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. A G(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 (g m r o ): Both g m and r o 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.g m r o is sufficiently large 3.Bias of the gate is not shown! Results: 1.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 variations of A. Aβ is called the loop gain. (Taylor series expansion)
Calculation of Loop Gain The input is set to 0. The negative of the transfer function is the loop gain. V F =-βAV t
Voltage Sensing How do you measure voltage across a port ? 1.Place a voltmeter in parallel 2.Use a voltmeter of high input impedance so It does not disturb the circuit. R1+R2 must be large enough so that A1 does feel the effect of the resistive divider.
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.