Operational Amplifiers

Transcription

1 Operational Amplifiers Spring 2008 Sean Lynch Lambros Samouris Tom Groshans

2 History of Op Amps Non Named for their originally intended functions: performing mathematical operations and amplification Addition Subtraction Integration Differentiation

3 History of Op Amps Non Op Amps were initially developed in the vacuum tube era, but later were made into IC s (Integrated Circuits) First integrated Op Amp to become widely available was the bipolar Fairchild µa709 Quickly superseded by the 741, a name that has stuck with Op Amps since

4 History of Op Amps Non The most common and most famous opamp is the ma741c or just 741, which is packaged in an 8pin minidip. The integrated circuit contains 20 transistors and 11 resistors Introduced by Fairchild in 1968, the 741 and subsequent IC opamps including FETinput opamps have become the standard tool for achieving amplification and a host of other tasks. Though it has some practical limitations, the 741 is an electronic bargain at less than a dollar.

5 Op Amp Features Non Op Amps have two different inputs, inverting,, and noninverting, s and s are the positive and negative power supplies out is the output

6 Diagram of the 741 Showed below is the 8pin version of the 741 op amp Non

7 Op Amps Non Infinite openloop gain Obtained when no feedback is used in the circuit On differential signal Applying feedback limits the gain to a usable range Zero gain for common mode input signal Infinite input impedance Thévenin equivalent of the IC looking into its input Current into the Op Amp is zero

8 Op Amps Non Infinite bandwidth Usable frequency range & Gain Infinite slew rate Zero output impedance The Thévenin equivalent impedance looking back into the output terminals Op amp can supply any current / voltage combination Zero noise

9 eal OpAmps OpAmp Typical OpAmp Non Input esistance infinity 10 6 Ω (bipolar) 10 9 Ω Ω (FET) Input Current A Output esistance Operational Gain Common Mode Gain Ω infinity Bandwidth infinity Attenuates and phases at high frequencies (depends on slew rate) Temperature independent Bandwidth and gain 9

10 eal Op Amps Non Open Loop Supply Limits (ails), Saturation Feedback educes Gain Bandwidth Gain Product 1 MHz gainbandwidth product would have a gain of 5 at 200 khz, and a gain of 1 at 1 MHz Analysis Feedback, positive, negative 0 Current, 0 oltage

11 Amplifier For an ideal opamp, the inverting amplifier gain is given by: Non The circuit that yields this equation is given on the diagram on the right

12 Amplifier Non For equal resistors, it has a gain of 1, and is used in digital circuits as an inverting buffer, or simply an inverter

13 Non Amplifier Non For an ideal opamp, the noninverting amplifier gain is given by A diagram of the circuit that yields the above equation is given on the right

14 Non Amplifier Non For an noninverting amplifier, the current rule tries to drive the current to zero at point A and the voltage rule makes the voltage at A equal to the input voltage. This leads to: and the amplification equation

15 Amplifier in C Non out

16 Amplifier in C Non out

17 Amplifier in i i = C d in = dt C out Non Use KCL to find current through each element and remember that the opamp uses no current. C d dt d dt out out out = = = C in t C 0 in in dt ( t = 0)

18 Amplifier out t = indt ( t 0 C = 0) Non in C out

19 Amplifier in C Non out = C d dt in out Similar to Integrator except and C have switched locations.

20 Amplifier Non 2 2 out 4 A more complex circuit. Can simplify using superposition of an inverting amplifier and a noninverting amplifier.

21 Amplifier Non out out _ inverting = 1 3 1

22 Amplifier Non 2 2 out 4

23 Amplifier 1 3 Non in out _ non inverting = in 1 in =? out 3 1

24 Amplifier Non 2 2 out 4

25 Amplifier Non 2 2 th = th 4 th

26 Non Amplifier = _ 1 inverting non out out in out = 1 3 _ 1 in inverting non out inverting out inverting non out out = = out = _ inverting out

27 Amplifier Non 2 2 out 4 out If 2=1 and 3=4, = out = ( )

28 Amplifier f Non 3.. n 3 n out

29 Amplifier f Non 3.. n 3 n out

30 1 2 3 n n f.. NODE i 1 i 2 i 3 i n i f Op Amps Non Amplifier f f n out in i i i i i i i i i KCL i i n n = = = = = = ,

31 Non Amplifier ( ) n n f f f out i out = = n n n f f out..

32 555 Timer Circuit, Open Loop, Logic Non

33 A/D converter, Open Loop, Logic Non

34 Non Transducers Microphones Strain Gauges PID Controllers Filters Low Pass High Pass Band Pass Butterworth Closed Loop, oltage Level

35 Closed Loop, Low Pass Filter, oltage Level Non Frequency range is governed by: 2 2 C2 f = π 1

36 Practical Tips Non Try to use single supply opamps in order to minimize need for a 10 difference from power supply Good low resistance, twisted, and shielded wire should be used when a sensor is located far away from the opamp circuit. Minimize current draw in sensor circuits to reduce thermal drift Filter power into opamp circuits using capacitors Design opamp circuits so output cannot be negative in order to protect 68HC11 A/D port. Isolate opamp circuit output with unity gain opamp if connected to an actuator. Make sure bandwidth of opamp is adequate Use trimmer potentiometers to balance resistors in differential opamp circuits Samples of opamps can be obtained from National Semiconductor ( Use the Net for circuit examples

37 eferences Wikipedia: The Art of Electronics, Horowitz and Hill Electrical Engineering, Hambley Previous Presentations Lab Notes Operational%20Amplifier.pdf

38 Non