Feedback: Part A - Basics Slides taken from: A.R. Hambley, Electronics, Prentice Hall, 2/e, 2000 1
Overview The Concept of Feedback Effects of feedback on Gain Effects of feedback on non linear distortion Effects of feedback on noise Effects of feedback on input and output impedance Types of feedback networks Design of feedback amplifiers Effect of Feedback on Bandwidth Transient and frequency response Effect of feedback on pole location Gain margin and phase margin Dominant-pole compensation 2
Feedback Consists of returning part of the output of a system to the input Negative Feedback: a portion of the output signal is returned to the input in opposition to the original input signal Positive Feedback: the feedback signal aids the original input signal 3
Negative Feedback Pro & Cons Negative Feedback Effects: Reduces gain Stabilizes gain Reduces non linear distortion Reduces certain types of noise Controls input and output impedances Extends bandwidth The disadvantage of reducing the gain can be overcome by adding few more stages of amplification 4
Effects of Feedback on Gain (1) Figure 9.1 Feedback amplifier. Note that the signals are denoted as x i, x f, x o, and so on. The signals can be either currents or voltages. 5
Effects of Feedback on Gain (2) 6
Problems with Positive Feedback 7
Gain Stabilization (1) If we design the amplifier so that Aβ >> 1, then the closed loop gain A f is approximately 1/β Under this condition A f depends only on the stable passive components (resistor or capacitors) used in the feedback network, instead of depending on the open loop gain A which in turn depends on active device parameters (g m ) which tend to be highly variable with operating point and temperature 8
Gain Stabilization (2) 9
The Summing Point Constraint 10
Reduction of non linear distortion (1) Figure 9.2 Transfer characteristic of a certain nonlinear amplifier. 11
Reduction of non linear distortion (2) Figure 9.3 Output of amplifier of Figure 9.2 for x in = sin(vt). Notice the distortion resulting from the nonlinear transfer characteristic. 12
Reduction of non linear distortion (3) Figure 9.4 Addition of a linear high-gain preamplifier and negative feedback to reduce distortion. 13
Reduction of non linear distortion (4) Figure 9.5 Predistorted input signal. 14
Reduction of non linear distortion (5) Figure 9.5 Predistorted input signal. 15
Example: crossover distortion (1) Figure 9.7 Nonlinear class-b power amplifier. Figure 9.8 Transfer characteristic for the amplifier of Figure 9.7. 16
Example: crossover distortion (2) Figure 9.9a Class-B power amplifier with feedback. 17
Example: crossover distortion (3) Figure 9.10 Waveforms for the circuit of Figure 9.9 with the switch in position A. Notice the crossover distortion in the output. Figure 9.11 Waveforms for the circuit of Figure 9.9 with the switch in position B. Notice the predistortion of the base drive voltage v B. 18
Noise Reduction SNR = signal to noise ratio Figure 9.12 Models that account for the addition of noise in amplifiers. 19
SNR for a Feedback Amplifier (1) Figure 9.13 Feedback amplifier with a noise source. 20
SNR for a Feedback Amplifier (2) 21
Types of Feedback There are 4 basic types of feedback that have different effects: series voltage series current parallel voltage parallel current 22
Series-Voltage Feedback 23
Series-Current Feedback 24
Parallel-Voltage Feedback 25
Parallel-Current Feedback 26
Sampling the output signal In complex circuits, sometimes it is not clear whether we have current or voltage feedback A simple test is to open or short the load If opening the load the feedback signal vanishes we have current feedback If shorting the load the feedback vanishes we have voltage feedback 27
Units of the feedback ratio The units of β are the inverse of the units of the amplifier gain For series-voltage feedback A=Av and β is unit less For series-current feedback A=Gm and β is in Ω For parallel-voltage feedback A=Rm and β is in Siemens For parallel-current feedback A=Ai and β is unit less 28
Effects of various types of feedback on gain series-voltage: series-current: parallel-voltage: parallel-current: 29
Input impedance: effect of series feedback Figure 9.15 Model for analysis of the effect of series feedback on input impedance. 30
Input impedance: effect of parallel feedback Figure 9.16 Model for analysis of the effect of parallel feedback on input impedance. 31
Output impedance: effect of voltage feedback Figure 9.17 Model for the analysis of output impedance with voltage feedback. 32
Output impedance: effect of current feedback Figure 9.18 Model for the analysis of output impedance with current feedback. 33
Summary: Effects of feedback 34
Analysis of feedback amplifiers Step 1 Identify negative feedback Step 2 Identify the type of feedback (current feedback vs. voltage feedback) Step 3 Determine the feedback ratio β = x f / x o 35
Examples of feedback amplifiers (1) 36
Examples of feedback amplifiers (2) 37
Examples of feedback amplifiers (3) 38
Examples of feedback amplifiers (4) 39
Design of feedback amplifiers (1) Step 1 Decide what type of feedback is required and determine the value of the feedback ratio Step 2 Select the appropriate circuit for the feedback network Step 3 select the appropriate valued for the components in the feedback network Step 4 Analyze the circuit to verify that all approximation were legitimate. Signal sources have nonzero internal resistance. The feedback network has non ideal input and output impedances. Consequently it loads the amplifier output and inserts impedance into the input circuit 40
Design of Feedback Amplifiers Series feedback try to select small resistance values, so that the network does not insert significant resistance into the input circuit Parallel feedback try to select large resistance values so that the feedback network does not tend to short out the input terminals Voltage feedback try to select large feedback resistance to do not load the amplifier Current feedback try to select small feedback resistances because the input of the feedback network is in series with the load 41