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Analog Electronics Lecture 7 Op-amp Circuits and Active Filters Electronic Devices, 9th edition Thomas L. Floyd Muhammad Amir Yousaf © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Lecture: How to compare an analog signal with certain voltage level. Comparing a noisy signal with certain (reference) level. Binding an signal to fixed +/- max levels. Analog to digital converters with comparators. Adding two analog signals. Adding weighted signals. Averaging on analog signals. Digital to Analog Converter with weighted additions. Integrating an analog waveform. Differentiating analog waveform. Logarithm on analog signal. Antilog of analog signal. Multiplying and diving analog signals. Converters. Peak Detectors. Filters. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Comparators A comparator is a specialized nonlinear op-amp circuit that compares two input voltages and produces an output state that indicates which one is greater. Because of the high open-loop voltage gain, a very small difference voltage between the two inputs drives the amplifier into saturation. Zero Level Detection: Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Comparators Non-Zero Level Detection: Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Noise on Comparator Noise contaminated signal may cause an unstable output. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Comparator with Hysteresis To avoid this, hysteresis can be used. Hysteresis is incorporated by adding regenerative (positive) feedback, which creates two switching points: The upper trigger point (UTP) and the lower trigger point (LTP). After one trigger point is crossed, it becomes inactive and the other one becomes active. VUTP Vin 0 t VLTP +Vout (max) –Vout(max) Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Output Bounding Some applications require a limit to the output of the comparator (such as a digital circuit). The output can be limited by using one or two Zener diodes in the feedback circuit. The circuit shown here is bounded as a positive value equal to the zener breakdown voltage. Vin Ri 0V +VZ – 0 + Electronic Devices, 9th edition Thomas L. Floyd –0.7 V © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Comparator Applications Flash analog-to-digital converters use 2n-1 comparators to convert an analog input to a digital value of n bits for processing. VREF R Vin (analog) Op-amp comparators + – R + – Flash ADCs are a series of comparators, each with a slightly different reference voltage. R R R The priority encoder produces an output equal to the highest value input. – (7) (6) + (5) (4) – (3) (2) + (1) (0) – R + – R Priority encoder + D2 D1 D0 Binary output Enable input + – R Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summing Amplifier A summing amplifier has two or more inputs; normally all inputs have unity gain. The output is proportional to the negative of the algebraic sum of the inputs. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Example Summing Amplifier What is VOUT if the input voltages are +5.0 V, -3.5 V and +4.2 V and all resistors = 10 kW? Rf R1 VOUT = -(VIN1 + VIN2 + VIN3) = -(+5.0 V - 3.5 V + 4.2 V) = -5.7 V Electronic Devices, 9th edition Thomas L. Floyd VIN1 R2 VIN2 10 kW – R3 VIN3 VOUT + © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Averaging Amplifier An averaging amplifier is basically a summing amplifier with the gain set to Rf /R = 1/n (n is the number of inputs). The output is the negative average of the inputs. What is VOUT if the input voltages are +5.0 V, -3.5 V and +4.2 V? Assume R1 = R2 = R3 = 10 kW and Rf = 3.3 kW? Rf R1 VIN1 R2 VOUT = -⅓(VIN1 + VIN2 + VIN3) = -⅓(+5.0 V - 3.5 V + 4.2 V) = -1.9 V Electronic Devices, 9th edition Thomas L. Floyd VIN2 3.3 kW – R3 VIN3 VOUT + © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Scaling Adder A scaling adder has two or more inputs with each input having a different gain. Rf R1 VIN1 R2 VIN2 – R3 VIN3 Electronic Devices, 9th edition Thomas L. Floyd VOUT + © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Scaling Adder: D/A Converter An application of a scaling adder is the D/A converter circuit shown here. The resistors are inversely proportional to the binary column weights. Because of the precision required of resistors, the method is useful only for small DACs. +V 8R 20 Rf 4R 21 2R – VOUT 2 2 + R 23 Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Integrator An op-amp integrator simulates mathematical integration, a summing process that determines total area under curve. I dQ dt C. Ii - If Vin dVout -C. R dt dVout 1 Vin . dt RC Vout= − C R – Vin Ii dV dt IC Vout + Ideal Integrator 1 0 The ideal integrator is an inverting amplifier that has a capacitor in the feedback path. The output voltage is proportional to the negative integral (running sum) of the input voltage. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Integrator Capacitor in the ideal integrator’s feedback is open to dc. Rf C This implies open loop gain with dc offset. That would lead to saturation. The practical integrator overcomes these issues– the simplest method is to add a relatively large feedback resistor. R Vin – Vout + Practical Integrator Rf should be large enough Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Differentiator R C An op-amp differentiator simulates mathematical differentiation, a process to determine instantaneous rate of change of a function. Vin – Vout + Ideal Differentiator The ideal differentiator is an inverting amplifier that has a capacitor in the input path. The output voltage is proportional to the negative rate of change of the input voltage. I= = . Ii - If . =− = -RC. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Instrumentation Amplifiers An instrumentation amplifier (IA) amplifies the voltage difference between its terminals. It is optimized for amplifying small differential signals that may be riding on a large common mode voltages. o High input impedance o High CMMR o Low output offset Input 1 + R3 R5 A1 Gain set – R1 – o Low output impedance R2 Gain set A3 Output + – R4 A2 Input 2 Electronic Devices, 9th edition Thomas L. Floyd + R6 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Instrumentation Amplifiers IC of instrumentation amplifier is made up of three op amps and several resistors. The gain is set by a single resistor that is supplied by the user. + Vcm Vin1Input 1 + R3 R5 A1 Gain set The output voltage is the closed loop gain set by RG multiplied by the voltage difference in the inputs. Electronic Devices, 9th edition Thomas L. Floyd – R1 – R2 RG Gain set + Output + – R4 A2 + Vcm Vin2Input 2 A3 Vout = Acl (Vin2 - Vin1) R6 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Instrumentation Amplifiers (IA) Applications: oUsed where a quantity is sensed by a remote sensor e.g. temperature, pressure transducer and sensed signal is sent over a long line. oElectrical noise produces common-mode voltages in the line. oIA at the end of line amplifies only the small differential signal and reject the common mode signal Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Example Instrumentation Amplifiers An IA that is based on the three op-amp design is the AD622. The formula for choosing RG is: RG 50.5 kW Av - 1 +V (7) What value of RG will set the gain to 35? RG 50.5 kW 50.5 kW Av - 1 35 - 1 = 1.5 kW Electronic Devices, 9th edition Thomas L. Floyd +IN (3) (1) RG –IN (6) AD622 (8) (5) (2) Output REF (Output signal common) (4) –V © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Logarithmic Amplifier A logarithmic (log) amplifier produces an output that is proportional to the logarithm of the input Log and antilog amplifiers are used in applications that require: o Compression of analog input data. o Linearization of transducers that have exponential outputs. o Analog multiplication and division, etc Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Logarithmic Amplifier Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Logarithmic Amplifier A semiconductor pn-junction in the form of either a diode or the baseemitter junction of a BJT provides a logarithmic characteristic. Voltage across the diode is proportional to the log of the current in the diode. Compare data for an actual diode on linear and logarithmic plots: Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Logarithmic Amplifier When a diode is placed in the feedback path of an inverting op-amp, the output voltage is proportional to the log of the input voltage. The gain decreases with increasing input voltage; therefore the amplifier is I I said to compress signals. F in Vin +V – F R1 0V – Op-amp Vout + Many sensors, particularly photosensors, have a very large dynamic range outputs. Current from photodiodes can range over 5 decades. A log amp will amplify the small current more than the larger current to effectively compress the data for further processing. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Example The Logarithmic Amplifier For the circuit shown, the equation for Vout is Vout - 0.025 V ln Vin (IR is a constant for a given diode.) I R R1 What is Vout? (Assume IR = 50 nA.) Vin Vin +11 V Iin R 1 IF ++ V –– VFF 1.0R1kW 0V Vout - 0.025 V ln 11 V 50 nA 1.0 kW –– Op-amp Op-amp VVout out ++ = -307 mV Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Antilog Amplifier The antilogarithm of a number is the result obtained when the base is raised to a power equal to the logarithm of that number. x= Ii - If I R Electronic Devices, 9th edition Thomas L. Floyd / = − = - . IR / © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Constant-current source A constant-current source delivers a load current that remains constant when the load resistance changes. IL = Ii Ri – + VIN Ii RL 0V 0A + – A basic circuit in which a stable voltage source (Vin) provides a constant current (Ii) through the input resistor (Ri) If RL changes, IL remains constant as long as Vin and Ri are held constant. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Current to Voltage Converter A current-to-voltage converter converts a variable input current to a proportional output voltage. A specific application of this circuit is where a photoconductive cell is used to sense changes in light level. As the amount of light changes, the cur-rent through the photoconductive cell varies because of the cell’s change in resistance. This change in resistance produces a proportional change in the output voltage. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Peak Detector The circuit is used to detect the peak of the input voltage and store that peak voltage on a capacitor. Ri Vin + – R1 Vout C The circuit can be used to detect and store the maximum value of a voltage surge. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Charge Sensitive Amplifier It is used in Radiation detection Charge on a photon is accumulated in the capacitor Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Active Filters Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Basic filter Responses A filter is a circuit that passes certain frequencies and rejects all others. The passband is the range of frequencies allowed through the filter. The critical frequency defines the end (or ends) of the passband and is normally specified at the point where the response drops -3dB (70.7%) from the passband response. Following the passband is a region called the transition region that leads into a region called the stopband. Gain Gain Gain f f Low-pass Electronic Devices, 9th edition Thomas L. Floyd High-pass Gain f Band-pass f Band-stop © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Basic Low-Pass Filter The low-pass filter allows frequencies below the critical frequency to pass and rejects other. The simplest low-pass filter is a passive RC circuit with the output taken across C. BW = fc Gain (normalized to 1) –3 dB 0 dB Actual response of a single-pole RC filter Passband –20 dB Transition region –40 dB –60 dB 0.01 fc Electronic Devices, 9th edition Thomas L. Floyd R –2 0d B/ de Stopband ca de region BW Vs Vout C f 0.1 fc fc 10 fc 100 fc 1000 fc © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Basic Low-Pass Filter o The ideal response is not attainable by any practical filter. o Actual filter responses depend on the number of poles, o Pole, a term used with filters to describe the number of RC circuits contained in the filter. o This basic RC filter has a single pole, and it rolls off at -20db/decade beyond the critical frequency. o20db/decade means that at a frequency of 10fc the output will be 20dB(10%) of the input. o This roll-off allows too much unwanted frequencies through the filter Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Basic Low-Pass Filter o Actual filters do not have a perfectly flat response up to the cutoff frequency. o More steeper response cannot be obtained by simply cascading the basic stages due to loading effect. o With combination of op-amps, the filters can be designed with higher roll-offs o In general, the more poles the filter uses, the steeper its transition region will be. The exact response depends on the type of filter and the number of pole. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Active Filters General Active Filters A single pole active filters The number of filter poles can be increases with cascading Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Basic High-Pass Filter The high-pass filter passes all frequencies above a critical frequency and rejects all others. The simplest high-pass filter is a passive RC circuit with the output taken across R. Gain (normalized to 1) –3 dB 0 dB –20 dB –40 dB –60 dB 0.001 fc Electronic Devices, 9th edition Thomas L. Floyd Actual response of a single-pole RC filter 0d –2 Passband C de ca e d B/ Vs Vout R f 0.01 fc 0.1 fc fc 10 fc 100 fc © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Band-Pass Filter A band-pass filter passes all frequencies between two critical frequencies. The bandwidth is defined as the difference between the two critical frequencies fc1 and fc2. The simplest band-pass filter is an RLC circuit. R Vout Bandwidth B.W= fc2 – fc1 Vs Center frequency fo= √ fc1 fc2 Quality Factor: In band pass filters it is ratio of center frequency to its bandwidth. Q = fo /B.W C L Vout (normalized to 1) 1 0.707 BW f fc1 Electronic Devices, 9th edition Thomas L. Floyd f0 fc2 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. The Band-Stop Filter A band-stop filter rejects frequencies between two critical frequencies; the bandwidth is measured between the critical frequencies. The simplest band-stop filter is an RLC circuit. Gain (dB) 0 –3 L C Vs fc1 f0 fc2 Vout R f BW Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Ideal vs Real Filters In comparison to ideal low pass filters, the real RC or RLC filters lack the following characteristics: Vout (normalized to 1) 1 o Flat passband o Sharp transition region oLinear phase response 0.707 BW f fc1 f0 fc2 Gain (dB) 0 –3 fc1 f0 fc2 f BW Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Active Filters Active filters include one or more op-amps in the design. One of the three characteristic can be achieved with active filters: o Flat band pass with Butterworth Av Chebyshev: rapid roll-off characteristic oSharp roll-off rate with Chebyshev oLinear phase response. Butterworth: flat amplitude response Bessel: linear phase response f Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. ID (mA) ID (mA) 10 8.0 7.0 6.0 1.0 5.0 4.0 0.1 3.0 2.0 0.01 1.0 0 0 0.1 0.2 Electronic Devices, 9th edition Thomas L. Floyd 0.3 0.4 0.5 0.6 0.7 0.8 VD (V) 0.001 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 VD (V) © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.