Lect2UP210_(100327) - Lecture 210 Output Amplifiers...

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Lecture 210 – Output Amplifiers (3/27/10) Page 210-1 CMOS Analog Circuit Design © P.E. Allen - 2010 LECTURE 210 – OUTPUT AMPLIFIERS LECTURE ORGANIZATION Outline • Introduction • Class A Amplifiers • Push-Pull Amplifiers • Bipolar Junction Transistor Output Amplifiers • Using Negative Feedback to Reduce the Output Resistance • Summary CMOS Analog Circuit Design, 2 nd Edition Reference Pages 218-229 Lecture 210 – Output Amplifiers (3/27/10) Page 210-2 CMOS Analog Circuit Design © P.E. Allen - 2010 INTRODUCTION General Considerations of Output Amplifiers f 1 ( v IN ) f 2 ( v IN ) i 1 i 2 R L v OUT V DD V SS Buffer v IN i OUT + - i 1 i 2 = I Q i OUT t Current i OUT t i 1 i 2 i OUT t i 1 i 2 Class A Class AB Class B Fig. 5.5-005 Requirements: 1.) Provide sufficient output power in the form of voltage or current. 2.) Avoid signal distortion. 3.) Be efficient 4.) Provide protection from abnormal conditions (short circuit, over temperature, etc.) Types of Output Amplifiers: 1.) Class A amplifiers 2.) Source followers 3.) Push-pull amplifiers 4.) Substrate BJT amplifiers 5.) Amplifiers using negative shunt feedback
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Lecture 210 – Output Amplifiers (3/27/10) Page 210-3 CMOS Analog Circuit Design © P.E. Allen - 2010 Output Current Requirements for an Output Amplifier Consider the current requirements placed by the load on the output amplifier: Output Amplifier + v OUT - C L R L i OUT 070422-01 I max due to R L I max due to R L I max due to C L v OUT t Result: | i OUT | > C L · SR | i OUT | > v OUT (peak) R L Fortunately, the maximum current for the resistor and capacitor do not occur at the same time. Lecture 210 – Output Amplifiers (3/27/10) Page 210-4 CMOS Analog Circuit Design © P.E. Allen - 2010 Output Resistance Requirements for an Output Amplifier In order to avoid attenuation of the amplifier voltage signal, the output resistance of the amplifier must be less than the load resistance. Output Amplifier + - R L v OUT 070422-02 t R out v OA v OA ( t ) v OA ( t ) Volts v OUT ( t ) = R L R L +R out v IN To avoid attenuation of the amplifier voltage signal, R out << R L .
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Lecture 210 – Output Amplifiers (3/27/10) Page 210-5 CMOS Analog Circuit Design © P.E. Allen - 2010 Separation of the Amplifier Bias from the Load Resistance Unfortunately, when a low load resistance is connected to the output of an amplifier, the bias conditions can be changed. Solution: 1.) Use a coupling capacitance for singled-ended power supplies. 2.) Redefine the output analog ground as ( V DD /2). 3.) Use dc coupling for split power supplies. V DD V BP 1 v OUT v IN I Q R L 070422-04 V DD V BP 1 v OUT v IN I Q R L V SS 0V I DC = 0 V DD V BP 1 v OUT v IN I Q R L 0.5 V DD V DD V BP 1 v OUT v IN I Q R L Loss of bias current through R L 070422-03 Lecture 210 – Output Amplifiers (3/27/10) Page 210-6 CMOS Analog Circuit Design © P.E. Allen - 2010 CLASS A AMPLIFIERS Current source load inverter A Class A circuit has current flow in the MOSFETs during the entire period of a sinusoidal signal.
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This note was uploaded on 03/19/2012 for the course EE 3050 at Georgia Institute of Technology.

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Lect2UP210_(100327) - Lecture 210 Output Amplifiers...

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