8 Macro Models of Amps in opamp and Op Amps.pptx

advanced electronics contents

1. Macro Models of Amplifiers, Op Amps, Comparators, Multipliers and Transconducting Devices N.J. Rao

2. Amplifiers June2014 N.J. Rao 2

3. Ideal Amplifiers controlled voltage/current sources and are two-port active networks have infinite power gain Infinite power gain can be achieved having zero input voltage (short circuit at the input) or zero input current (open circuit at the input) finite output (voltage or current) June2014 N.J. Rao 3

4. Differential Input Differential Output Amplifiers DIDO Amplifiers Several DIDO amplifiers are available from Analog Devices (ex. SLOA054, SLOA072) They can also be realized using DISOs June2014 N.J. Rao 4

5. Differential Input Single Output Amplifier DISO Differential input single ended output is the common general purpose IC Op amp available today Some general purpose Op Amps 741, 747, 356, TL071, TL081, TL082 June2014 N.J. Rao 5

6. Single Input Single Output Amplifier June2014 N.J. Rao 6 SISO Three terminal active devices including FETs and BJTs Mainly used in designing analog integrated circuits

7. Single Input Differential Output Amplifiers SIDO These are constructed using DIDOs June2014 N.J. Rao 7

8. Types of ideal amplifiers Voltage Controlled Voltage Source (VCVS) - Voltage Amplifier Voltage Controlled Current source (VCCS) – Transconductance Amplifier Current Controlled Current Amplifier (CCCS) – Current Amplifier Current Controlled Voltage Amplifier (CCVS) – Transresistance Amplifier June2014 N.J. Rao 8

9. VCVS is represented as The equivalent network and input-output relationship June2014 N.J. Rao 9

10. VCCS June2014 N.J. Rao 10 also known as Transconductance Amplifier The equivalent network and input-output relationship

11. CCCS also known as Current Amplifier Equivalent network and input-output relationship June2014 N.J. Rao 11

12. CCVS also known as Trans-resistance Amplifier Equivalent network and input-output relationship June2014 N.J. Rao 12

13. Amplifiers as Integrated Circuits All the four amplifiers can be realized as integrated circuits The most dominant commercial amplifier is Voltage Amplifier It is available as wide band amplifier, audio amplifiers, instrumentation amplifiers and voltage controlled amplifiers. Availability is more due to historical reasons rather than any technical reasons. While amplifiers are available commercially, they are not that widely used. Operational Amplifiers can meet practically all requirements of analog signal processing. June2014 N.J. Rao 13

14. Ideal Operational Amplifiers The transfer parameter ( g f , h f , i f and z f ) of an amplifier goes to infinity They are called so as they are used to perform several mathematical operations including addition, subtraction, integration and differentiation. The input-output characteristic of an ideal Op Amp June2014 N.J. Rao 14

15. Model of an Ideal Op Amp For any finite output, the input voltage and input current are zero. Such behaviour can only be modelled by a pair of elements called nullator and norator . Nullator represents the input element Norator represents the output element that provides any finite output voltage and current. June2014 N.J. Rao 15

16. Representation of Ideal Op Amp Ideal Op Amp is represented by a nullator-norator pair Every nullator has its companion norator A nullator along with its companion norator is called ‘ nullor ’. June2014 N.J. Rao 16

17. DIDO with infinite gain June2014 N.J. Rao 17

18. DISO with infinite gain June2014 N.J. Rao 18 One of its output ( norator ) terminals is grounded

19. SISO with infinite gain One of the input ( nullator ) and one of its output ( norator ) terminals are grounded June2014 N.J. Rao 19

20. Commercial Op Amps Designed with BJTs and JFETs or BJTs and MOSFETs as active devices Op Amp requires single or dual DC power supply voltages for it to satisfactorily operate in the active region, with output restricted to values in between the power supply voltages. June2014 N.J. Rao 20

21. Commercial Op Amps have finite, but large, dc gain gain dependent on the frequency and signal level (non-linearity) off-set voltages and currents finite input impedance non-zero output impedance. All these parameters of an Op Amp also depend on the temperature June2014 N.J. Rao 21

22. Non Ideal Op Amps (Op Amps ) have finite gain ranging from 10 3 to 10 6 (60 -120 dB) have several independent parasitic capacitances associated with internal nodes Each capacitance reduces the gain at 20 dB/decade . The gain transfer function of an Op Amp can be approximated in the range where magnitude of the gain is more than one (above 0dB) is June2014 N.J. Rao 22

23. Gain transfer function of an Op Amp can be approximated in the range where magnitude of the gain is more than one (above 0dB) as where A is the dc gain, and are the corner frequencies. June2014 N.J. Rao 23

24. Gain Transfer Function If w z is absent, each corner frequency causes, when fully effective, 20 dB/decade attenuation and phase shift. At the corner frequency, the magnitude reduces by 3 dB and phase shift becomes 45 O (at as long as w p2 and w p3 are much greater than w p1 ). At w p2 the phase shift can become 90 O +45 O , and at w p3 it can become .180 O +45 O there can be a frequency between w p2 and w p3 , at which phase shift becomes 180 O . At frequencies greater than w p3 , phase shift approaches 270 O . June2014 N.J. Rao 24

25. Gain Transfer Function If w z is present the Op Amp will be designed for it to be greater than w p1 . If w z is present the phase shift may never reach 180 O , depending on its value in relation to the other corner frequencies. June2014 N.J. Rao 25

26. Gain of Op amp All the active devices of an Op Amp may go into saturation as the output voltage levels approach the supply voltages of the Op Amp, making the gain go toward zero. is maximum when the output voltage is at , where V + is the positive supply voltage and V - is the negative supply voltage. approaches zero as the output approaches V + or V - June2014 N.J. Rao 26

27. Input-Output Parameters mismatches at the inputs cause input offset voltage, which could be of the order of a few milli volts to a few tens of milli volts input offset current, which could be a few nano amperes in case of Op Amps designed with BJTs. If the Op Amp is a voltage amplifier, the input impedance can be from a few M W to a few hundred M W . The output impedance can be few hundred Ohms. The output current of majority Op Amps is limited to 20 mA. June2014 N.J. Rao 27

28. Slew Rate The maximum rate of change of output of an Op Amp is called ‘slew rate’ and is expressed as Volts/ m sec. It is one to 20 Volts/ m sec with the commercially available Op Amps. It arises out of limiting current available for charging the capacitor at the output. All commercially available Op Amps are protected against short circuit at the output. This limits the output current to 20 to 50 mA. June2014 N.J. Rao 28

29. Gain-Bandwidth Op Amps are also characterized by their gain-bandwidth (first corner frequency) (GB) product, a parameter that will be used extensively when designing Op Amp based circuits. It is typically1MHz to 20 MHz. June2014 N.J. Rao 29

30. TL081C (1977) $0,2 for >1000 June2014 N.J. Rao 30 Parameter Value Total Supply Voltage 7 to 36 Volts All the parameters are defined for + 15V 1. Gain-Bandwidth Product at 25 O C 3 MHz 2. Slew Rate 13 V/  sec 3. CMRR 70 dB 4. Input Offset Voltage 20mV(max) 5. Input Offset Voltage Temperature Coefficient 18  V/ O C 6. Input Offset Current 2 nA (max) 7. Input Bias Current 10 nA (max ) 8. Input Resistance 10 12 Ohms 9. Output Resistance 200 Ohms

31. Comparators Comparator is an interface component. Its input is analog and its output is digital (1/0 or high/low). The output of the comparator changes its state when the input voltage crosses, while decreasing or increasing, a reference value. June2014 N.J. Rao 31

32. I/O characteristic of a Voltage Comparator June2014 N.J. Rao 32

33. I/O characteristic of a Current Comparator June2014 N.J. Rao 33 An ideal comparator will have infinite gain in the active region while transiting from one state to the other.

34. LM311 - $0.2 >1000 Nos June2014 N.J. Rao 34 Value 1. Total Supply Voltage 3.5 to 30 Volts 2. Rise Time 115 ns 3. Input Offset Voltage 7.5 mV(max) 4. Input Offset Current 70 nA (max) 5. Input Bias Current 300 nA (max)

35. Offset Voltage in comparator June2014 N.J. Rao 35 Even when the forward gain is infinity, the behaviour of a comparator differs from that of an ideal one because of input offset voltage (7.5 mV in case of 311).

36. Comparator with finite gain June2014 N.J. Rao 36 The commercial comparators are designed to have a forward gain of the order of 100 rather than the high values (> 10 6 ) associated with Op Amp. If the forward gain of the comparator is finite, the active transition region will be instead of zero in case of infinite gain.

37. Multipliers Multipliers are analog components that provide multiplication of two input voltages or currents. Multipliers can perform a variety of signal processing applications including mixing, modulating, and demodulating. They can also act as voltage controlled amplifiers, filters and oscillators, and phase detectors. Multipliers are available commercially in several forms including voltage controlled amplifier, current controlled amplifier and digitally controlled amplifier. June2014 N.J. Rao 37

38. I/O Characteristic of a Multiplier June2014 N.J. Rao 38

39. I/O Relationship where V offset is the DC offset voltage K X and K Y are feed through components and K O is the multiplier constant and has the dimension per volts. June2014 N.J. Rao 39

40. Precision multiplier MPY634 June2014 N.J. Rao 40 Parameter Value 1. Bandwidth 6 MHz (min) 2. Slew rate 20V/sec 3. Output Offset Voltage + 100 mV(max) 4. Output Short Circuit Current 30 mA

41. Transconducting Devices June2014 N.J. Rao 41

42. VCCS An ideal voltage controlled current source (VCCS) is also known as ‘transconductance amplifier’. The input-output relationship a transconductance amplifier If a transconductor is designed with a given value of g m then it becomes an amplifier with a fixed gain. June2014 N.J. Rao 42

43. Variable g m g m can be made a function of one of two variables available - ‘output current’ or ‘input voltage’ A device that provides a transfer parameter that is linearly varying with output current or input voltage will meet all analog signal processing functions. June2014 N.J. Rao 43

44. Square Law Device Relationship gives where V T is known as Threshold Voltage It is a square law relationship between output current and input voltage. June2014 N.J. Rao 44

45. Exponential Law Device Relationship gives where I S is known as Reverse Saturation Current It is an exponential relationship between the output current and input voltage. June2014 N.J. Rao 45

46. Transconducting Devices Semiconductor devices Field Effect Transistors (FETs) Bipolar Junction Transistors (BJTs) exhibit square law and exponential law relationships. FET exhibits a square law relationship in the region above threshold voltage, and an exponential relationship in sub-threshold region (Vi < V T ). BJT exhibits exponential relationship. June2014 N.J. Rao 46

47. History of FET and BJT The field-effect transistor was first patented by Julius Edgar Lilienfeld  in 1926 and by Oskar Heil  in 1934 Practical semiconducting devices (the JFET) were developed only much later. The MSOFET (Metal Oxide on Semiconductor Field Effect Transistor), which largely superseded the JFET was invented by  Dawon Kahng  and Martin Atalla in 1960. The bipolar point-contact transistor was invented in December 1947 at the Bell Telephone Laboratories by John Bardeen and Walter Brattain under the direction of William Shockley.   June2014 N.J. Rao 47

48. History of FET and BJT The junction version known as the bipolar junction transistor, invented by Shockley in 1948, enjoyed three decades as the device of choice in the design of discrete and integrated circuits. At present discrete MOSFETS are not commercially made available because of problems associated with electro static discharge. JFET are available, but their use is not popular in signal processing. MOSFETs and MOSFET technology is dominant in both digital and analog integrated circuits. June2014 N.J. Rao 48

49. History of FET and BJT Discrete BJTs were commercially made available for several decades, but their usage at present in signal processing functions has practically stopped. This was mainly due to ready availability of Op Amps and the requirement of smaller footprints for the electronic systems. Discrete semiconductor devices at present are mainly available as power devices including Power MOSFETs and IGBTs . June2014 N.J. Rao 49

50. History of FET and BJT Integrated circuits today are predominantly use CMOS (Complementary MOSFET) technology and BiCMOS technology to a limited extent. All digital integrated circuits are manufactured using CMOS technology, and some mixed signal circuits are made with BiCMOS technology. Some Op Amps based on bipolar devices are still produced today because of their popularity with the users. June2014 N.J. Rao 50

51. Field Effect Transistors there are six types of FETs n-channel JFET p-channel JFET n-channel depletion mode FET p-channel depletion mode FET n-channel enhancement mode FET p-channel enhancement mode FET Enhancement mode FETs are the preferred devices as they are normally off-devices (no channel between source and drain) when no voltage is applied to gate with respect to the substrate. June2014 N.J. Rao 51

52. Field Effect Transistors Depletion mode FET technology would have been the natural choice for analog ICs because the devices are in the active region with zero DC bias. As the enhancement mode FET technology is used mainly for digital ICs, the same technology is also used for analog ICs in view of higher reliabilities, yields and used for a single technology for mixed signal processing ICs. June2014 N.J. Rao 52

53. Structure of n-channel enhancement MOSFET June2014 N.J. Rao 53

54. FET (n-channel enhancement ) A voltage applied to the gate with respect to the substrate which is positive causes a channel to exist between source and drain is known as threshold voltage, . When a voltage higher than the threshold is applied to the gate, the current through the channel increases in proportion to square of the voltage June2014 N.J. Rao 54

55. Large signal macro model of FET June2014 N.J. Rao 55

56. Small signal macro model of FET June2014 N.J. Rao 56

57. Nullator-norator model of a FET June2014 N.J. Rao 57

58. Bipolar Junction Transistor June2014 N.J. Rao 58 Collector current I C = a x emitter current (I E ) with a being very near to 1 (typically equal to 0.995)

59. BJT June2014 N.J. Rao 59

60. Small signal micro model of a BJT June2014 N.J. Rao 60

61. Ideal nullator-norator model of BJT June2014 N.J. Rao 61