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Feedback (2) Section
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
General Comment Parallel Connection: Impedance fall by 1+loop gain. Series Connection: Impedance Rises by 1+loop gain
Voltage-Voltage Feedback Sense Vout in parallel Return Vin in series Alternative name: Return-Sense=Series-Shunt feedback
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.
Example (R1+R2=large so as not to disturb V out )
Input Resistance (non-ideal) (ideal) Without feedback: With feedback:
Output Resistance (ideal)
Voltage-Voltage Feedback Sense Vout in parallel Return Vin in series
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
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)
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.
Calculation of Input Impedance (small resistance)
Example (Open loop input-impedance) R 0 =R D1 (-g m2 R D2 ) I RF =V out /R F K=-1/RF
Calculation of Output Impedance (small resistance) V A =(-I F )R o R out (Current drawn by the feedback network is neglected)
Example Rout=RD2 R 0 =R D1 (-g m2 R D2 ) I RF =V out /R F K=-1/RF
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)
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.
Example (For sensing current) (polarity check) (Calculate the open loop gain)
Calculation of Input Impedance (V in -V F )/R in =I in V F =KI in R in G m
Example Open Loop Input impedance: 1/g m
Calculation of Output Impedance
Example Open Loop Input impedance: 1/g m2
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
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)
Ideal Forward Current Amplifier Zero input impedance in order to maximize current transfer. Infinite output impedance in order to behave as a current source.
Polarity of Feedback
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.
Calculation of Input Impedance
Calculation of Output Impedance AIAI
Inclusion of I/O Effects
Rules for Breaking the Feedback Network (1)
Rules for Breaking the Feedback Network (2)
Voltage-Voltage Feedback K is driven by a zero source impedance. K sees the infinite input impedance of the forward amplifier.
Voltage-Current Feedback K is driven by a zero source impedance. K sees a zero input impedance of the forward amplifier.
Current-Voltage Feedback K is driven by an infinite source impedance. K sees the infinite input impedance of the forward amplifier.
Current-Current Feedback K is driven by an infinite source impedance. K sees the zero input impedance of the forward amplifier.
Rules for Breaking the Feedback Network Applicable for both sense and return duplicate. – Open for series connection – Shorted for parallel connection
Calculate the Feedback Factor
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.
Voltage-Voltage Example (1) (R1+R2 is not much larger than RD)
Voltage-Voltage Example (2)
Voltage-Current Example (1)
Current-Voltage Example (1)
Current-Current Example (1)
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