Feedback Section 8.1. Topics General Feedback Examples of Feedback Circuits – Bandwidth Extension...
-
Upload
cameron-wimbrow -
Category
Documents
-
view
254 -
download
0
Transcript of Feedback Section 8.1. Topics General Feedback Examples of Feedback Circuits – Bandwidth Extension...
Topics
• General Feedback• Examples of Feedback Circuits– Bandwidth Extension– Gain Sensitivity– Input and Output Impedance
• Types of Amplifiers
General Feedback System
H(s)=Feedforward network, represents an amplifier, “Open-loop” transfer function, a.k.a. AG(s)=Feedback network, usually frequency independent, a.k.a, β.Y(s)/X(s)=“Closed-loop” transfer function
Behavior of a well-designed Negative Feedback System
The error term is minimized!
An accurate copy of the input.
The input of H(s) is a virtual ground because the error term is minimized!
Elements of Feedback System
• Feed forward amplifier• Means of Sensing the Output• Feedback network• Means of Generating Feedback Error
C2 senses the output voltage, converts it to a current feedback signal, which is added tot eh current produced by Vin through C1.
Common-Source Stage
Poor definition of the gain (gmro): Both gm and ro depend vary with process and temperature.
Gain Desensitization Example
Assumption:1. The frequency is low enough that C2 does not load the output node and CGS behaves as an open circuit.2. gmro is sufficiently large3. Bias of the gate is not shown!
Results:4. If C1 and C2 are made of the same material, then Process and temperature variations do not change C1/C2.
Gain Desensitization
The closed loop gain is approximately, 1/β.If Aβ is sufficiently large, Y/X is relatively insensitive to variationsof A. Aβ is called the loop gain.
(Taylor series expansion)
Calculation of Loop GainThe input is set to 0.
The negative of the transfer function is the loop gain.
VF=-βAVt
Common Gate Loop Gain Example
Feed forward: M1 and RDFeedback: R1 and R2R1+R2>>RD
M1 operates as a subtractor.
(Loop gain)
Achieving High gain and High Bandwidth
1. Apply feedback to Improve speed of each amplifier2. Cascade to improve gain!
Feedback Mechanism
(Feed forward amplifier)
Subtraction occurs in the current domain at the Input terminal.
Realistic Model
(Voltage) Transimpedance
Transimpedance
TransconductanceCurrent amplifier
More than one model is possible.
Voltage Sensing
How do you measure voltage across a port?1. Place a voltmeter in parallel2. Use a voltmeter of high input impedance soIt does not disturb the circuit.
R1+R2 must be largeenough so that A1does feel the effectof the resistive divider.
Current Sensing
(Current Meter resistance)Current is sensed by measuring voltageacross r.
(Implementation)RS is ideally small.
Polarity of Feedback
1. Assume the input signal goes up or down
2. Follow the change through the forward amplifier and the feedback network
3. Determine whether the returned quantity opposes or enhances the original effect produced by the input change.