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Published byKylan James Modified over 2 years ago

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Feedback (2) Section

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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 parallel Return Vin in series Alternative name: Return-Sense=Series-Shunt feedback

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Ideal A 0 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 V out )

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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 parallel Return current in parallel Alternative name: Return-Sense=Shunt-Shunt feedback K has a dimension of conductance: K=I F /V out

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Example (RF is large in order to return a current) (Open-loop gain) Assumption: RF is large! Or R F >>R D2 I RF =V out /R F K=-1/RF (- comes from the The direction of IF)

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Ideal R 0 Zero input impedance so that it can Measure currents as an ideal current meter. Zero output resistance so as to behave as an ideal voltage source.

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Calculation of Input Impedance (small resistance)

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Example (Open loop input-impedance) R 0 =R D1 (-g m2 R D2 ) I RF =V out /R F K=-1/RF

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Calculation of Output Impedance (small resistance) V A =(-I F )R o R out (Current drawn by the feedback network is neglected)

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Example Rout=RD2 R 0 =R D1 (-g m2 R D2 ) I RF =V out /R F K=-1/RF

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Current-Voltage Feedback Sense Iout in series Return Vin in series Alternative name: Return-Sense=series-series feedback (K=V F /I out, hence a dimension of resistance)

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GmGm Infinite input resistance so it can sense voltage as an ideal voltmeter. Infinite output resistance in order to behave as 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 (V in -V F )/R in =I in V F =KI in R in G m

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Example Open Loop Input impedance: 1/g m

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Calculation of Output Impedance

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Example Open Loop Input impedance: 1/g m2

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Current-Current Feedback Sense Vout in parallel Return current in parallel Alternative name: Return-Sense=Shunt-Shunt feedback K has a dimension of conductance: K=I F /V out

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Current-Current Feedback Sense Iout in series Return current in parallel Alternative name: Return-Sense=Shunt-series feedback K has a dimension of conductance: K=I F /I out (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 Feedback RM is small, therefore V P is small. Vp is I out R M (RF>>1/g m1 ) 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 AIAI

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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-Current Example (1)

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Current-Voltage Example (1)

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Current-Current Example (1)

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