General Purpose Operational Amplifiers

Transcription

1 General Purpose Operational Amplifiers OUTLINE Lecture 0, 0/7/05 Corrected 0/9/05 Op-Amp from -Port Blocks Op-Amp Model and its Idealization Negative Feedback for Stability Components around Op-Amp define the Circuit Function eading Hambley 4.-4., pp. 644 Lecture 0, Slide General Purpose Amplifier General purpose amplifiers called Operational Amplifiers (Op-Amps) were developed before general purpose computers (Microprocessors). These Op-Amps can be viewed as being built from twoport circuits. in Port out in in Port out in Port out in Port out out Stages => EE 05 Differential High Gain Low 0 Assemble as amplifier in EE 40 Lecture 0, Slide

2 High Quality Dependent Source In an Amplifier AMPLIFIE SYMBOL Differential Amplifier 0 A 0 = A( ) AMPLIFIE MODEL Circuit Model in linear region i A 0 0 depends only on input ( - ) See the utility of this: this Model when used correctly mimics the behavior of an amplifier but omits the complication of the many many transistors and other components. Lecture 0, Slide 3 Op Amp Terminals 3 signal terminals: inputs and output IC op amps have additional terminals for DC power supplies Common-mode signal= (v v )/ Differential signal = v -v positive power supply Inverting input v Non-inverting input v - v 0 output negative power supply Lecture 0, Slide 4

3 Op Amp Terminal oltages and Currents All voltages are referenced to a common node. Current reference directions are into the op amp. v i i v i c cc i o i c- v o common node (external to the op amp) Lecture 0, Slide 5 cc Model for Internal Operation A is differential gain or open loop gain Ideal op amp A i = 0 o Common mode gain = 0 ( v v) vcm =, vd = vv vo = Acmvcm Advd Since v = A( v v ), A = 0 o cm v v Circuit Model i i o i o i v o _ A(v v ) Lecture 0, Slide 6 3

4 Model and Feedback Negative feedback connecting the output port to the negative input (port ) Positive feedback connecting the output port to the positive input (port ) v Circuit Model i o i o i v o v i _ A(v v ) Lecture 0, Slide 7 Op-Amp and Use of Feedback A very high-gain differential amplifier can function in an extremely linear fashion as an operational amplifier by using negative feedback. IN Summing Point 0 IN Negative feedback Stabilizes the output Hambley Example pp. 644 for Power Steering i - Circuit Model A 0 We can show that that for A and i, 0 IN Stable, finite, and independent of the properties of the OP AMP! Lecture 0, Slide 8 4

5 Negative Feedback Familiar examples of negative feedback: Thermostat controlling room temperature Driver controlling direction of automobile Photochromic lenses in eyeglasses Fundamentally pushes toward stability Familiar examples of positive feedback: Microphone squawk in room sound system Mechanical bi-stability in light switches Thermonuclear reaction in H-bomb Fundamentally pushes toward instability or bi-stability Lecture 0, Slide 9 Summing-Point Constraint Check if under negative feedback Small v i result in large v o Output v o is connected to the inverting input to reduce v i esulting in v i =0 Summing-point constraint v = v i = i =0 irtual short circuit Not only voltage drop is 0 (which is short circuit), input current is 0 This is different from short circuit, hence called virtual short circuit. Lecture 0, Slide 0 5

6 Ideal Op-Amp Analysis Technique Applies only when Negative Feedback is present in circuit! Assumption : The potential between the op-amp input terminals, v () v (-), equals zero. Assumption : The currents flowing into the op-amp s two input terminals both equal zero. No Currents No Potential Difference IN EXAMPLE 0 Lecture 0, Slide Ideal Op-Analysis: Non-Inverting Amplifier Yes Negative Feedback is present in this circuit! Assumption : The potential between the op-amp input terminals, v () v (-), equals zero. Assumption : The currents flowing into the op-amp s two input terminals both equal zero. IN EXAMPLE KCL with currents in only two branches 0 IN appears here v v in v in v out = 0 out = v in Non-inverting Amplifier Lecture 0, Slide 6

7 Ideal Op-Amp Analysis: Inverting Amplifier Yes Negative Feedback is present in circuit! I oltage is IN - L OUT Only two currents for KCL IN OUT = 0 ( ) OUT = in Inverting Amplifier with reference voltage Lecture 0, Slide 3 The Operational Amplifier The operational amplifier ( op amp ) is a basic building block used in analog circuits. Its behavior is modeled using a dependent source. When combined with resistors, capacitors, and inductors, it can perform various useful functions: amplification/scaling of an input signal sign changing (inversion) of an input signal addition of multiple input signals subtraction of one input signal from another integration (over time) of an input signal differentiation (with respect to time) of an input signal analog filtering nonlinear functions like exponential, log, sqrt, etc Lecture 0, Slide 4 7