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UNIT III – IC 741 OP-AMP By: Ajay Kumar Gautam Asst. Prof. ECED, DBITW, Dehradun Wednesday, April 25, 2012 SYLLABUS The 741 OPAMP Circuit: Bias circuit, short circuit protection, the input stage, the second stage, the output stage, the Device parameters DC Analysis of 741: Reference bias current, input stage bias, input bias and offset current, input offset voltage, input common range, second stage bias, output stage bias Small Signal Analysis of 741: The input stage, second stage, the output stage Gain, Frequency Response and Slew rate of 741: Small signal gain, frequency response, a simplified model, slew rate, relationship between Ft and SR Analog Integrated Circuits, Unit - III, IC 741 2 Wednesday, April 25, 2012 3 Analog Integrated Circuits, Unit - III, IC 741 BASICS OF IC 741 Wednesday, April 25, 2012 MANUFACTURERS OF IC 741 It is possible to identify the manufacturer by looking at the number printed on the OP-AMP IC. Fairchild first produced it and sold it as “μA741” where “μA” represent the initials for Fairchild and 741 as OP-AMP. Fairchild μA741 National semiconductor LM741 Motorola MC1741 RCA CA3741 Texas instrument SN52741 Signetics N5741 Analog Integrated Circuits, Unit - III, IC 741 4 Wednesday, April 25, 2012 PIN CONFIGURATION Analog Integrated Circuits, Unit - III, IC 741 5 Wednesday, April 25, 2012 OP-AMP IC 741 - DESCRIPTION This IC is an 8 pin IC in the dual in line (DIP) package. This is the one of the oldest and one of the most popular Op-amp IC. It is a high performance monolithic operational amplifier. It has wide range of applications such as integrator, differentiator, summing amplifier etc. Analog Integrated Circuits, Unit - III, IC 741 6 Wednesday, April 25, 2012 DEFINITION OF 741-PIN FUNCTIONS Pin 1 (Offset Null): Offset voltage is nulled by application of a voltage of opposite polarity to the offset. Pin 2 (Inverted Input): All input signals at this pin will be inverted at output pin 6. Pin 3 (Non-Inverted Input): All input signals at this pin will be processed normally without inversion. Pin 4 (-V): The V- pin (also referred to as Vcc) is the negative supply voltage terminal. Pin 5 (Offset Null): Same pin 1. Analog Integrated Circuits, Unit - III, IC 741 7 Wednesday, April 25, 2012 Pin 6 (Output): Output signal's polarity will be the opposite of the input's when this signal is applied to the op-amp's inverting input. Pin 7 (+V): The V+ pin (also referred to as Vcc) is the positive supply voltage terminal of the 741 Op-Amp IC. Pin 8 (N/C): The 'N/C' stands for 'Not Connected'. There is no other explanation. There is nothing connected to this pin, it is just there to make it a standard 8-pin package Analog Integrated Circuits, Unit - III, IC 741 8 Wednesday, April 25, 2012 ABSOLUTE MAXIMUM RATINGS Supply voltage : 18V Internal power dissipation :500 mW :510 mW :570 mW Storage temperature range Metal package DIP Analog Integrated Circuits, Unit - III, IC 741 Metal package DIP Flat pack :-65oC to +150oC :-55oC to +125oC Operating temp range Military Commercial :- 55oC to +125oC :0oC to +70oC 9 Characteristics Value for IC 741 Ideal Value 1. Input Resistance 2 MΩ ∞ 2. Output Resistance 75 Ω 0 3. Voltage Gain 2x105 ∞ 4. B.W. 1 MHz ∞ 5. Slew Rate 0.5 V/μs ∞ Analog Integrated Circuits, Unit - III, IC 741 Sr. No. Wednesday, April 25, 2012 IMPORTANT CHARACTERISTICS OF IC 741 10 Wednesday, April 25, 2012 11 Bias Circuit, Short Circuit Protection, The Input Stage, The Second Stage, The Output Stage, The Device Parameters Analog Integrated Circuits, Unit - III, IC 741 THE 741 OP-AMP CIRCUIT Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Figure 1: The 741 op-amp circuit. Q11, Q12, and R5 generate a reference bias current, IREF. Q10, Q9, and Q8 bias the input stage, which is composed of Q1 to Q7. The second gain stage is composed of Q16 and Q17 with Q13B acting as active load. The class AB output stage is formed by Q14 and Q20 with biasing devices Q13A, Q18, and Q19, and an input buffer Q23. Transistors Q15, Q21, Q24, and Q22 serve to protect the amplifier against output short circuits and are normally cut off. 12 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Initially IC 741 was manufactured by “Fairchild Corporation”. It consists of 24 transistors, 11 resistors and 1 capacitor. IC 741 requires two power supplies, +VCC and – VEE. Normally +VCC = +15 V and –VEE = -15V. The IC 741 is capable of operating at much lower power supply voltages ( upto 5 V) 13 Wednesday, April 25, 2012 14 Analog Integrated Circuits, Unit - III, IC 741 BIAS CIRCUIT Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 The reference bias current of the 741 circuit, IREF, is generated in the branch at the extreme left of Fig. 1, Q11 and Q12 and the resistance R5. 15 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Widlar current source is formed by Q11 and Q10 and the resistance R4 . The bias current for the first stage is generated in the collector of Q10 . 16 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 There is another current mirror formed by Q8 and Q9 which is responsible for the biasing in the first stage. 17 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Q13 is double-collector lateral pnp transistor. The transistors Q12 and Q13 form a two-output current mirror. Collector of Q13A provides the bias current for the output stage of the op amp. The purpose of Q18 and Q19 is to establish the two VBE drops between the bases of the output transistors Q14 and Q20. 18 Wednesday, April 25, 2012 CIRCUITRY 19 Analog Integrated Circuits, Unit - III, IC 741 SHORT-CIRCUIT PROTECTION Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 The 741 circuit includes large number of transistors that are normally off and conduct only when large output current is required. The large current can be achieved at the output terminals if the output terminal is short-circuited to one of the two supplies. This circuit protects the IC if an excess load current is drawn from it. The short-circuit protection network consists of R6, R7, Q15, Q21, Q24, R11 and Q22. 20 Wednesday, April 25, 2012 21 Analog Integrated Circuits, Unit - III, IC 741 THE INPUT STAGE Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Input stage consists of transistors through Q1 to Q 7. The biasing is performed by transistors Q8, Q9 and Q10. Transistors Q5, Q6 and Q7 and resistors R1, R2 and R3 form the load circuit of the input stage. Every OP-AMP circuit uses a level shifter. The function of level shifter is to shift the dc level of the signal so that the signal at the OP-AMP output can swing positive and negative. In 741, level shifting is done in the first stage using the lateral pnp transistors Q3 and Q4. 22 Wednesday, April 25, 2012 23 Analog Integrated Circuits, Unit - III, IC 741 THE SECOND STAGE Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 The second stage or intermediate stage is composed of Q16, Q17, Q13B, and two resistors R8 and R9. Transistor Q16 acts as an emitter follower. So it provides high input resistance to the second stage. This minimizes the loading on the input stage and avoids the loss of gain. Transistor Q17 acts as an common-emitter amplifier with a 100 Ώ resistance in its emitter. 24 Analog Integrated Circuits, Unit - III, IC 741 Capacitor CC is small in value. The chip are for Capacitor CC is about 13 times that of a standard npn transistor. Wednesday, April 25, 2012 Its load is composed of the high output resistance of the pnp current source Q13B in parallel with the input resistance with the output stage. The output of the second stage is taken at the collector of Q17. Capacitor CC is connected in the feedback path of the second stage to provide frequency compensation. 25 Wednesday, April 25, 2012 26 Analog Integrated Circuits, Unit - III, IC 741 THE OUTPUT STAGE Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 741 uses class AB output stage. The purpose of the output stage is to provide the amplifier with low output resistance. Emitter follower circuit is the class A output stage. The drawback of the class A output stage is large power dissipated in the transistor. This power dissipation can be reduced by arranging the transistor to turn on only when an input signal is applied. 27 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Figure 2 (a) The emitter follower is a class A output stage. (b) Class B output stage (c) The output of a class B output stage fed with an input sinusoid. Observe the crossover distortion. (d) Class AB output stage. 28 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 So in order to reduce the power dissipation two transistors are required. An npn to source the output current and a pnp transistor to sink the output current. This kind of arrangement is called class B output stage. Both the transistors will be cutoff when vI = 0. When vI goes positive QN conducts while QP remains off. When vI goes negative transistors reverse roles. Class B output stage is efficient in power dissipation, but the output signal is distorted. 29 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Output signal is distorted when |vI|is less then about 0.5, neither of the transistors will conduct. This is called crossover distortion. Crossover distortion can be reduced by biasing the output stage transistors at low current. In this case, the output stage transistors will remain conducting when vI is small. 30 Wednesday, April 25, 2012 31 Analog Integrated Circuits, Unit - III, IC 741 THE DEVICE PARAMETERS For the standard npn and pnp transistors, the following parameters will be used: In 741 circuit the nonstandard devices are Q13, Q14 and Q20. For transistor Q13, ISA = 0.25x10-14 A ISB = 0.75x10-14 A, Transistors Q14 and Q20 have an area three time that of a standard device. Analog Integrated Circuits, Unit - III, IC 741 npn: Is = 10-14 A, β = 200, VA = 125 V pnp: Is = 10-14 A, β = 50, VA = 50 V Wednesday, April 25, 2012 32 I 3 I1 IS 3IS 4 I S1I S 2 Analog Integrated Circuits, Unit - III, IC 741 For the circuit shown Fig, neglect base currents and use the exponential ic - vBE relationship to show that Wednesday, April 25, 2012 EXERCISE 33 Wednesday, April 25, 2012 34 Reference bias current, input stage bias, input bias and offset current, input offset voltage, input common range, second stage bias, output stage bias Analog Integrated Circuits, Unit - III, IC 741 DC ANALYSIS OF 741 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 For the dc analysis of an op-amp circuit, the Input terminals are grounded. This should result in zero dc voltage at the output. However, because the op amp has very large gain, the output voltage is close to either +VCC or –VEE. To overcome this problem In the dc analysis, it will be assumed that the op amp is connected in a negative feedback loop that stabilizes the output dc voltage to zero volts. 35 Wednesday, April 25, 2012 36 Analog Integrated Circuits, Unit - III, IC 741 REFERENCE BIAS CURRENT The reference Bias current IREF can be obtained as: For VCC = VEE = 15 V and VBE12 = VEB12 = 0.7 V, we have IREF = 0.73 mA. Analog Integrated Circuits, Unit - III, IC 741 I REF VCC VEB12 VBE11 (VEE ) R5 Wednesday, April 25, 2012 37 Wednesday, April 25, 2012 38 Analog Integrated Circuits, Unit - III, IC 741 INPUT-STAGE BIAS Figure The Widlar current source that biases the input stage Analog Integrated Circuits, Unit - III, IC 741 Transistor Q11 is biased by IREF, and the voltage developed across it is used to bias Q10 which has a series emitter resistance R4. Wednesday, April 25, 2012 39 From the circuit, and assuming β10 to be large, we have, Now assume that, IS11=IS10 we get, I REF VT ln I C10 R4 I C10 At room temperature VT = 25 mV. For our case IC10 = 19 µA. Analog Integrated Circuits, Unit - III, IC 741 VBE11 VBE10 IC10 R4 Wednesday, April 25, 2012 40 Analog Integrated Circuits, Unit - III, IC 741 Design the Widlar current source to generate a current IC10 = 10μA given that IREF = 1mA. If at a collector current of 1mA, VBE = 0.7 V, find VBE11 &VBE10. Wednesday, April 25, 2012 EXERCISE 41 Now we can determine dc current in each of the input-stage transistors. Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Figure: The dc analysis of the 741 input stage. 42 From the Fig shown on previous slide, from symmetry we see that, IC1 = IC2 = I If βp is high, then for transistors Q3 & Q4, IE3 = IE4 = I And base currents of Q3 & Q4 are equal, i.e., IB3 = IB4 Now consider current mirror formed by Q8 & Q9, at node X, if βp is very larger than 1, IC10 = I/βp. = 2I For the 741 circuit, IC10 = 19 µA. So, we can determine that, IC1 = IC3 = IC3 = IC4 = 9.5 µA Analog Integrated Circuits, Unit - III, IC 741 Wednesday, April 25, 2012 43 2I N VBE 6 IR2 R3 The vale of VBE6 can be determined as, VBE 6 VT ln I Is Now substituting IS = 10-14A, I = 9.5 µA, gives, VBE6 = 517 mV & IC7 = 10.5 µA. Analog Integrated Circuits, Unit - III, IC 741 IC 7 I E 7 Wednesday, April 25, 2012 Now consider the remainder circuit of the 741 input stage, This part of the circuit is fed by IC3 = IC4 = I Transistors Q5 & Q6 are identical so, IC3 = IC4 = I The bias current of the Q7 is given by, 44 Wednesday, April 25, 2012 45 Analog Integrated Circuits, Unit - III, IC 741 INPUT BIAS AND OFFSET CURRENTS IB B1 B2 2 For the 741 we obtain I B I N 47.5 nA Because of possible mismatches in the β values of Q1 and Q2 the input base currents will not be equal. In this case, input offset current is defined as, I B I B1 I B 2 Analog Integrated Circuits, Unit - III, IC 741 The input bias current of an op amp is defined as I I Wednesday, April 25, 2012 46 Wednesday, April 25, 2012 47 Analog Integrated Circuits, Unit - III, IC 741 INPUT OFFSET VOLTAGE In other words, it is the amount of the input voltage that should be applied between two input terminals in order to force the output voltage to zero. In the 741 circuit, Input offset voltage is due to mismatch between Q1 and Q2, between Q3 and Q4, between Q5 and Q6 and between R1 and R2. Analog Integrated Circuits, Unit - III, IC 741 Input offset voltage is the differential voltage that exists between two input terminals of an opamp, without any external inputs applied. Wednesday, April 25, 2012 48 Wednesday, April 25, 2012 49 Analog Integrated Circuits, Unit - III, IC 741 INPUT COMMON MODE RANGE For 741 circuit the input common-mode range is determined at the upper end by saturation of Q1 and Q2 and at the lower end by saturation of Q3 and Q4. Analog Integrated Circuits, Unit - III, IC 741 The input common-mode range is the range of Input common-mode voltages over which the input stage remains in the linear active mode. Wednesday, April 25, 2012 50 Wednesday, April 25, 2012 51 Analog Integrated Circuits, Unit - III, IC 741 SECOND STAGE BIAS Analog Integrated Circuits, Unit - III, IC 741 VBE17 I C17 VT ln 618mV IS Wednesday, April 25, 2012 If we neglect the base current of Q23, then we see that the collector current of Q17 is approximately equal to the current supplied by current source Q13B. Because Q13B, has a scale current 0.75 times that of Q12. Its collector current will be IC13B ≈ 0.75IREF, where we have assumed that βP >>1. Thus IC13B = 550µA & IC17 ≈ 550µA. Base-emitter voltage of Q17 is 52 The collector current of Q16 is given as, I C16 I E16 I E17 R8 VBE17 I B17 R9 Which gives, IC16 = 16.2µA Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 53 Wednesday, April 25, 2012 54 Analog Integrated Circuits, Unit - III, IC 741 OUTPUT-STAGE BIAS Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Figure The 741 output stage without the short-circuit protection devices. 55 IC23 ≈ IE23 ≈ 0.25IREF = 180 µA. The base current IB18 = 165/200 = 0.8 µA. So IE18 ≈ IC18 = 180 - 15 = 165 µA & VBE18 = 588 mV. IC19 ≈ IE19 = 15.8 µA VBE19 is determined as VBE19 I C19 VT ln 530mV IS Analog Integrated Circuits, Unit - III, IC 741 If we assume that VBE18 is approximately 0.6 V. we can determine the current in R10 as 15 µA. The emitter current of Q18 is therefore Wednesday, April 25, 2012 Current source Q13A delivers a current of 0.25IREF (because IS of Q13A is 0.25 times the IS of Q12). Neglecting base currents of Q14 & Q20, 56 VBB = VBE18 + VBE19 = 588+530 = 1.118V Where VBB I C14 I C 20 VT ln VT ln I S14 I S 20 Now putting values of VBB, and IS14 = IS20 = 3x1014 A, we get, I C14 = IC20 = 154 µA. This is the small current at which class AB output stage is biased. Analog Integrated Circuits, Unit - III, IC 741 Wednesday, April 25, 2012 As, we know that the purpose of the transistors Q18 - Q19 network is to provide two VBE drops between the bases of the output transistors Q14 & Q20. This voltage drop VBB is given by, 57 Wednesday, April 25, 2012 58 Analog Integrated Circuits, Unit - III, IC 741 SUMMARY Wednesday, April 25, 2012 SUMMARY Analog Integrated Circuits, Unit - III, IC 741 59 Wednesday, April 25, 2012 60 OF The input stage, second stage, the output stage Analog Integrated Circuits, Unit - III, IC 741 SMALL SIGNAL ANALYSIS 741 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 61 INPUT STAGE THE Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 62 Figure Small-signal analysis of the 741 input stage. Where re is emitter resistance of the four transistors shown in figure. Where, re vI 4re VT 25 mV 2.63 k I 9.5 A Input differential resistance is given by, Rid 4( N 1)re 2.11 M Analog Integrated Circuits, Unit - III, IC 741 ie Wednesday, April 25, 2012 The differential signal vI applied between the input terminals. The four transistors shown in figure are connected in series. Emitter signal currents flow as indicated in Fig. 63 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Figure The load circuit of the input stage fed by the two complementary current signals generated by Q1 through Q4 in Fig. shown on previous slide Circled numbers indicate the order of the analysis steps. 64 Assuming the base current of Q7 to be equal to zero, so the collector current of Q5 will be, IC 5 ie IC 6 IC 5 ie So the load circuit behaves like a current mirror. Consider output node of the input stage, the output current is given by, i0 ie IC 6 ie ie 2 ie Transconductance of the input stage is given by, Gm1 i0 2 ie 1 1.9 x104 A / V vI 4ie re 2re 2(2.63 k ) Analog Integrated Circuits, Unit - III, IC 741 Transistors Q5 and Q6 are identical and have identical emitter resistances, therefore, Wednesday, April 25, 2012 65 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Figure Simplified circuits for finding the two components of the output resistance Ro1 of the first stage. 66 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Output resistance (R01) of the input stage is the resistance seen looking back onto the collector of transistor Q6. From the figure shown on slide 64, we can say that, R01 is equal to the parallel combination of the output resistance of the current source ie and the output resistance of Q6. Assume that the base of Q4 is virtual ground. The R04 is given by, R04 ro [1 gm (re r )] So, for Q4, re=2.63 kΩ, ro=VA/I, VA= 50V, I = 9.5 µA , r = (β+1)re>>re, so neglecting it. So, R04 = 10.5 MΩ. 67 The R04 is given by, R06 ro [1 gm ( R2 r )] So, for Q4, re= 2.63 kΩ, ro=VA/I, VA= 50V, I = 9.5 µA, r = (β+1)re>>re, so neglect it. So, R06 = 18.2 MΩ. Hence, the output resistance of the input stage is given by, R01 = 6.7 MΩ. Figure Small-signal equivalent circuit for the input stage of the 741 op amp. Analog Integrated Circuits, Unit - III, IC 741 Wednesday, April 25, 2012 68 Wednesday, April 25, 2012 69 Analog Integrated Circuits, Unit - III, IC 741 SECOND STAGE Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 70 Figure The 741 second stage prepared for small-signal analysis Transconductance: from the small-signal equivalent circuit model of the second stage, we can observe that, Gm 2 ic17 vi 2 Analog Integrated Circuits, Unit - III, IC 741 Figure Small-signal equivalent circuit model of the second stage. Wednesday, April 25, 2012 This is used to determine the values of the parameters of the equivalent circuit. Input Resistance: The resistance Ri2 is given by Ri 2 (16 1)[re16 R9 (17 1)(re17 R8 )] So, Ri2 = 4MΩ. 71 vb17 re17 R8 ( R9 Ri17 ) vi 2 vi 2 ( R9 Ri17 ) re17 Analog Integrated Circuits, Unit - III, IC 741 So, Gm2 = 6.5 mA/V Output Resistance: output resistance of the second stage is given by, R02 ( R013B R017 ) Where, R013B = re13, for 741 re13 = 90.9 kΩ. Where, R017 r0 [1 g m R8 ] = 787 kΩ. Therefore, R02 = 81kΩ. Wednesday, April 25, 2012 ic17 vb17 72 Wednesday, April 25, 2012 73 Analog Integrated Circuits, Unit - III, IC 741 OUTPUT STAGE Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 74 Figure The 741 output stage. Wednesday, April 25, 2012 75 Small signal gain, frequency response, a simplified model, slew rate, relationship between Ft and SR Analog Integrated Circuits, Unit - III, IC 741 GAIN, FREQUENCY RESPONSE AND SLEW RATE OF 741 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 76 GAIN SMALL SIGNAL The overall small-signal gain can be found from the cascade of the equivalent circuits. Load resistance RL = 2 kΩ. The overall gain can be expressed as, vo v v v i 2 o 2 o Gm1 ( Ro1 vi vi vi 2 vo 2 Ri 2 )(Gm 2 Ro 2 )Gvo 3 So, Ao= -476.1X(-526.5)X0.97=243,147V/V RL RL Rout Analog Integrated Circuits, Unit - III, IC 741 Figure Cascading the small-signal equivalent circuits of the individual stages for the evaluation of the overall voltage gain. Wednesday, April 25, 2012 77 Wednesday, April 25, 2012 78 Analog Integrated Circuits, Unit - III, IC 741 FREQUENCY RESPONSE Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Figure High frequency model of OP-AMP 79 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Frequency response of the op-amp is the plot of its open loop gain versus frequency. The open loop gain changes with frequency. To plot the frequency response we need to refer high frequency model of OP-AMP. After a certain frequency the rolloff decreases after certain frequency. The capacitor is due to BJT used in the 741. The BJTs has parasitic capacitances so the capacitances is too small. So in order to reduces the effect of this parasitic capacitances the compensated capacitor is used in 741. 80 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 81 Wednesday, April 25, 2012 82 Analog Integrated Circuits, Unit - III, IC 741 A SIMPLIFIED MODEL Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 83 Vo ( s) Gm1 Vi ( s) sCC A( j ) Gm1 jCC Analog Integrated Circuits, Unit - III, IC 741 A( s) Wednesday, April 25, 2012 In simplified model of 741, the high-gain second stage, with its feedback capacitance CC is modeled by an ideal integrator. In this model, the gain of the second stage is assumed to be very large. That’s why the output resistance of the input stage and the input resistance of the second stage have been omitted. The output stage is assumed to be an ideal unity gain follower. 84 CC So, ft t 2 1 MHz Analog Integrated Circuits, Unit - III, IC 741 Where, ft is called unity gain frequency. At f >> f3dB, the gain falls off with a slope of 20dB/decade, just like an integrator. Wednesday, April 25, 2012 The magnitude of the gain becomes unity at ω=ωt G Where, t m1 85 Wednesday, April 25, 2012 86 Analog Integrated Circuits, Unit - III, IC 741 SLEW RATE Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 87 Wednesday, April 25, 2012 Analog Integrated Circuits, Unit - III, IC 741 Consider the unity-gain follower shown on previous slide. 10 volt step is applied at the input. The entire value of the step signal will appear as a differential signal between the two input terminals. 88 From the circuit shown on previous slide, wee see that, output voltage ramp with a slope of 2I/CC. v0 (t ) 2I t CC Analog Integrated Circuits, Unit - III, IC 741 So the slew rate is given by: SR 2 I CC For 741 SR = 0.63 V/µs. Wednesday, April 25, 2012 89 Wednesday, April 25, 2012 90 Analog Integrated Circuits, Unit - III, IC 741 RELATIONSHIP BETWEEN FT AND SR Relationship between ft and SR can be found by: t Gm1 CC 2I CC 1 Gm1 2re 2re SR 2VT I So, t 2CCVT And we get, t Finally we get, SR 4VT ft SR 8 VT Analog Integrated Circuits, Unit - III, IC 741 As we know that So, Gm1 I VT & re I Wednesday, April 25, 2012 91 Wednesday, April 25, 2012 92 Analog Integrated Circuits, Unit - III, IC 741 ALL THE BEST FOR INTERNAL EXAMS