Feedback
8 Feedback Why Feedback? Desensitize the gain: make the value of the gain less sensitive to variations in the value of circuit components, such as might be caused by changes in temperature Reduce nonlinear distortion: make the output proportional to the input Reduce the effect of noise: minimize the contribution to the output of unwanted electric signals generated, either by the circuit components themselves, or by extraneous interference Control the input and output impedances: raise or lower the input and output impedance by the selection of an appropriate feedback topology Extend the bandwidth of the amplifier
8 Feedback 8.1 The General Feedback Structure
8.2 Some Properties of Negative Feedback 8.2.1 Gain Desensitivity Assume is constant. Taking differentials of both sides results in desensitivity factor
8.2 Some Properties of Negative Feedback 8.2.2 Bandwidth Extension Midband Gain: 3-dB Frequency:
8.2 Some Properties of Negative Feedback 8.2.3 Noise Reduction Signal-to-Noise Ratio
8.2 Some Properties of Negative Feedback 8.2.3 Reduction in Nonlinear Distortion = 0.01 Open Gain: 1000 Open Gain: 100
8.3 The Four Basic Feedback Topologies 8.3.1 Voltage Amplifiers Sample Voltage Mix Feedback Input: Voltage Output: Voltage Voltage-mixing voltage sampling Series - shunt feedback
8.3 The Four Basic Feedback Topologies 8.3.2 Current Amplifiers Sample Current Mix Feedback Input: Current Output: Current Current-mixing current sampling Shunt - series feedback
8.3 The Four Basic Feedback Topologies 8.3.3 Transconductance Amplifiers Sample Current Mix Feedback Input: Voltage Output: Current Voltage-mixing current sampling Series - series feedback
8.3 The Four Basic Feedback Topologies 8.3.4 Transresistance Amplifiers Sample Voltage Mix Feedback Input: Current Output: Voltage Current-mixing Voltage sampling Shunt - shunt feedback
8.4 The Series-Shunt Feedback Amplifier 8.4.1 The Ideal Situation
8.4 The Series-Shunt Feedback Amplifier 8.4.1 The Ideal Situation (cont.) Series mixing
8.4 The Series-Shunt Feedback Amplifier 8.4.1 The Ideal Situation (cont.) Shunt sampling
8.5 The Series-Series Feedback Amplifier 8.5.1 The Ideal Situation Series mixing
8.5 The Series-Series Feedback Amplifier 8.5.1 The Ideal Situation (cont.) Series sampling
8.6 The Shunt-Shunt and Shunt-Series Feedback Amplifier 8.6.1 The Shunt-Shunt Configuration
8.6 The Shunt-Shunt and Shunt-Series Feedback Amplifier 8.6.3 The Shunt-Series Configuration
8.6 The Shunt-Shunt and Shunt-Series Feedback Amplifier 8.6.3 Summary Ri Rif: Mixing Voltage (series) mixing always increases the input resistance. Current (shunt) mixing always reduces it. Ro Rof: Sampling Voltage (shunt) sampling always reduces the output resistance Current (series) sampling increases it
Homework: 10.1, 10.3, 10.8, 10.16, 10.24, 10.26, 10.31, 10.32