feedback (2)
DESCRIPTION
Feedback (2). Section 8.2-8.4. Topics. Feedback topologies Loading Effects Effect of Feedback on Noise. Feedback Topologies. Types Voltage-voltage Voltage-Current Current-Voltage Current-Current Parameters Closed Loop Gain Input Impedance Output Impedance. Summary. - PowerPoint PPT PresentationTRANSCRIPT
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Feedback (2)
Section 8.2-8.4
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Topics
• Feedback topologies• Loading Effects• Effect of Feedback on Noise
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Feedback Topologies
• Types– Voltage-voltage– Voltage-Current– Current-Voltage– Current-Current
• Parameters– Closed Loop Gain– Input Impedance– Output Impedance
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Summary
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General Comment
• Parallel Connection: Impedance fall by 1+loop gain.
• Series Connection: Impedance Rises by 1+loop gain
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Voltage-Voltage Feedback
Sense Vout in parallelReturn Vin in series
Alternative name:Return-Sense=Series-Shunt feedback
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Ideal A0
Infinite input resistance so it can sense voltage as an ideal voltmeter.Zero output resistance so as to serve as an ideal voltage source.
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Example
(R1+R2=large so as not to disturb Vout)
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Input Resistance
(non-ideal)
(ideal)
Without feedback:
With feedback:
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Example
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Output Resistance
(ideal)
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Example
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Voltage-Voltage Feedback
Sense Vout in parallel
Return Vin in series
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Voltage-Current Feedback
Sense Vout in parallelReturn current in parallel
Alternative name:Return-Sense=Shunt-Shunt feedback
K has a dimension of conductance:K=IF/Vout
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Example
(RF is largein order to returna current)
(Open-loop gain)Assumption:RF is large!Or RF>>RD2
IRF=Vout/RF
K=-1/RF (- comes from the The direction of IF)
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Ideal R0
Zero input impedance so that it can Measure currents as an ideal current meter.
Zero output resistance so as to behaveas an ideal voltage source.
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Calculation of Input Impedance
(small resistance)
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Example
(Open loop input-impedance)
R0=RD1(-gm2RD2)
IRF=Vout/RF
K=-1/RF
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Calculation of Output Impedance
(small resistance)
VA=(-IF)RoRout
(Current drawn by the feedback network is neglected)
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Example
Rout=RD2
R0=RD1(-gm2RD2)
IRF=Vout/RF
K=-1/RF
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Current-Voltage Feedback
Sense Iout in seriesReturn Vin in series
Alternative name:Return-Sense=series-series feedback
(K=VF/Iout, hence a dimension of resistance)
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Gm
Infinite input resistance so it can sense voltage as an ideal voltmeter.
Infinite output resistance in order to behaveas an ideal current source.
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Example
(For sensing current)
(polarity check)
(Calculate the open loop gain)
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Calculation of Input Impedance
(Vin-VF)/Rin=Iin
VF=KIinRinGm
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Example
Open Loop Input impedance: 1/gm
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Calculation of Output Impedance
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Example
Open Loop Input impedance: 1/gm2
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Current-Current Feedback
Sense Vout in parallelReturn current in parallel
Alternative name:Return-Sense=Shunt-Shunt feedback
K has a dimension of conductance:K=IF/Vout
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Current-Current Feedback
Sense Iout in seriesReturn current in parallel
Alternative name:Return-Sense=Shunt-series feedback
K has a dimension of conductance:K=IF/Iout
(current gain)
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Ideal Forward Current Amplifier
Zero input impedance in order to maximize current transfer.Infinite output impedance in order to behave as a current source.
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Polarity of Feedback
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Current and Current FeedbackRM is small, therefore VP is small.Vp is IoutRM
(RF>>1/gm1)RF is large in order for K to behave as a current source.
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Calculation of Input Impedance
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Example
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Calculation of Output Impedance
AI
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Example
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In Summary
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Inclusion of I/O Effects
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Rules for Breaking the Feedback Network (1)
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Rules for Breaking the Feedback Network (2)
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Voltage-Voltage Feedback
K is driven by a zero source impedance. K sees the infinite input impedance of the forward amplifier.
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Voltage-Current Feedback
K is driven by a zero source impedance. K sees a zero input impedance of the forward amplifier.
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Current-Voltage Feedback
K is driven by an infinite source impedance. K sees the infinite input impedance of the forward amplifier.
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Current-Current Feedback
K is driven by an infinite source impedance. K sees the zero input impedance of the forward amplifier.
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Rules for Breaking the Feedback Network
• Applicable for both sense and return duplicate.– Open for series connection– Shorted for parallel connection
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Calculate the Feedback Factor
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Voltage-Voltage Feedback
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Voltage-Current Feedback
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Current-Voltage Feedback
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Current-Current Feedback
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Rules for Determining the Feedback
• If the output of the feedback depends on voltage, open it.
• If the output of the feedback depends on current, short it.
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Voltage-Voltage Example (1)(R1+R2 is not much larger than RD)
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Voltage-Voltage Example(1)
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Voltage-Voltage Example (2)
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Voltage-Voltage Example (2)
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Voltage-Current Example (1)
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Voltage-Current Example (1)
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Current-Voltage Example (1)
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Current-Voltage Example (1)
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Current-Current Example (1)
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Current-Current Example (1)