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6.10: Other Specialized Devices

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  • Op amps have been further refined into other specialized devices. Along with these, a wide range of specialized linear integrated circuits have emerged. In the area of simple amplifiers, devices are available that are designed to work primarily from single polarity power supplies. Also, a large array of specialty amplifiers exist for such applications as low noise audio pre-amplifiers. For very low offset applications, specialized “auto zero” amplifiers are available. These amplifiers continually monitor their DC offset and actively correct it.

    Moving away from the strict confines of op amps, linear ICs are available for most popular functions, making the designer's job even easier. Some of these items will be explored in later chapters. Other devices are for very specific applications and will not be covered here.

    Some manufacturers develop ICs solely for specific market segments. As an example, Analog Devices, THAT Corporation and Maxim make very specialized ICs for audio and musical equipment manufacturers. The Analog Devices SSM line includes high precision microphone pre-amps and low power loudspeaker and headphone amplifiers. In like fashion, THAT Corporation makes voltage-controlled amplifiers, balanced line drivers, dynamics processors and other items aimed at the audio industry. Maxim makes a series of audio subsystem ICs for notebook PCs and mobile communications. Of course, nothing prevents a designer from using one of these ICs in a non-musical product.




    In this chapter we have examined operational amplifiers that exhibit extended performance, and those that have been tailored to more specific applications. In the area of precision differential amplification comes the instrumentation amplifier. Instrumentation amplifiers may be formed from three separate op amps, or they may be purchased as single hybrid or monolithic ICs. Instrumentation amplifiers offer isolated high impedance inputs and excellent common-mode rejection characteristics.

    Programmable op amps allow the designer to set desired performance characteristics. In this way, an optimum mix of parameters such \(f_{unity}\) and slew rate versus power consumption is achieved. Programmable op amps can also be set to a very low power consumption standby level. This is ideal for battery powered circuits. Generally, programming is performed by either a resistor for static applications, or via an external current or voltage for dynamic applications.

    The output drive capabilities of the standard op amp have been pushed to high levels of current and voltage. Power op amps may be directly connected to low impedance loads such as servo motors or loudspeakers. Moderate power devices exist that have been designed for line drivers and audio applications. Due to the higher dissipation requirements these applications produce, power op amps are often packaged in TO-220 and TO-3 type cases.

    Along with higher power devices, still other devices show increased bandwidth and slewing performance. These fast devices are particularly useful in video applications. Perhaps the fastest amplifiers are those that rely on current feedback and utilize a transimpedance output stage. These amplifiers are a significant departure from the ordinary op amp. They do not suffer from strict gain-bandwidth limitations, and can achieve very wide bandwidth with moderate gain.

    Operational transconductance amplifiers, or OTAs, may be used as building blocks for larger circuits such as voltage-controlled amplifiers or filters. Norton amplifiers rely on a current mirror to perform a current-differencing operation. They are relatively inexpensive and operate directly from single-polarity power supplies with little support circuitry. Because they are current-sensing, input limiting resistors are required.


    Review Questions


    1. What are the advantages of using an instrumentation amplifier versus a simple op amp differential amplifier?

    2. How might an instrumentation amplifier be constructed from general purpose op amps?

    3. What are the advantages of using programmable op amps?

    4. What are the results of altering the programming current in a programmable op amp?

    5. Give at least two applications for a high-power op amp.

    6. Give at least two applications for a high-speed op amp.

    7. What is an OTA?

    8. How is an OTA different from a programmable op amp?

    9. Give an application that might use an OTA.

    10. Give at least three applications that could benefit from specialty linear integrated circuits.

    11. Describe how a Norton amplifier achieves input differencing.

    12. List a few of the major design differences that must be considered when working with Norton amplifiers versus ordinary op amps.

    13. Explain why a current feedback amplifier does not suffer from the same gain-bandwidth limitations that ordinary op amps do.




    Analysis Problems


    1. An instrumentation amplifier has a differential input signal of 5 mV and a common-mode hum input of 2 mV. If the amplifier has a differential gain of 32 dB and a CMRR of 85 dB, what are the output levels of the desired signal and the hum signal?

    2. Determine \(V_{out}\) in Figure 6.34 if \(V_{in+} = +20 mV\) DC and \(V_{in-} = -10 mV\) DC.


    3. Repeat Problem 2 for a differential input signal of 10 mV peak to peak.

    4. Repeat Problem 3 for a differential input signal of 20 mV peak to peak and a common-mode signal of 5 mV peak to peak. Assume the system CMRR is 75 dB.

    5. Determine the programming current in Figure 6.11 if \(R_{set} = 1 MΩ\). Assume standard ±15 V supplies.

    6. Using an LM4250 programmable amplifier, determine the following parameters if \(I_{set} = 5 μA\): slew rate, \(f_{unity}\), input noise voltage, input bias current, standby supply current, and open loop gain.

    7. Determine the voltage gain, \(f_2\), and power bandwidth (assume \(V_p\) = 10 V) for the circuit of Figure 6.35.


    8. The circuit of Figure 6.36 uses a light-dependent resistor to enable or disable the amplifier. Under full light, this LDR exhibits a resistance of 1 kΩ, but under no light conditions, the resistance is 50 MΩ. What are the standby current and \(f_{unity}\) values under full light and no light conditions?


    9. Determine the required capacitance to set PSRR to at least 20 dB at 100 Hz for an LM386 power amp.

    10. What is the power bandwidth for a 5 V peak signal in Figure 6.20?

    11. Determine the device dissipation for an LM386 delivering 0.5 W into an 8 ohm load. Assume a 12 V power supply is used.

    12. Determine the input resistance, output resistance and transconductance for an LM13700 OTA with \(I_{abc} = 100\) μA.

    13. Determine the value of \(I_{abc}\) in Figure 6.22 if \(V_{control}\) = 0.5 V DC.

    14. Recalculate the value of \(R_i\) for the circuit of Figure 6.26 if a voltage gain of 45 is desired.

    15. Recalculate the input capacitance value for the circuit of Figure 6.26 if a lower break frequency of 15 Hz is desired.


    Design Problems


    16. Design an instrumentation amplifier with a gain of 20 dB using the LT1167

    17. Utilizing the LM3900, design an inverting amplifier with a gain of 12 dB, an input impedance of at least 100 kΩ, and a lower break frequency no greater than 25 Hz.

    18. Utilizing the LM3900, design a noninverting amplifier with a gain of 24 dB, an input impedance of at least 40 kΩ, and a lower break frequency no greater than 50 Hz.

    19. Utilizing the CLC1606, design a noninverting amplifier with a gain of 6 dB

    20. Utilizing the CLC1606, design an inverting amplifier with a gain of 12 dB.


    Challenge Problems


    21. Using Figure 6.34 as a guide, design an instrumentation amplifier with a differential gain of 40 dB. The system bandwidth should be at least 50 kHz. Indicate which op amps you intend to use.


    22. Determine the gain, \(f_2\), power bandwidth (\(V_p = 10 V\)), and standby current for the circuit of Figure 6.37 if \(V_b= 0 V\), and if \(V_b = -15 V\).

    23. Design a power amplifier with a voltage gain of 32 dB using the LM386.

    24. Determine \(V_{out}\) in Figure 6.22 if \(V_{control} = 1 V DC \) and \(V_{in} = 100\) mV sine wave.

    25. Sketch \(V_{out}\) for Figure 6.22 if \(V_{control}(t) = 1 sin 2 π 1000 t\), and \(V_{in}(t) = 0.5 sin 2 π 300,000 t.\)

    26. Alter Figure 6.22 such that a 100 mV input signal will yield a 1 V output signal when \(V_{control}\) is +2 V.

    27. Derive Equation 6.9 from the text.


    Computer Simulation Problems


    28. Use a simulator to verify the design produced in Problem 21.

    29. Simulate the output of the circuit shown in Figure 6.26. Use a +15 V DC power supply with \(V_{in}(t) = 0.01 sin 2 π 100 t\).

    30. Use a simulator to verify the gain of the design produced in Problem 18.

    31. Device mismatching can adversely affect the CMRR of an instrumentation amplifier. Rerun the simulation of the instrumentation amplifier (Figure 6.5) with a ±5% tolerance applied to the value of \(R_{i}^{'}\) (Rip). Based on these simulations, what do you think would happen if all of the circuit resistors had this same tolerance?