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electronics fundamentals circuits, devices, and applications THOMAS L. FLOYD DAVID M. BUCHLA chapter 20 Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Instrumentation amplifiers An instrumentation amplifier (IA) is a special integrated circuit designed to for applications where small signals are in noisy environments. They have high input impedance and a high CMRR for excellent noise rejection. R3 to R6 are equal Vin1 + Vcm values. If R1 = R2, then the closed-loop gain is set by a single R external resistor, RG, G supplied by the user. The gain is: 2 R Vin2 + Vcm Acl 1 RG Electronics Fundamentals 8th edition Floyd/Buchla R3 + - R 1 - R2 + R5 R4 Vout = Acl (Vin2-Vin1) + R6 © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Instrumentation amplifiers For R1 = R2 = 10 kW, what value of RG will set the gain to 40? Vin1 2R Acl 1 RG Solving for RG, 2R RG Acl - 1 2 10 kW RG 513 W 40 - 1 R3 + - R 1 RG - R2 Vin2 + R5 R4 Vout + R6 (510 W is the nearest standard value). Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Instrumentation amplifiers The bandwidth of instrumentation amplifiers decreases when higher gain is selected. The frequency response is published by the manufacturer. For example the frequency response of an INA333 is shown. If the required frequency response is less than about 300 Hz, a gain of 1000 can be selected. The INA333 can operate on as little as 1.8 V, so is used in portable medical, handheld instrumentation, weigh scales and data acquisition applications. Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Isolation amplifiers An isolation amplifier provides dc isolation for applications where electrical separation between input and output is necessary. A block diagram of a isolation amplifier is shown. This one uses pulse width modulation (PWM) to send a signal across the isolation barrier. +V -V +V -V Modulation is the process of modifying Input stage Output stage the input with another waveform in order to Op-amp Modulator Demodulator Op-amp transmit the signal across the barrier. Notice the separate Oscillator power supplies and Isolation barrier with grounds for each stage. capacitive coupling Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Isolation amplifiers Signals for pulse width modulation are shown. This is the method used by the ISO124, discussed in the text. After transmitting the modulated signal across the barrier, the original signal is recovered. +V -V Original input +V -V Input stage Op-amp Modulator Output stage Demodulator Op-amp AM Oscillator PWM Modulated signal Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Isolation amplifiers Transformer coupling can also be used in isolation amplifiers. The Burr-Brown 3656KG is an example of a versatile IC that has gain control as well as three-port isolation (with three separate grounds). It can also supply isolated power on both the input and the output side for external devices. The 3656KG is suited to applications such as interfacing the signals in an electrocardiogram, or to isolate the input signals for fetal heart monitoring as shown here in in the text. Electrode for sensing fetal heartbeat Common electrode Electronics Fundamentals 8th edition Floyd/Buchla Shielded cable 3656KG Input Output Heart monitor Com © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Operational Transconductance Amplifier (OTA) An operational transconductance amplifier is a voltage to current amplifier. The symbol shows a current source on the output which is dependent on bias current. The gain of an OTA is given as a transconductance parameter (similar to a FET): g I out m Vin IBIAS The transconductance is dependent on the bias current (IBIAS) and a constant (K): Inputs OTA + gm = KIBIAS Output Iout = KIBIASVin Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Operational Transconductance Amplifier (OTA) A typical curve is shown. Notice that both axes are logarithmic. For example, a transconductance of 1000 mS (103), requires a bias current of about 63 mA. 100 k Transconductance, gm (mS) The specification sheet shows a graph of the relationship between transconductance and bias current. The user can then set the bias to the desired transconductance. 10 k 1.0 k 100 10 Electronics Fundamentals 8th edition Floyd/Buchla 1 10 100 Bias current (mA) 1000 © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Operational Transconductance Amplifier (OTA) The bias current also affect the input and output resistance. If you require a very high input resistance, you can choose a smaller bias current. Output resistance 10 M Resistance (W) What is the input resistance indicated by the plot if the bias current is 20 mA? 100 M 1.0 M Input resistance 100k Reading the graph, the input resistance is approximately 1.5 MW Electronics Fundamentals 8th edition Floyd/Buchla 10k 1 100 10 Bias current (mA) 1000 © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Operational Transconductance Amplifier (OTA) The LM13700 is a representative OTA which has a gm that can be set over a six decade range. There are many applications1 for this OTA including modulators, function generators, and voltage controlled circuits including amplifiers, filters, and resistors. +15 V Vin R1 10 kW The bias current for the LM13700 is found from the formula RBIAS OTA I BIAS Vout VBIAS - -V - 1.4 V RBIAS + R2 10 kW RL 20 kW -15 V Electronics Fundamentals 8th edition Floyd/Buchla 1 see: http://cache.national.com/ds/LM/LM13700.pdf © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Operational Transconductance Amplifier (OTA) What value of bias current will produce a voltage gain of 40 for the OTA? The transconductance curve is shown. +15 V Vin R1 10 kW RBIAS OTA Vout + R2 10 kW RL 20 kW -15 V Electronics Fundamentals 8th edition Floyd/Buchla Transconductance, gm (mS) The required gm is Av/RL = 40/20 kW = 2,000 mS. 105 From the graph, IBIAS ≈ 125 mA 104 103 102 10 1 0.1 1 10 100 1000 Bias current (mA) © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Operational Transconductance Amplifier (OTA) What value of bias resistor will set the bias current at 125 mA for the previous circuit? I BIAS VBIAS - -V - 1.4 V RBIAS RBIAS +15 V Vin R1 10 kW OTA RBIAS 220 kW Vout + R2 10 kW RL 20 kW VBIAS - -V - 1.4 V I BIAS 15 V - -15 V - 1.4 V 125 μA = 228 kW The nearest standard 5% value is 220 kW. -15 V Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Clamping circuits A basic clamping circuit (also called a dc restorer) adds a dc level to a signal voltage using a diode and capacitor. Vp(in) - 0.7 V - C + Vin 0V Vout D RL 0V Diode conducts The first negative cycle of the signal biases the diode on and causes the capacitor to charge to Vp(in) - 0.7 V. The long time constant keeps the capacitor charged, which adds a dc voltage to the signal voltage at the output. Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Clamping circuits The 0.7 V diode drop in the basic circuit can be a problem for low-level signals, which are common in signal processing applications. 0 V Vin 0V Vin - C + Vout R1 Active diode circuits use an op-amp and feedback to closely approximate the behavior of an ideal diode. The same clamping circuit with an active diode has a nearly ideal response. Electronics Fundamentals 8th edition Floyd/Buchla + D RL © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Clamping circuits Active clamping circuits can produce negative 0V clamping action by reversing the diode. Vin 0V C Vin + - Vout R1 The first positive cycle of the signal charges the capacitor as shown, adding a negative dc voltage to the input. Electronics Fundamentals 8th edition Floyd/Buchla + D RL © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Clamping circuits An application for a clamping circuit is shown. C1 Vs +15 V R1 D1 R2 This circuit adds or subtracts a dc voltage to the output, depending on the setting of the DC Offset control. _ DC Offset Control R3 + Vout RL R4 -15 V Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Limiting circuits Diode limiters (clippers) are circuits that limit voltage above or below a specified level. A basic limiter circuit that clips voltages below 0.7 V is shown. Vin 0V If the input is above 0.7 V, the diode conducts, causing the output to be limited to this level. Electronics Fundamentals 8th edition Floyd/Buchla Vout 0V R1 D 0.7 V RL © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Limiting circuits Reversing the diode causes the limiter to clip signal voltages that are below –0.7 V. Vin 0V Vout R1 -0.7 V D RL If the input is below -0.7 V, the diode conducts, causing the output to be limited (clipped) for voltages less than this level. Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Limiting circuits An active limiter uses an op-amp and diode circuit to form an almost ideal diode. This means the 0.7 V forward drop of the diode does not affect the output. Vin 0V Vout 0V R - This circuit limits the positive waveform (clipping voltages above ground) because the + input reference is at 0 V. Electronics Fundamentals 8th edition Floyd/Buchla + D A different reference level is easy to achieve by putting the desired reference voltage on the non-inverting input. © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Summary Peak detector Another useful active diode circuit is the peak detector. The purpose of the circuit is to store the maximum positive value of a voltage on a capacitor and hold the value for a certain time. The op-amp is set up as a comparator. If Vin > VC , the diode is forward biased and charges to the peak of Vin. For example if a 1.0 Vpp sine wave is the input, the output will be a dc level of +0.5 V. Electronics Fundamentals 8th edition Floyd/Buchla Vin Ri + - R1 D VOUT C © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Selected Key Terms Instrumentation An amplifier specifically designed for amplifier amplifying small differential signals and rejecting large common-mode voltages. Isolation An amplifier with electrically isolated amplifier internal stages. Operational A voltage-to-current amplifier in which the transconductance gain is set by a bias current. amplifier (OTA) Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Selected Key Terms Clamper A circuit that adds a dc level to an ac signal; a dc restorer. Limiter A circuit that removes part of a waveform above or below a specified level; a clipper. Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Quiz 1. Selecting the highest gain for an instrumentation amplifier means that a. the bandwidth will be less. b. the CMRR is higher. c. both of the above. d. none of the above. Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Quiz 2. The block diagram for an instrumentation amplifier is shown. The resistor shown in the blue box a. determines the CMRR. R3 + b. determines the gain. c. both of the above. d. none of the above. - R 1 RG + Electronics Fundamentals 8th edition Floyd/Buchla - R2 - R5 R4 + R6 © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Quiz 3. For an isolation amplifier, the input and output stages cannot a. have a common power supply. b. be connected with a conductive path. c. both of the above. d. none of the above. Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Quiz 4. In an isolation amplifier the purpose of the modulator is to a. increase the signal-to-noise ratio. b. increase the bandwidth. c. remove high frequency noise from the signal. d. modify the signal for transmission. Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Quiz 5. The gain of an operational transconductance amplifier (OTA), is specified as the ratio of a. output current to input current. b. output voltage to input current. c. output voltage to input voltage. d. output current to input voltage. Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Quiz 6. To increase the gain of an operational transconductance amplifier (OTA), you would a. change the ratio of the feedback resistors. b. reduce the size of the bias resistor. c. increase the size of the gain resistor. d. increase the size of the load resistor. Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Quiz 7. Another name for a clamping circuit is a a. dc restorer. b. clipping circuit. c. limiter. d. peak detector. Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Quiz 8. The dc voltage you would expect to measure across the capacitor is equal to 0.7 V less than the a. peak-to-peak value of the input voltage. b. rms value of the input voltage. c. peak value of the input voltage. d. average value of the input voltage. Vin Electronics Fundamentals 8th edition Floyd/Buchla C + Vout D RL © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Quiz 9. Reversing the diode in a clipping circuit causes a. the opposite side of the input to be clipped. b. a dc level shift in the output. c. the clipping level to increase. d. the ground reference to change. Electronics Fundamentals 8th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Quiz 10. Assume the circuit has an ac input as shown. The output will be a. an amplified sine wave. Ri b. 0.5 Vdc +1.0 V c. 1.0 Vdc d. 2.0 Vdc Electronics Fundamentals 8th edition Floyd/Buchla Vin 0V + - R1 D VOUT -1.0 V C © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. Chapter 20 1 Quiz Answers: Electronics Fundamentals 8th edition Floyd/Buchla 1. a 6. b 2. b 7. a 3. c 8. c 4. d 9. a 5. d 10. c © 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.