to the original noise to allow oscillation Similarly, a loop gain of unity (or greater) is also required to enable growth of the oscillation amplitude Summary A negative feedback system may oscillate at ω 1 if: the phase shift around the loop at this frequency is so much that the feedback becomes positive, and the loop gain is still enough to allow signal buildup

27 Stability Issues in Feedback Amplifiers (contd.) Unstable System Stable System To make the system stable, the idea is to minimize the total phase shift so that for βh =1, <βh is still more positive than -180 О

28 Stability Issues in Feedback Amplifiers (contd.) For a unity gain (β=1) Feedback Gain crossover, GX β increases Phase crossover, PX If β is reduced (i.e., less feedback is applied), the magnitude plot will shift down essentially moves GX closer to origin in turn makes the system more stable

29 Stability Issues in Feedback Amplifiers (contd.) Stability Test: Nyquist Plot Intersection on the left of this point makes the feedback circuit unstable Polar plot of loop gain with varying frequency Intersection of Hβ with negative real axis: ω=ω 1

30 Stability Issues in Feedback Amplifiers (contd.) Stability and Pole Location the transient response of an amplifier with a pole pair s p = σ p ± jω p subjected to disturbance will show a transient response: pt j pt j pt pt x( t) e e e 2e cos( pt) Envelope Sinusoid For poles in right half of the s-plane the oscillations will grow exponentially considering that σ p will be positive For poles with σ p =0, the oscillation will be sustained For poles in the left half of the s-plane, the term σ p will be negative and therefore the oscillation will decay exponentially towards zero

31 Stability Issues in Feedback Amplifiers (contd.) Stability and Pole Location Stable Negative σ p Unstable Positive σ p Sustained Oscillation - Unstable Zero σ p Obviously, the presence of zeros have been ignored

32 Poles of the Feedback Amplifier Study of single-pole feedforward amplifier Closed-loop transfer function Then the closed-loop transfer function becomes A closed A0 / (1 A0 ) () s 1 s/ (1 A) P 0 Y H () s Aclosed ( s) ( s) X 1 H ( s) Where, H() s It is apparent that the feedback moves the pole frequency from ω p to: A 0 1 s / p (1 ) p, closed p A0

33 Amplifier with a Single Pole (contd.) The frequency response of amplifier with and without feedback A 0 A closed p p, closed A closed This demonstrates that gain although drops and pole frequency becomes bigger but with phase lag of only -90 О single-pole system is stable by default 45 Pole Frequency 90

34 Amplifier with a Single Pole (contd.) Root Locus (1 ) p, closed p A0 The original pole and its movement with feedback It is apparent that the pole never enters the right half of the s-plane unconditionally stable scenario!

35 Amplifier with Two Poles Open-loop transfer function of an amplifier with two pole is given as: The closed-loop poles are obtained from: Therefore the closedloop poles are: 1 As ( ) 0 As () A 1 s/ 1 s/ 0 P1 P2 ( ) (1 ) 0 2 s s P 1 P2 A0 P 1 P2 1 1 s ( ) ( ) 4(1 A ) P1 P2 P1 P2 0 P1 P2 As the loop gain A 0 β is increased from zero, the poles come closer At certain A 0 β the poles will coincide Further increase in A 0 β make poles complex conjugate which move along a vertical line

36 Amplifier with Two Poles (contd.) Root-locus Diagram 1 1 s ( ) ( ) 4(1 A ) P1 P2 P1 P2 0 P1 P2 Increase in β Increase in β Root-locus shows that the poles never enter the right half of s- plane Unconditionally stable!!! Reason is simple: the maximum phase shift of A(s) is -180 О (-90 О per pole) [that too when ω p ] There is no finite frequency at which the phase shift reaches О therefore no polarity reversal of feedback Poles when no feedback (i.e., β = 0)

37 Amplifier with Two Poles (contd.) Frequency Response GX happens before PX unconditionally stable The system is stable since the loop gain is less than 1 at a frequency For which the angle(βh(ω))=-180. With reduced feedback GX moves closer to origin doesn t affect PX the system becomes more stable

38 Amplifier with Three Poles Frequency Response Possibility of oscillation There is a finite frequency at which the loop gain can be more than 180 О phase shift (3 poles can bring a max phase shift of 270 О )

39 Amplifier with Three Poles Highest frequency pole moves leftward with increase in A 0 β do not bring instability Increase in A 0 β bring the other two poles together Further increase in A 0 β make the poles complex and then conjugate At a definite A 0 β the pair of complex-conjugate poles enter the right half of s-plane bring instability!!! Highest Frequency Pole

40 Amplifier with Three Poles (contd.) In order to maintain the stability of amplifiers it is imperative to keep loop gain A 0 β smaller than the value corresponding to the poles entering right half s- plane In terms of Nyquist diagram, the critical value of A 0 β is that for which the diagram passes through the (-1, 0) point Reducing A 0 β below this value causes the Nyquist plot to shrink the plot intersects the negative real axis to the right of (-1, 0) point indicates stable amplifier Increasing A 0 β above this value causes expansion of Nyquist plot plot encircles the (-1, 0) point unstable performance Case Study: Relative Location of GX and PX Case 1: <βh(jω p )=-175 o Case 2: <βh(jω p ) such that GX<<PX Case 3: <βh(jω p )=-135 o

41 Case 1: <βh(jω p )=-175 o 1 j175 Y X H( j ) ( j p p) 1 H ( j ) p Y X Y 11.5 ( j p) X e ( j p) 1 1 e Since at low frequencies, Y/X = 1/β: the closed loop freq response exhibits a sharp peak in the vicinity of ω=ω p j175 p The system is technically stable, but it suffers from ringing

42 Case 2: <βh(jω p ) such that GX<<PX Higher is the spacing between GX and PX (while GX remains below PX), the more stable is the system Alternatively, phase of βh at the GX frequency can serve as the measure of stability: the smaller <βh at GX, the more stable the system Leads to the concept of phase margin (PM) PM 180 H ( ) Where, ω 1 is the GX frequency 1

43 Case 3: <βh(jω1)=-135 How much PM is adequate? PM 45 H ( 1) 135 H ( ) 1 1 Y X H ( j ) ( j ) 1 1 e 1 1 j135 Closed-Loop Y X 1.3 Suffers a 30% peak at ω 1

44 Case 3: <βh(jω1)=-135 Peaking is associated with ringing in time domain Ringing in time domain Little ringing but fast settling Free from ringing but slow settling You design your system to achieve PM of around 60 О Caution PM is useful for small signal analysis. For large signal step response of a feedback system, the nonlinear behavior is usually such that a system with satisfactory PM may still exhibit excessive ringing. Transient analysis should be used to analyze large signal response.

45 Frequency Compensation Open loop transfer function is modified such that the closed-loop circuit is stable and the time response is well behaved Reason for frequency compensation: βh(ω) does not drop to unity when <βh(ω) reaches -180 o. Possible Solutions: (Push PX Out) (push GX in)

46 Option 1: Push PX OUT Minimize the # of poles What s the problem? Each stage contributes a pole. Reduction in # of stages implies difficult trade-off of gain versus output swings.

47 Option 2: Push GX In Problem: Bandwidth is sacrificed for stability Typical Approach Minimize the number of poles first to push PX out Use compensation to move the GX towards the origin next

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