Microelectronic Circuits SJTU Yang Hua Chapter 8 Feedback Introduction 8.1 The general feedback structure 8.2 Some properties of negative feedback 8.3 The four basic feedback topologies 8.4 The series-shunt feedback amplifier 8.5 The series-series feedback amplifier 8.6 The shunt-shunt and shunt-series feedback amplifier 8.10 Stability study using bode plot 8.11 Frequency compensation

SJTU Yang HuaMicroelectronic Circuits Introduction It ’ s impossible to think of electronic circuits without some forms of feedback. Negative feedback  Desensitize the gain  Reduce nonlinear distortion  Reduce the effect of noise  Control the input and output impedance  Extend the bandwidth of the amplifier The basic idea of negative feedback is to trade off between gain and other desirable properties. Positive feedback will cause the amplifier oscillation.

SJTU Yang HuaMicroelectronic Circuits Three Parts: PartI: The basic concept and some Properties of negative feedback PartII: The four basic feedback and analysis PartIII: The loop gain, stability problem and frequency compensation

SJTU Yang HuaMicroelectronic Circuits PartI The basic concept Judgment and Properties of feedback examples

SJTU Yang HuaMicroelectronic Circuits PartI: The basic concept and some Properties of negative feedback This is a signal-flow diagram, and the quantities x represent either voltage or current signals. In electronic circuits, part of or all output signal is fed back to input, and affects the input signal value, which is called feedback. 8.1 The General Feedback Structure

SJTU Yang HuaMicroelectronic Circuits The feedback judgment for amplifier circuits

SJTU Yang HuaMicroelectronic Circuits Negative feedback and positive feedback ： According to the effecting of feedback 1) positive feedback increases the signal that appears at the input of the basic amplifier 2) negative feedback reduces the signal that appears at the input of the basic amplifier DC feedback and AC feedback ： 1)Feedback quantity only contains DC quantity ， is called DC feedback 2) Feedback quantity only contains AC quantity ， is called AC feedback Usually AC feedback and DC feedback are concomitant

SJTU Yang HuaMicroelectronic Circuits The feedback judgment ： (a) No feedback (b) Feedback exists (c) No feedback

SJTU Yang HuaMicroelectronic Circuits Instantaneous polarity method ： The judgment of feedback parity 1) Regulate the polarity of input signal relative to ground at sometime. 2) Decide all points’ parity step by step, at last get the parity of output signal. 3) According to the parity of output signal decides the parity of amount of feedback. 4) If amount of feedback increases the signal that appears at the input of the basic amplifier, the circuit inducts the positive feedback. Otherwise, it inducts the negative feedback.

SJTU Yang HuaMicroelectronic Circuits To integrated operational amplifiers ， the input quantity can be U D or i N （ i P ）

SJTU Yang HuaMicroelectronic Circuits To discrete components amplifiers ， the input quantity can be Ube or i b

SJTU Yang HuaMicroelectronic Circuits AC feedback ， no DC feedback DC feedback ， no AC feedback The judgment of DC feedback and AC feedback

SJTU Yang HuaMicroelectronic Circuits Example ： feedback ？ Positive or negative ？ DC or AC ？ AC and DC negative feedback

SJTU Yang HuaMicroelectronic Circuits The General Feedback Equation  Open loop gain A  Feedback factor β  Loop gain Aβ  Closed loop gain A f  Amount of feedback (1+ Aβ)

SJTU Yang HuaMicroelectronic Circuits The General Feedback Equation  If Aβ >>1, The gain of the feedback amplifier is almost entirely determined by the feedback network.  If Aβ >>1, which implies that the signal Xi at the input of the basic amplifier is reduced to almost zero.

SJTU Yang HuaMicroelectronic Circuits 8.2 Some Properties of Negative Feedback 1. Gain desensitivity the percentage change in A f (due to variations in some circuit parameter) is smaller than the percentage change in A by the amount of feedback. For this reason the amount of feedback, 1 + Aβ, is also known as the desensitivity factor.

SJTU Yang HuaMicroelectronic Circuits 2. Bandwidth extension Some Properties of Negative Feedback Note that the amplifier bandwidth is increased by the same factor by which its midband gain is decreased, maintaining the gain-bandwidth product at a constant value.

SJTU Yang HuaMicroelectronic Circuits Some Properties of Negative Feedback 3. Noise reduction

SJTU Yang HuaMicroelectronic Circuits Some Properties of Negative Feedback 4. Reduction in nonlinear distortion

SJTU Yang HuaMicroelectronic Circuits Some Properties of Negative Feedback 4. Reduction in nonlinear distortion

SJTU Yang HuaMicroelectronic Circuits Homework: May 11th, ; 8.7;

SJTU Yang HuaMicroelectronic Circuits PartII: The four basic feedback and analysis

SJTU Yang HuaMicroelectronic Circuits 8.3 The Four Basic Feedback Topologies Voltage amplifier---series-shunt feedback voltage mixing and voltage sampling

SJTU Yang HuaMicroelectronic Circuits The Four Basic Feedback Topologies Current amplifier---shunt-series feedback Current mixing and current sampling

SJTU Yang HuaMicroelectronic Circuits Example: Figure 8.5 A transistor amplifier with shunt– series feedback. (Biasing not shown.)

SJTU Yang HuaMicroelectronic Circuits Transconductance amplifier---series-series feedback Voltage mixing and current sampling The Four Basic Feedback Topologies

SJTU Yang HuaMicroelectronic Circuits Example: Figure 8.6 An example of the series–series feedback topology. (Biasing not shown.)

SJTU Yang HuaMicroelectronic Circuits Transresistance amplifier---shunt-shunt feedback Current mixing and voltage sampling The Four Basic Feedback Topologies

SJTU Yang HuaMicroelectronic Circuits Example: Figure 8.7 (a) The inverting op-amp configuration redrawn as (b) an example of shunt–shunt feedback.

SJTU Yang HuaMicroelectronic Circuits Homework: May 11nd, ; 8.15; 8.17; 8.19

SJTU Yang HuaMicroelectronic Circuits 8.4 The Series-Shunt Feedback Amplifier The ideal situation The practical situation Summary

SJTU Yang HuaMicroelectronic Circuits The Ideal Situation  A unilateral open- loop amplifier (A circuit).  An ideal voltage mixing voltage sampling feedback network (β circuit).  Assumption that the source and load resistance have been included inside the A circuit.

SJTU Yang HuaMicroelectronic Circuits The Ideal Situation Equivalent circuit. R if and R of denote the input and output resistance with feedback.

SJTU Yang HuaMicroelectronic Circuits Input and Output Resistance with Feedback Input resistance In this case, the negative feedback increases the input resistance by a factor equal to the amount of feedback. Output resistance In this case, the negative feedback reduces the output resistance by a factor equal to the amount of feedback.

SJTU Yang HuaMicroelectronic Circuits The Practical Situation  Block diagram of a practical series–shunt feedback amplifier.  Feedback network is not ideal and load the basic amplifier thus affect the values of gain, input resistance and output resistance.

SJTU Yang HuaMicroelectronic Circuits The Practical Situation The circuit in (a) with the feedback network represented by its h parameters. Omit the controlled source h 21 I 1

SJTU Yang HuaMicroelectronic Circuits The Practical Situation The circuit in (b) with h 21 neglected.

SJTU Yang HuaMicroelectronic Circuits The Practical Situation The load effect of the feedback network on the basic amplifier is represented by the components h 11 and h 22. The loading effect is found by looking into the appropriate port of the feedback network while the port is open-circuit or short-circuit so as to destroy the feedback. If the connection is a shunt one, short-circuit the port. If the connection is a series one, open-circuit the port. Determine the β.

SJTU Yang HuaMicroelectronic Circuits

SJTU Yang HuaMicroelectronic Circuits Summary R i and R o are the input and output resistances, respectively, of the A circuit. R if and R of are the input and output resistances, respectively, of the feedback amplifier, including R s and R L. The actual input and output resistances exclude R s and R L.

SJTU Yang HuaMicroelectronic Circuits Example of Series-Shunt Feedback Amplifier

SJTU Yang HuaMicroelectronic Circuits Example of Series-Shunt Feedback Amplifier Op amplifier connected in noninverting configuration with the open-loop gain μ, R id and r o Find expression for A, β, the closed-loop gain V o /V i, the input resistance R in and the output resistance R out Find numerical values

SJTU Yang HuaMicroelectronic Circuits Example of Series-Shunt Feedback Amplifier

SJTU Yang HuaMicroelectronic Circuits Example of Series-Shunt Feedback Amplifier

SJTU Yang HuaMicroelectronic Circuits 8.5 The Series-Series Feedback Amplifier The ideal situation The practical situation Summary

SJTU Yang HuaMicroelectronic Circuits The Ideal Situation Transconductance gain

SJTU Yang HuaMicroelectronic Circuits The Ideal Situation Tranresistance feedback factor

SJTU Yang HuaMicroelectronic Circuits Input and Output Resistance with Feedback Input resistance In this case, the negative feedback increases the input resistance by a factor equal to the amount of feedback. Output resistance In this case, the negative feedback increases the output resistance by a factor equal to the amount of feedback.

SJTU Yang HuaMicroelectronic Circuits The Practical Situation Block diagram of a practical series–series feedback amplifier. Feedback network is not ideal and load the basic amplifier thus affect the values of gain, input resistance and output resistance.

SJTU Yang HuaMicroelectronic Circuits The Practical Situation The circuit of (a) with the feedback network represented by its z parameters.

SJTU Yang HuaMicroelectronic Circuits The Practical Situation A redrawing of the circuit in (b) with z 21 neglected.

SJTU Yang HuaMicroelectronic Circuits The Practical Situation The load effect of the feedback network on the basic amplifier is represented by the components Z 11 and Z 22. Z 11 is the impedance looking into port 1 of the feedback network with port 2 open-circuited. Z 22 is the impedance looking into port 2 of the feedback network with port 1 open-circuited. Determine the β.

SJTU Yang HuaMicroelectronic Circuits

SJTU Yang HuaMicroelectronic Circuits Summary R i and R o are the input and output resistances, respectively, of the A circuit. R if and R of are the input and output resistances, respectively, of the feedback amplifier, including R s and R L. The actual input and output resistances exclude R s and R L.

SJTU Yang HuaMicroelectronic Circuits Example of Series-Series Feedback Amplifier

SJTU Yang HuaMicroelectronic Circuits Example of Series-Series Feedback Amplifier

SJTU Yang HuaMicroelectronic Circuits Example of Series-Series Feedback Amplifier

SJTU Yang HuaMicroelectronic Circuits Example of Series-Series Feedback Amplifier

SJTU Yang HuaMicroelectronic Circuits 8.6 The Shunt-Shunt and Shunt-Series Feedback Amplifiers Fig8.19. Ideal structure for the shunt-shunt feedback amplifier.

SJTU Yang HuaMicroelectronic Circuits The practical Shunt-Shunt feedback amplifier

SJTU Yang HuaMicroelectronic Circuits

SJTU Yang HuaMicroelectronic Circuits Example 8.3

SJTU Yang HuaMicroelectronic Circuits

SJTU Yang HuaMicroelectronic Circuits The shunt-series configuration Fig 8.22 Ideal structure for the shunt–series feedback amplifier.

SJTU Yang HuaMicroelectronic Circuits A practical shunt-series feedback amplifier

SJTU Yang HuaMicroelectronic Circuits

SJTU Yang HuaMicroelectronic Circuits Example 8.4

SJTU Yang HuaMicroelectronic Circuits

SJTU Yang HuaMicroelectronic Circuits summary

SJTU Yang HuaMicroelectronic Circuits The method of finding A circuit  h,z,r,g parameter method for two-port feedback network.  Equivalent circuit method:  Find the feedback network;  Find the feedback network equivalent load Resistance to amplifier input, for output voltage sampling feedback, output short-circuit (Vo=0); for current sampling feedback, output Io open-circuit (Io=0).  Find the feedback network equivalent load Resistance to amplifier output, for input voltage mixing feedback, it should be disconnected from input to feedback network (Ii=0); for input current mixing feedback, let input connect to ground to disconnect from input signal to feedback network.

SJTU Yang HuaMicroelectronic Circuits 引入负反馈的一般原则 为为为为稳定静态工作点，应引入直流负反馈； 为为为为改善电路动态性能，应引入交流负反馈； 当当当当信号源为恒压源或内阻较小的电压源时， 应引入串联负反馈； 当当当当信号源为恒流源或内阻较大的电压源时， 应引入并联负反馈；

SJTU Yang HuaMicroelectronic Circuits 当当当当负载需要稳定的电压信号时，应引入 电压负反馈； 当当当当负载需要稳定的电流信号时， 应引入 电流负反馈； 若若若若将电流信号转换成电压信号，应引入 电压并联负反馈(shunt-shunt)； 若若若若将电压信号转换成电流信号， 应引入 电流串联负反馈(series-series)。

SJTU Yang HuaMicroelectronic Circuits Example: 1 ）减小放大电路从信号源索取的电流并增强带负载的能力 2 ）将输入电流转换成与之成稳定稳定线性关系的输出电流 3 ）将输入电流转换成稳定的输出电压 答：引入 series-shunt ( 电压串联负反馈 ) ： 8 连 10 ， 3 连 9 ， 4 连 6 答：引入 shunt-series( 电流并联负反馈 ) ： 7 连 10 ， 2 连 9 ； 4 连 6 答： shunt-shunt( 电压并联负反馈 ) ： 2 连 9 ， 8 连 10 ， 5 连 6

SJTU Yang HuaMicroelectronic Circuits Homework: May 13th, ; 8.25; 8.30; D8.37

SJTU Yang HuaMicroelectronic Circuits The Stability Problem

SJTU Yang HuaMicroelectronic Circuits The Stability Problem The condition for negative feedback to oscillate Any right-half-plane poles results in instability.  Amplifier with a single-pole is unconditionally stable.  Amplifier with two-pole is also unconditionally stable.  Amplifier with more than two poles has the possibility to be unstable. Stability study using bode plot

SJTU Yang HuaMicroelectronic Circuits Balance condition: Magnitude condition: Phase Condition: Start up oscillation condition: Judgment method of amplifier stability: 1) If ω180 is not exist, then the Amplifier is stable; 2) If ω180 is exist and ω180< ω1, then the amplifier is not stable; 3) If ω180 is exist and ω180> ω1, then the amplifier is stable;

SJTU Yang HuaMicroelectronic Circuits The Definitions of the Gain and Phase margins  Gain margin represents the amount by which the loop gain can be increased while stability is maintained.  Unstable and oscillatory  Stable and non- oscillatory  Only when the phase margin exceed 45º or gain margin exceed 6dB, can the amplifier be stable.

SJTU Yang HuaMicroelectronic Circuits Example:

SJTU Yang HuaMicroelectronic Circuits Effect of phase margin on closed- loop response To see the relationship between phase margin and Close-Loop gain, consider a feedback amplifier with a large low-frequency loop gain.

SJTU Yang HuaMicroelectronic Circuits Stability analysis using Bode plot of |A|.

SJTU Yang HuaMicroelectronic Circuits Stability Analysis Using Bode Plot of |A| Gain margin and phase margin The horizontal line of inverse of feedback factor in dB. A rule of thumb: The closed-loop amplifier will be stable if the 20log(1/β) line intersects the 20log|A| curve at a point on the – 20dB/decade segment. The general rule states: At the intersection of 20log[1/ | β (jω)| ] and 20log |A(jω)| the difference of slopes should not exceed 20dB/decade.

SJTU Yang HuaMicroelectronic Circuits Frequency Compensation The purpose is to modifying the open-loop transfer function of an amplifier having three or more poles so that the closed-loop amplifier is stable for any desired value of closed-loop gain. Theory of frequency compensation is the enlarge the – 20dB/decade line. Implementation  Capacitance C c added  Miller compensation and pole splitting

SJTU Yang HuaMicroelectronic Circuits Frequency Compensation

SJTU Yang HuaMicroelectronic Circuits

SJTU Yang HuaMicroelectronic Circuits Frequency Compensation-Miller compensation  A gain stage in a multistage amplifier with a compensating capacitor connected in the feedback path  An equivalent circuit.  Miller compensation can reduce the value of C

SJTU Yang HuaMicroelectronic Circuits Homework: May 20 th, ; 8.54; 8.63; 8.66