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Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. Learning Objectives: At the end of this topic you will be able to; draw the circuit diagram for a non-inverting amplifier; select and use the formula G 1 RF to calculate the gain of a non R1 inverting amplifier; draw the circuit diagram for an inverting amplifier; select and use the formula G RF to calculate the gain of an R IN inverting amplifier; in each case, use the gain formulae to select resistors to produce the given gain; interpret and produce graphs showing input and output signals. 1 GCSE Electronics. Unit E2 : Applications of Electronics 2.4.2 Op-Amp Voltage Amplifiers All of the amplifier circuits we are going to consider in this topic are built around the operational amplifier or Op-Amp for short. The circuit symbol for an operational amplifier (or Op-amp) is shown below: +V Inverting input - Non-inverting input + VOUT -V There are several things to be aware of when dealing with Op-amps; i. ii. iii. iv. v. The ‘+’ & ‘-‘ signs do not refer to power supply connections. There are two inputs; the non-inverting input ‘+’ and the inverting input ‘-‘. There is one output, labelled VOUT. There are two power supply connections labelled +V and –V, since an op-amp requires a dual rail power supply. This provides both a positive and a negative voltage (e.g. ±12V) to allow the AC signal to swing above and below 0V. The saturation voltage of an op-amp is usually 1 or 2 volts less than the power supply voltage. Typically for a supply of ±9V, the saturation voltage would be ±8V Two commonly used op-amps are the LM741 and the much better TLO81. Both have the same pinout as the industry standard op-amp, the 741. The op-amp is usually packaged in an 8-pin DIL package as shown below 2 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. We need to consider two types of voltage amplifier in this topic, the noninverting amplifier and the inverting amplifier. Non-inverting Amplifier The input impedance of a non-inverting op-amp is very high which makes it ideal to be used as a pre-amplifier. The circuit diagram for non-inverting amplifier is shown below: RF +V Note : The power supply connections are often left off the diagram of the amplifier to make the circuit easier to follow. + -V VIN VOUT R1 0V The voltage gain of this amplifier is given by the following formulae. Voltage Gain VOUT R 1 F VIN R1 Important things to remember for this amplifier are: i. ii. iii. Voltage gain can be determined if either VOUT and VIN are known or if RF and R1 are known. The gain is positive so at any instant, if the input voltage is positive, the output will be also be positive, and vice versa. If you are designing an amplifier of this type then all resistors chosen must be greater than 1kΩ, to reduce power dissipation. We will now look at some examples: 3 GCSE Electronics. Unit E2 : Applications of Electronics Example 1: A non-inverting amplifier is required to act as a preamplifier for a microphone. The amplifier requires a voltage gain of +100. (a) Draw the circuit diagram for a non-inverting amplifier. RF +V + -V VIN VOUT R1 0V (b) Determine a suitable resistor for RF if R1 = 1kΩ In the question we are told that the gain needs to be +100, so we now apply the gain formula as shown below; Gain 1 RF R1 100 1 RF R1 100 1 RF R1 R F 99 R1 R F 99 1k 99k OR if you find it difficult to rearrange the formula try this method: For a non-inverting op-amp with a gain of 100, RF is 99 times bigger than RIN so: R F 99 1k 99k 4 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. Example 2: The following circuit shows a non-inverting amplifier connected to a ±10V power supply. The saturation voltage is ±8V. RF = 47kΩ +10V + -10V VIN R1= 1kΩ VOUT 0V (a) (b) What is the voltage gain of this amplifier? Gain 1 RF R1 Gain 1 47 48 1 If the peak value of VIN = 100mV, determine the peak value of VOUT. Gain VOUT VIN VOUT 100 48 100 4800mV 4.8V 48 VOUT 5 GCSE Electronics. Unit E2 : Applications of Electronics (c) The graph below shows the 100mV AC signal applied to VIN. On the axes below sketch the graph of VOUT. VIN/mV 100 75 50 25 0 -25 -50 -75 -100 VOUT/V Output graph not inverted - i.e. positive i/p gives positive o/p etc. Peak output voltage = peak input voltage x gain 4.8 3.6 2.4 1.2 0 -1.2 -2.4 -3.6 -4.8 Frequency remains the same 6 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. (d) The AC signal applied to VIN is now increased to 200mV. On the axes below sketch the new graph of VOUT. VIN/mV 200 155 100 50 0 -50 -100 -150 -200 Output graph not inverted - i.e. positive i/p gives positive o/p etc. VOUT/V Peak output voltage = 200mV x 48 = 9.6V Saturation occurs at 8V, therefore output voltage is clipped at saturation values. 9.6 7.2 4.8 2.4 0 -2.4 -4.8 -7.2 -9.6 Frequency remains the same HORRIBLE SCALES – I ENCOURAGE MINE TO DRAW IN THE FULL SINE WAVE, USING A DOTTED LINE, WITH THE CALCULATED AMPLITUDE, AND THEN CLIP IT AT THE SATURATION VOLTAGES. Here are a couple of examples for you to do! 7 GCSE Electronics. Unit E2 : Applications of Electronics Exercise 1: 1. The following circuit shows a non-inverting amplifier connected to a ±10V power supply. RF = 78kΩ +V + -V VIN R1= 2kΩ VOUT 0V (a) What is the voltage gain of this amplifier? ....................................................................................................................................... ....................................................................................................................................... (b) If the peak value (amplitude) of VIN = 75mV, determine the peak value of VOUT. ....................................................................................................................................... ....................................................................................................................................... ....................................................................................................................................... 8 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. (c) The graph below shows the AC signal applied to VIN. On the axes below sketch the graph of VOUT. Add a suitable scale to the VOUT axis. VIN/mV 100 75 50 25 0 -25 -50 -75 -100 Vout/mV 0 9 GCSE Electronics. Unit E2 : Applications of Electronics 2. A non-inverting amplifier is required to act as a preamplifier for a microphone in a public address system. The amplifier requires a gain of 35. (a) Draw the circuit diagram for a non-inverting amplifier. (b) Determine a suitable resistor value for R1 if RF = 68kΩ. ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ 10 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. Inverting Amplifier One use of the inverting amplifier is as part of a mixer circuit which we will look at later. First we consider the inverting amplifier on its own. The circuit diagram is shown below: RF +V RIN Note : The power supply connections are often left off the diagram of the amplifier to make the circuit easier to follow. VIN + -V VOUT 0V The voltage gain of this amplifier is given by the following formulae. Voltage Gain VOUT R F VIN R IN Important things to remember for this amplifier are: i. ii. iii. Voltage gain can be determined if either VOUT and VIN are known or if RF and RIN are known. The ‘-‘ sign in the formula indicates the inverting action of this amplifier, so at any moment if the input voltage is positive, the output will be negative, and vice versa. If you are designing an amplifier of this type then all resistors chosen must be greater than 1kΩ, to reduce power dissipation. We will now look at some examples: 11 GCSE Electronics. Unit E2 : Applications of Electronics Example 3: An inverting amplifier is required with a voltage gain of -20. (a) Draw the circuit diagram for an inverting amplifier. RF RIN + VIN VOUT 0V (b) Determine a suitable resistor for RF if RIN has a value of 10kΩ. We apply the gain formula as shown below; Gain RF R IN RF 10 R F 20 10 20 R F 200k OR if you find it difficult to rearrange the formula try this method: For an inverting op-amp with a voltage gain of -20, RF is 20 times bigger than RIN so: R F 20 10k 200k 12 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. Example 4: The graph below shows the input and output waveforms of an inverting amplifier VIN/mV 10 7.5 5 2.5 0 -2.5 -5 -7.5 -10 VOUT/mV 800 600 400 200 0 -200 -400 -600 -800 13 GCSE Electronics. Unit E2 : Applications of Electronics (a) What is the voltage gain of this amplifier From the graph: The amplitude of the input signal is 10mV The amplitude of the output signal is 600mV Gain VOUT 600mV 60 VIN 10mV but, the output signal is inverted, so the voltage gain is -60 (b) Either: Here is the circuit diagram for the inverting amplifier. Calculate the value of RF Gain RF R IN RF 2 k R 60 F 2 k R F 60 2k 120k Or: For an inverting op-amp with a gain of -60, RF is 60 times bigger than RIN so: 60 14 R F R IN 60 2k 60 120k Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. Exercise 2: 1. The following circuit shows an inverting amplifier connected to a ±12V power supply. The output saturates at ±10V. 390kΩ 13kΩ VIN + VOUT 0V (a) What is the voltage gain of this amplifier? ....................................................................................................................................... ....................................................................................................................................... 15 GCSE Electronics. Unit E2 : Applications of Electronics 16 (b) Graph 1 below shows the signal applied to VIN. On graph 2 axes sketch the graph of VOUT. Label the graph with suitable values. (c) VIN is increased to 0.5V. On graph 3 axes sketch the graph of the new signal at VOUT. Label the graph with suitable values. Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. 2. An inverting amplifier is required with a voltage gain of -50. (a) Draw the circuit diagram for an inverting amplifier. (b) Determine a suitable resistor for RIN if RF =75k. ....................................................................................................................................... ....................................................................................................................................... ....................................................................................................................................... ....................................................................................................................................... ....................................................................................................................................... ....................................................................................................................................... 17 GCSE Electronics. Unit E2 : Applications of Electronics Solutions to Student Exercises Exercise 1: 1. (a) Gain 1 RF R1 Gain 1 78 40 2 (b) Gain VOUT 75 40 75 3000mV 3V 40 Vout (c) VOUT/V 4 3 2 1 0 -1 -2 -3 -4 18 VOUT VIN Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. 2. (a) RF +V + -V VIN VOUT R1 0V (b) In the question we are told that the voltage gain needs to be 35, so applying the gain formula; Gain 1 RF R1 35 1 68 R1 35 1 68 R1 68 34 R 1 R1 68 2 k 34 OR if you find it difficult to rearrange the formula using the other method: For a non-inverting op-amp with a gain of 35, RF is 34 times bigger than R1 so: R1 68 2k 34 19 GCSE Electronics. Unit E2 : Applications of Electronics Exercise 2: 1. (a) (b) VOUT/V 600 400 200 0 -200 -400 -600 -800 20 RF R IN Gain 390 30 13 For a voltage gain of-30 the peak value of VOUT will be 30 times larger than VIN and will also be inverted: Graph 2 800 Gain Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. (c) When VIN is increased to 0.5V, the output will try to reach an amplitude of 30 x 0.5V = 15V The op-amp saturates at ±10V, so the output will be clipped at 10V, so Graph 3 should look like that shown below. Vout/V Graph 3 12 9 6 3 0 -3 -6 -9 -12 2. RF (a) RIN VIN + VOUT 0V (b) Either: (b) Or: Gain RF R IN 50 75k R IN R IN 75k 1.5k 50 For an inverting op-amp with a gain of 50, RF is 50 times bigger than RIN so: R F R IN 50 75k R IN 50 R IN 75k 1.5k 50 Now for some Examination Style Questions 21 GCSE Electronics. Unit E2 : Applications of Electronics Examination Style Questions 1. A voltage amplifier is needed which will produce the output waveform shown below, when the input signal shown is applied to it. (a) Which of the following describes the performance of this amplifier? A. B. C. D. E. F. (b) It is an inverting amplifier with a voltage gain of 39. It is a non-inverting amplifier with a voltage gain of 39 It is an inverting amplifier with a voltage gain of 40. It is a non-inverting amplifier with a voltage gain of 40. It is an inverting amplifier with a voltage gain of 390. It is a non-inverting amplifier with a voltage gain of 390. Answer: …… [1] Complete the following circuit diagram for the amplifier you chose in part (a) based on an op-amp. [2] (c) The input voltage is increased to 1V.When testing the amplifier, it is found that the output voltage never goes above +12V, or below –12V. Give an explanation for this finding. ……………………………………………………………………………………………… ……………………....……………………………………………………………………… [2] 22 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. 2. The following diagram shows an inverting amplifier. The power supply is ±15V and the output saturates at ±12V (a) A voltage gain of 100 is required. Choose a suitable values for RF if R1 = 2.2k. .................................................................................................................................................. .................................................................................................................................................. (b) ................................................................................................................................................ [2] An AC voltage of peak value 2V is applied to the input. Complete the graph showing the corresponding output voltage. [3] 23 GCSE Electronics. Unit E2 : Applications of Electronics 3. The graph shows the input signal applied to four different voltage amplifiers, A, B, C and D. These are the graphs of outputs of the four amplifiers. 24 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. Use the graphs to answer the following questions. (a) What is the amplitude of the output of amplifier A? The answer is .................................... [1] (b) What is the voltage gain of amplifier A? The answer is .................................... [1] (c) Which amplifier, A, B, C, or D, is clipping the output? The answer is .................................... [1] (d) What is the saturation voltage of amplifier D? The answer is .................................... [1] 25 GCSE Electronics. Unit E2 : Applications of Electronics 4. The following diagram shows a non-inverting amplifier. (a) On the circuit diagram, clearly label: The output of the amplifier with the letter O; The feedback resistor with the letter F; The inverting input of the op-amp, with the letter I. [2] (b) Use the values for RF and R1 to calculate the gain of this amplifier. ................................................................................................................................................ (c) ................................................................................................................................................ [2] The gain of the amplifier is changed to 100. Use the following frequency response graph to determine the bandwidth of the amplifier. Show clearly on the graph how you obtain your result. Bandwidth = ............................. 26 [2] Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. 5. (a) Draw a diagram showing how an op-amp and 2 resistors can be used to produce a noninverting amplifier [3] (b) One undesirable effect which can occur in this amplifier is clipping of the output waveform. Describe one cause of clipping. ……………………………………………………………………………………………….. (c) ……………………………………………………………………………………………….. [1] The amplifier has a voltage gain of 100 The same voltage gain can be obtained by using two amplifiers each with a gain of 10. Explain how this might be better than a single amplifier with a gain of 100. ............................................................................................................................................... ............................................................................................................................................... [2] 27 GCSE Electronics. Unit E2 : Applications of Electronics 6. The diagram shows an incomplete non-inverting amplifier circuit. (a) (i) Which input, C or D, on the diagram is the inverting input? .................... [1] (ii) Complete the diagram for the non-inverting amplifier. [2] (iii) Which resistor, A or B, is the feedback resistor? .................... [1] 28 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. (b) The resistors A and B are chosen to give a gain of 40. The following signal is the input voltage V1. (i) Calculate the peak value (amplitude) of the output voltage V2 in mV. ................................................................................................................................................ ................................................................................................................................................ ................................................................................................................................................ [2] (ii) Draw a graph of this output voltage V2. [3] (c) The Gain-Bandwidth Product of the amplifier is 1.6MHz. Calculate the bandwidth of the amplifier. …………………………………………………………………………………… [1] 29 GCSE Electronics. Unit E2 : Applications of Electronics 7. The following diagram shows a non-inverting amplifier. (a) Use the values for RF and R1 to calculate the gain of this amplifier. ................................................................................................................................................ ................................................................................................................................................ [2] (b) The resistors are now changed to give a gain of 10. The following signal is then applied to VIN. (i) Calculate the maximum value of the output voltage VOUT in mV. ................................................................................................................................................ ................................................................................................................................................ [2] 30 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. (ii) Draw a graph of this output voltage VOUT on the following grid. [3] 31 GCSE Electronics. Unit E2 : Applications of Electronics 8. A non-inverting amplifier is used as a pre-amplifier. (a) (i) Which input, C or D, on the diagram is the non-inverting input? ................................................................................................................................................ [1] (ii) Complete the diagram for the non-inverting amplifier. [2] (iii) Choose suitable values for resistors A and B to give a gain of 50. ................................................................................................................................................ ................................................................................................................................................ ................................................................................................................................................ [2] 32 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. (b) The following signal is the input voltage V1. (i) Calculate the peak value (amplitude) of the output voltage V2 in mV. ................................................................................................................................................ ................................................................................................................................................ [2] (ii) Draw a graph of this output voltage V2. [3] 33 GCSE Electronics. Unit E2 : Applications of Electronics 9. (a) Complete the following diagram to show an inverting amplifier by adding two resistors R IN and RF and any necessary connections. [3] (b) RIN = 11k and RF = 330kWhat is the gain of the amplifier? ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. [3] (c) 34 RF is changed to give a gain of -80 The following signal is then applied to the input. The amplifier output saturates at ±9V. Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. (i) Use the graph to find the maximum value of the input voltage VIN in mV. ................................................................................................................................................. [1] (ii) Calculate the maximum value of the corresponding output voltage VOUT in mV. ................................................................................................................................................. [1] (iii) Draw a graph of this output voltage VOUT on the following grid. [3] 35 GCSE Electronics. Unit E2 : Applications of Electronics (d) The power supply changes, and the amplifier is found to saturate at ±7V. Draw a graph of the new output voltage on the following grid. [2] 36 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. 10. An audio pre-amplifier is shown below. The amplifer has a voltage gain of 250. The op-amp is powered from a ±15V supply and saturation occurs at ±12V. (a) A test signal of amplitude 8mV is applied to the amplifier input. (i) Calculate the amplitude of the output voltage. [1] ................................................................................................................................................. ................................................................................................................................................. 37 GCSE Electronics. Unit E2 : Applications of Electronics (ii) The test signal is shown on the following graph. Draw the graph of the amplifier output on the axes below. Mark on the graph any significant voltage. [2] 38 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. (b) The amplitude of the test signal is increased to 80mV. Use the axes provided to sketch the resulting output signal. [The time axis has the same scale.] [2] (c) The amplifier (voltage gain 250) has a bandwidth of 25 kHz. Use the axes provided to sketch the frequency response of this amplifier. [2] 39 GCSE Electronics. Unit E2 : Applications of Electronics 11. A non-inverting voltage amplifier has a gain of 25 The amplifier uses a ±15V power supply. The diagrams below show two input signals applied to this amplifier. In each case, draw the corresponding output signal using the axes provided. In the part (a) the voltage scale for the output signal is provided. In the part (b), you must label the output signal with appropriate voltages. [5] (a) 40 Topic 2.4 – Analogue Communications. 2.4.2 Op-Amp Voltage Amplifiers. (b) 41 GCSE Electronics. Unit E2 : Applications of Electronics Self Evaluation Review My personal review of these objectives: Learning Objectives draw the circuit diagram for a noninverting amplifier; select and use the formula G 1 RF R1 to calculate the gain of a non inverting amplifier; draw the circuit diagram for an inverting amplifier; select and use the formula G RF to calculate the gain of an R IN inverting amplifier; in each case, use the gain formulae to select resistors to produce the given gain; interpret and produce graphs showing input and output signals. draw the circuit diagram for a noninverting amplifier; Targets: 1. ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… 2. ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… 42