Post on 29-Mar-2020
Lecture #2 Feedback Effects on Op-Amp
Instructor: Dr. Ahmad El-Banna
Faculty of Engineering Department of Electronics and Communications
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GEE336 Electronic Circuits II
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Agenda
Gain With Feedback
Effect of Feedback on Gain and Bandwidth
Op-Amps with Negative Feedback
Open & Closed Loop Frequency Responses 2
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GAIN WITH FEEDBACK 3
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Voltage-Series Feedback
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overall voltage gain with feedback:
Voltage-Shunt Feedback
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EFFECT OF FEEDBACK ON GAIN AND BANDWIDTH
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Effect of Feedback on Gain and Bandwidth
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The amplifier with negative feedback has more bandwidth (Bf) than the amplifier without feedback ( B ).
Gain Stability with feedback
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OP-AMPS WITH NEGATIVE FEEDBACK 9
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Why Use Negative Feedback?
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• Negative feedback is the process whereby a portion of the output voltage of an amplifier is returned to the input with a phase angle that opposes (or subtracts from) the input signal.
• Open-loop voltage gain of a typical op-amp is very high.
• Therefore, an extremely small input voltage drives the op-amp into its saturated output states.
• In fact, even the input offset voltage of the op-amp can drive it into saturation.
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OP-AMPS WITH NEGATIVE FEEDBACK
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• An op-amp can be connected using negative feedback to stabilize the gain and increase frequency response.
• The closed-loop voltage gain is the voltage gain of an op-amp with external feedback.
• The closed-loop voltage gain is determined by the external component values and can be precisely controlled by them.
• Non-inverting Amplifier
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OP-AMPS WITH NEGATIVE FEEDBACK Non-inverting Amplifier
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OP-AMPS WITH NEGATIVE FEEDBACK..
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• Inverting Amplifier
• Voltage-Follower
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OP-AMPS WITH NEGATIVE FEEDBACK.. Inverting Amplifier
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EFFECTS OF NEGATIVE FEEDBACK ON OP-AMP IMPEDANCES
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• Non-inverting Amplifier
• Voltage Follower
• Inverting Amplifier
Assignment: Derive the impedance equations for Non-inverting and Inverting Amplifiers.
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OPEN & CLOSED LOOP FREQUENCY RESPONSES
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Open-Loop Frequency & Phase Responses
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Overall Frequency & Phase Responses (Open-Loop)
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CLOSED-LOOP FREQUENCY RESPONSE
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• The gain-bandwidth product is always equal to the frequency at which the op-amp’s open-loop gain is unity or 0 dB (unity-gain bandwidth, fT).
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• For more details, refer to:
• Chapter 14, Boylestad, Electronic Devices and Circuit theory, 11th edition
• Chapter 12, T. Floyd, Electronic Devices, 9th edition.
• The lecture is available online at:
• http://bu.edu.eg/staff/ahmad.elbanna-courses/12884
• For inquires, send to:
• ahmad.elbanna@feng.bu.edu.eg
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