Chapter 12 Flashcards
Components of feedback system
Feedforward
Output and sensing mechanism
Feedback
Comparison mechanism
Feedforward System
Original system without FB
Feedback system
Generates a portion of output to be compared to (subtracted from) input
Generates feedback signal Xf from sensed output
Comparison mechanism
A means of subtracting the feedback signal from the input to obtain the error
Loop gain
Product of feedback factor and open loop gain
Break circuit, apply test voltage, see the gain around the loop
Properties of negative feedback
Gain desensitization
Bandwidth extension
Linearity improvement
Closed loop gain less sensitive to load resistance
Lowers gain but makes it stable and improves linearity
Gain desensitization
Makes system immune to open loop gain variations
If one loop gain is very high
Closed loop gain is independent of open loop gain and hence variations of A1
Bandwidth extension
Gain decreases so bandwidth increases
Sustains higher frequencies
Linearity improvement
For non-linear circuits, gain at different points may be different
Shape of line with negative FB is flatter
Feedback summary
Trade off between BW and gain
Linearity is a great advantage of negative feedback
If system is linear, small signals can be larger
BW increases, linearity improves
Voltage amp
Converts: V to V
Measured as: V/V
Gain: Vout/Vin
Measure voltage, deliver voltage
FB: Input - Series, output - parallel
Current amp
Converts: I to I
Measured as: A/A
Gain: Iout/Iin
Measure current, deliver current
FB: Input - parallel, output - series
Transimpedance amp
Converts: I to V
Measured as: V/A
Gain: Vout/Iin
Measure current, deliver voltage
FB: Input - parallel, output - parallel
Transconductance amp
Converts: V to I
Measured as: A/V
Gain: Iout/Vin
Measure voltage, deliver current
FB: Input - Series, output - Series
