Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Chapter 19

Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Comparators Op-amps can be used to compare the amplitude of one voltage with another. Although general-purpose op-amps can be used as comparators, special op-amps are available to optimize speed and add features. An example of a comparison circuit is shown. The input is compared with a reference set by the voltage- divider. Notice that there is no feedback; the op-amp is operated in open-loop, so the output will be in saturation. V in R1R1 V out +  R2R2 +V

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Sketch the output of the comparator in relationship to the input; assume the maximum output is ±13 V. V in R1R1 V out +  R2R2 V = +15 V 10 k  3.9 k  The threshold is +4.2 V. The output is in positive saturation when V in > +4.2 V V in +10 V  10 V 0 V +4.2 V +13 V  13 V 0 V

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Show the output of the comparator for the last example if the inputs to the op-amp are reversed. V in R1R1 V out +  R2R2 V = +15 V 10 k  3.9 k  The threshold is still +4.2 V but now the output is in negative saturation when V in > +4.2 V. V in +10 V  10 V 0 V +4.2 V +13 V  13 V 0 V

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Summing amplifier There are a number of useful applications for the basic inverting amplifier configuration. One is the summing amplifier that uses two or more inputs and one output. R1R1 V IN1 V IN2 V IN3 V INn R2R2 R3R3 RnRn RfRf +  V OUT The virtual ground isolates the inputs from each other. Input current from each input is passed to R f, which develops an output voltage that is proportional to the algebraic sum of the inputs. Virtual ground

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Averaging amplifier An averaging amplifier is a variation of the summing amplifier in which all input resistors are equal. The feedback resistor is the reciprocal of the number of inputs times the input resistor value. R1R1 V IN1 V IN2 V IN3 R2R2 R3R3 RfRf +  V OUT For example, if there are three input resistors, each with a value of 10 k , then R f = 3.3 k  to form an averaging amplifier. 10 k  3.3 k 

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Scaling adder A scaling adder is another variation of the summing amplifier in which the input resistors are adjusted to weight inputs differently. The input “weight” is proportional to the current from that input. R1R1 V IN1 V IN2 V IN3 R2R2 R3R3 RfRf +  V OUT Larger resistors will allow less current for a given input voltage, so they have less “weight” than smaller resistors. In the case shown, V IN3 is “weighted” 2 times more than V IN2, which is 2 times more than V IN1. 10 k  5.0 k  2.5 k  10 k 

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Scaling adder R1R1 V IN1 V IN2 V IN3 R2R2 R3R3 RfRf +  V OUT 10 k  5.0 k  2.5 k  10 k  What is V OUT for the scaling adder if all inputs are V? By Ohm’s law, the currents into R f are I 1 = 0.1 mA, I 2 = 0.2 mA and I 3 = 0.4 mA. Using the superposition theorem, the current in R f is 0.7 mA. From Ohm’s law, V OUT = 7 V

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Integrators Mathematical integration is basically a summing process. Within certain limitations, an integrator circuit simulates this process. The ideal integrator is essentially a summing amplifier with a capacitor in place of the feedback resistor. R C V in V out In practical circuits, a large value resistor is usually in parallel with the capacitor to prevent the output from drifting into saturation. RfRf + 

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Integrators For the ideal integrator, the rate of change of the output is given by R C V in V out The minus sign in the equation is due to the inverting amplifier. If the input is a square wave centered about 0 V, the output is a negative triangular wave (provided saturation is not reached). V in V out 0 V + 

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary R C V in V out A 5 kHz square wave with 10 V pp is applied to a practical integrator. Show the output waveform voltages. 33 nF 2.7 k  270 k  RfRf During the positive input (½ the period), the change in the output is 5.6 V The feedback resistor (R f ) is large compared to R, so has little effect on the shape of the waveform. In a practical circuit, it will cause the output waveform to center on zero as shown on the following slide. + 

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary R C V in V out 33 nF 2.7 k  270 k  RfRf The results of a computer simulation on Multisim confirm the calculated change (5.6 V) in output voltage (blue line). continued… + 

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Differentiators In mathematics, differentiation is the process of finding the rate of change. An idea differentiator circuit is shown. It produces an inverted output that is proportional to the rate of change of the input. +  C V in V out In practical circuits, a small value resistor is added in series with the input to prevent high frequency ringing. R in V in RfRf

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Differentiators The output voltage for the ideal differentiator is given by +  RfRf C V in V out The minus sign in the equation is due to the inverting amplifier. If the input is a ramp, the output is a negative dc level for the positive slope and a positive dc level for the negative slope. V in V out

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary A 1.0 kHz, 10 V pp triangular wave is applied to a practical differentiator as shown. Show the output in relationship to the input. When the input has a positive slope, the output is RfRf C V in 100 nF 2.7 k  120  +  V out R in V in +5.0 V  5.0 V 0 V  5.4 V By symmetry, when the input has a negative slope, the output will be +5.4 V. See next slide for waveforms… 0 1 ms 2 ms

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary continued… The results of a computer simulation on Multisim confirm the calculated output voltages (±5.4 V). The output voltage is the blue line.

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall JFET bias circuit Negative feedback with JFET gain control Lead-lag circuit Summary Oscillators The feedback oscillators introduced in Chapter 17 and other types of feedback oscillators can be implemented with op-amps. One type of feedback oscillator is called the Wien-bridge oscillator. This circuit is useful for generating low distortion sine waves. +  R1R1 R2R2 R3R3 RfRf Q1Q1 C1C1 C2C2 C3C3 D1D1 R4R4 V out

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall +  R3R3 RfRf Q1Q1 C3C3 D1D1 R4R4 V out Summary Oscillators R1R1 R2R2 C1C1 C2C2 The lead-lag circuit in the Wien-bridge oscillator has a maximum response at the resonant frequency given by This equation is valid when R’s and C’s in the lead-lag circuit are equal. The lead-lag circuit response is… Because the attenuation is ⅓ at f r, the gain of the Wien bridge must set for 3. V out V in frfr f ⅓V in

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Wien-bridge oscillator +  R1R1 R2R2 R3R3 RfRf Q1Q1 C1C1 C2C2 C3C3 D1D1 R4R4 V out 47 nF 6.8 k  The frequency is given by 498 Hz 1.0 k  10 k  1.0  F What is the frequency of the bridge?

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Triangular-wave oscillator A triangular-wave oscillator can be made from a comparator and an integrator. The integrator produces a ramp due to the constant current charging of the capacitor. When the ramp reaches a trip point, the comparator suddenly switches to opposite level and the ramp changes direction. V out (triangle) V out (square) ComparatorIntegrator +  +  R3R3 R2R2 R1R1 C

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Square-wave relaxation oscillator The square-wave relaxation oscillator uses a comparator to switch the output based on the charging and discharging of a capacitor. C V out +  R3R3 R2R2 R1R1

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Active filters By reversing the resistors and capacitors in the low-pass circuit, a high-pass filter is created. This filter has a gain of 1 at frequencies where f > f c. V out +  R2R2 R1R1 C2C2 C1C1 V in Gain (dB) f fcfc 33 0  40 dB/decade +  R2R2 R1R1 C2C2 C1C1 V in Gain (dB) fcfc 33 0  40 dB/decade A filter selects certain frequencies and excludes others. Active filters use op-amps to optimize the frequency response. A 2-pole low-pass filter and its response is shown. The gain for this filter is 1 (0 dB) for f < f c.

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Voltage regulators Voltage regulators are made from integrated circuits. A basic series IC regulator has four blocks: Control element V OUT V IN Sample circuit Error detector Reference voltage

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Voltage regulators A series regulator uses a comparator to compare the output voltage with a reference voltage. The series transistor drops more or less voltage to keep the output constant. V OUT +  R3R3 R2R2 R1R1 V IN Q1Q1 The voltage at the inverting input is forced to be the same as the reference voltage by feedback action. Therefore,

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Voltage regulators V OUT +  R3R3 R2R2 R1R1 V IN =Q1Q1 What is the output voltage of the series regulator? 5.1 V 4.7 k  +24 V 3.3 k  6.8 k  15.6 V

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Voltage regulators A shunt regulator also has four blocks; it controls the current in the parallel control element. A series resistor drops more or less voltage to keep the output constant. Control element V OUT V IN Sample circuit Error detector Reference voltage R1R1

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Voltage regulators V OUT V IN +  R3R3 R2R2 R1R1 Q1Q1 R4R4 Shunt regulators are not as efficient as series regulators, but have the advantage of short circuit protection. Can you identify each element in this circuit? Sample circuit Reference voltage Control element Error detector

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summing amplifier Averaging amplifier Scaling adder Selected Key Terms An amplifier with several inputs that produces an output voltage proportional to the algebraic sum of the inputs. A special type of summing amplifier with weighed inputs. An amplifier with several inputs that produces an output voltage that is the mathematical average of the input voltages.

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Integrator Differentiator Active filter Series regulator Selected Key Terms A frequency selective circuit consisting of active devices such as transistors or op-amps combined with reactive (RC) circuits. A circuit that produces an inverted output that approaches the mathematical integral of the input. A circuit that produces an inverted output that approaches the mathematical derivative of the input, which is the rate of change. A type of voltage regulator with the control element in series between the input and output.

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 1. When an op-amp is configured as a comparator, the gain is equal to a. 0. b. 1. c. a ratio of two resistors. d. the open-loop gain.

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 2. The approximate voltage at the inverting input of the op-amp shown is equal to R1R1 V IN1 V IN2 V IN3 R2R2 R3R3 RfRf +  V OUT 10 k  3.3 k  a.the average of the input voltages. b.the sum of the input voltages c.0 V d.

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 3. For the scaling adder shown, the input with the greatest weight is a. V IN1 b. V IN2 c. V IN3 d.they are all equal R1R1 V IN1 V IN2 V IN3 R2R2 R3R3 RfRf +  V OUT 10 k  5.0 k  2.5 k  10 k 

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 4. In a practical integrator, the purpose of the feedback resistor (R f ) is to a. limit the gain. b. prevent drift. c. prevent oscillations. d. all of the above. R C V in V out +  RfRf

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 5. Assume the top waveform represents the input to a differentiator circuit. Which represents the expected output? V in a. b. c. d.

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 6. The lead-lag network in a Wien bridge with equal value R’s and C’s attenuates the signal by a factor of a. 2 b. 3 c. 5 d. 10

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 7. A Wien-bridge is used to produce a. sine waves. b. square waves. c. triangle waves. d. all of the above.

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 8. For the circuit shown, the two outputs (in red) produce a. sine and square waves. b. triangle and square waves. c. sine and triangle waves. d. sawtooth and triangle waves. ComparatorIntegrator +  +  R3R3 R2R2 R1R1 C V out

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 9. The purpose of the op-amp in the series regulator is a. to sample the output. b. to establish a reference. c. as a control element. d. error detection. V OUT +  R3R3 R2R2 R1R1 V IN Q1Q1

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 10. An advantage of a shunt regulator is a. short circuit protection. b. efficiency. c. no need for a reference voltage. d. all of the above.

Chapter 19 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz Answers: 1. d 2. c 3. c 4. b 5. c 6. b 7. a 8. b 9. d 10. a