Op Amp Supplemental Questions. Consists of inverting, non-inverting, summing and differencing op amp circuits.

Transcription

1 Op Amp Supplemental Questions. Consists of inverting, non-inverting, summing and differencing op amp circuits. This worksheet and all related files are licensed under the Creative Commons Attribution License, version 1.0. To view a copy of this license, visit or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. The terms and conditions of this license allow for free copying, distribution, and/or modification of all licensed works by the general public. 1

2 Question 1 Questions For all practical purposes, how much voltage exists between the inverting and noninverting input terminals of an op-amp in a functioning negative-feedback circuit? file Question 2 Complete the table of voltages for this opamp voltage follower circuit: 15 V -15 V 0 volts 0 volts 5 volts 10 volts 15 volts 20 volts -5 volts -10 volts -15 volts -20 volts file

3 Question 3 Calculate the overall voltage gain of this amplifier circuit (A V ), both as a ratio and as a figure in units of decibels (db). Also, write a general equation for calculating the voltage gain of such an amplifier, given the resistor values of R 1 and R 2 : 27 kω R 1 27 kω R 2 file Question 4 What would have to be altered in this circuit to increase its overall voltage gain? () R 1 R 2 file

4 Question 5 Calculate all voltage drops and currents in this circuit, complete with arrows for current direction and polarity markings for voltage polarity. Then, calculate the overall voltage gain of this amplifier circuit (A V ), both as a ratio and as a figure in units of decibels (db): 22 kω 47 kω =??? R 2 R 1 = 3.2 volts file Question 6 Calculate all voltage drops and currents in this circuit, complete with arrows for current direction and polarity markings for voltage polarity. Then, calculate the overall voltage gain of this amplifier circuit (A V ), both as a ratio and as a figure in units of decibels (db): =??? R a 10 kω = V R b 2.7 kω file

5 Question 7 Determine both the input and output voltage in this circuit: 5 kω 18 kω I = 2 ma file Question 8 Calculate the voltage gain for each stage of this amplifier circuit (both as a ratio and in units of decibels), then calculate the overall voltage gain: Stage 1 Stage 2 1 kω 3.3 kω 470 Ω 2.7 kω file Question 9 There is something wrong with this amplifier circuit. Note the relative amplitudes of the input and output signals as measured by an oscilloscope: 12 V 12 kω 7.9 kω 0 V -12 V 0.4 V RMS This circuit used to function perfectly, but then began to malfunction in this manner: producing a clipped output waveform of excessive amplitude. Determine the approximate amplitude that the output voltage waveform should be for the component values given in this circuit, and then identify possible causes of the problem and also elements of the circuit that you know cannot be at fault. file

6 Question 10 The equation for voltage gain (A V ) in a typical inverting, single-ended opamp circuit is as follows: A V = R 1 R 2 Where, R 1 is the feedback resistor (connecting the output to the inverting input) R 2 is the other resistor (connecting the inverting input to voltage signal input terminal) Suppose we wished to change the voltage gain in the following circuit from 3.5 to 4.9, but only had the freedom to alter the resistance of R 2 : 7k7 R 1 2k2 R 2 Algebraically manipulate the gain equation to solve forr 2, then determine the necessary value of R 2 in this circuit to give it a voltage gain of 4.9. file Question 11 Calculate the necessary resistor value (R 1 ) in this circuit to give it a voltage gain of 15: R 1 22 kω file

7 Question 12 Calculate the output voltage of this op-amp circuit (using negative feedback): 1.5 V 5 kω 27 kω Also, calculate the DC voltage gain of this circuit. file Question 13 Calculate the voltage gain for each stage of this amplifier circuit (both as a ratio and in units of decibels), then calculate the overall voltage gain: Stage 1 Stage 2 10 kω 15 kω 3.3 kω 5.1 kω file

8 Question 14 Operational amplifier circuits employing negative feedback are sometimes referred to as electronic levers, because their voltage gains may be understood through the mechanical analogy of a lever. Explain this analogy in your own words, identifying how the lengths and fulcrum location of a lever relate to the component values of an op-amp circuit: Fulcrum Lever Fulcrum Lever file Question 15 Calculate the voltage gain for each stage of this amplifier circuit (both as a ratio and in units of decibels), then calculate the overall voltage gain: Stage 1 Stage 2 10 kω 15 kω 3.3 kω 5.1 kω file

9 Question 16 Operational amplifier circuits employing negative feedback are sometimes referred to as electronic levers, because their voltage gains may be understood through the mechanical analogy of a lever. Explain this analogy in your own words, identifying how the lengths and fulcrum location of a lever relate to the component values of an op-amp circuit: Fulcrum Lever Fulcrum Lever file Question 17 Compare and contrast inverting versus noninverting amplifier circuits constructed using operational amplifiers: Inverting amplifier circuit Noninverting amplifier circuit How do these two general forms of opamp circuit compare, especially in regard to input impedance and the range of voltage gain adjustment? file

10 Question 18 Determine the amount of current from point A to point B in this circuit: 1 kω 2 V 1 kω A B 3.5 V 1 kω I =??? 1 V file Question 19 Determine all current magnitudes and directions, as well as voltage drops, in this circuit: 8.33 kω 25 kω 5 kω 5 kω 5 kω 5 kω 10 V 3 V 5 V 11 V file

11 Question 20 Determine the amount of current from point A to point B in this circuit, and also the output voltage of the operational amplifier: 1 kω 2 V 1 kω A B 1 kω 3.5 V 1 kω I =??? 1 V file Question 21 Complete the table of values for this opamp circuit, calculating the output voltage for each combination of input voltages shown: V 1 10 kω 10 kω V 2 10 kω 10 kω V 1 V 2 0 V 0 V 1 V 0 V 0 V 1 V 2 V 1.5 V 3.4 V 1.2 V -2 V 4 V 5 V 5 V -3 V -3 V What pattern do you notice in the data? What mathematical relationship is there between the two input voltages and the output voltage? file

12 Question 22 How does the operation of this difference amplifier circuit compare with the resistor values given (2R = twice the resistance of R ), versus its operation with all resistor values equal? R 2R R 2R Describe what approach or technique you used to derive your answer, and also explain how your conclusion for this circuit might be generalized for all difference amplifier circuits. file

13 Answer 1 Zero volts Answers Answer 2 0 volts 0 volts 5 volts 5 volts 10 volts 10 volts 15 volts 15 volts 20 volts 15 volts -5 volts -5 volts -10 volts -10 volts -15 volts -15 volts -20 volts -15 volts Follow-up question: the output voltage values given in this table are ideal. A real opamp would probably not be able to achieve even what is shown here, due to idiosyncrasies of these amplifier circuits. Explain what would probably be different in a real opamp circuit from what is shown here. Answer 3 A V = 2 = 6:02 db A V = R 1 R 2 1 (expressed as a ratio, not db) Follow-up question: explain how you could modify this particular circuit to have a voltage gain (ratio) of 3 instead of 2. Answer 4 The voltage divider would have to altered so as to send a smaller proportion of the output voltage to the inverting input. Answer 5 I R2 = I R1 = µa V R2 = V V R1 = 3.2 V 22 kω 47 kω = V Current arrows drawn in the direction of conventional flow notation. = 3.2 volts A V = = db 13

14 Answer 6 Current arrows drawn in the direction of conventional flow = V V Ra = V 10 kω I Ra = I Rb = µa = V 2.7 kω V Rb = 2.35 V A V = = db Follow-up question: how much input impedance does the volt source see as it drives this amplifier circuit? Answer 7 = 10 V = 46 V Answer 8 Stage 1: A V = 4.3 = db Stage 2: A V = = db Overall: A V = = db Answer 9 (ideal) = 1.01 volts RMS I ll let you determine possible faults in the circuit! From what we see here, we know the power supply is functioning (both V and -V rails) and that there is good signal getting to the noninverting input of the opamp. Answer 10 R 2 = R 1 A V For the circuit shown, R 2 would have to be set equal to kω. 14

15 Answer 11 R 1 = kω Answer 12 = -8.1 volts A V = 5.4 Follow-up question: the midpoint of the voltage divider (connecting to the inverting input of the op-amp) is often called a virtual ground in a circuit like this. Explain why. Answer 13 Stage 1: A V = 1.5 = db Stage 2: A V = = db Overall: A V = = db Answer 14 The analogy of a lever works well to explain how the output voltage of an op-amp circuit relates to the input voltage, in terms of both magnitude and polarity. Resistor values correspond to moment arm lengths, while direction of lever motion (up versus down) corresponds to polarity. The position of the fulcrum represents the location of ground potential in the feedback network. Answer 15 Stage 1: A V = 1.5 = db Stage 2: A V = = db Overall: A V = = db Answer 16 The analogy of a lever works well to explain how the output voltage of an op-amp circuit relates to the input voltage, in terms of both magnitude and polarity. Resistor values correspond to moment arm lengths, while direction of lever motion (up versus down) corresponds to polarity. The position of the fulcrum represents the location of ground potential in the feedback network. Answer 17 The noninverting configuration exhibits a far greater input impedance than the inverting amplifier, but has a more limited range of voltage gain: always greater than or equal to unity. Answer 18 I = 6.5 ma 15

16 Answer V V 8.33 kω 25 kω 870 µa 870 µa 7.25 V (no current) V 5 kω 5 kω 5 kω 5 kω 550 µa 850 µa 450 µa 750 µa 10 V 3 V 5 V 11 V Conventional flow notation used for all current arrows Follow-up question: what would be required to get this circuit to output the exact sum of the four input voltages? Answer 20 I = 6.5 ma = -6.5 V Answer 21 V 1 V 2 0 V 0 V 0 V 1 V 0 V -1 V 0 V 1 V 1 V 2 V 1.5 V -0.5 V 3.4 V 1.2 V -2.2 V -2 V 4 V 6 V 5 V 5 V 0 V -3 V -3 V 0 V 16

17 Answer 22 It is very important that you develop the skill of exploring a circuit configuration to see what it will do, rather than having to be told what it does (either by your instructor or by a book). All you need to have is a solid knowledge of basic electrical principles (Ohm s Law, Kirchhoff s Voltage and Current Laws) and know how opamps behave when configured for negative feedback. As for a generalized conclusion: R mr A V(differential) = m R mr 17