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| university:courses:electronics:text:chapter-4 [30 Aug 2013 17:07] – [4.9 Operational amplifier Schmitt trigger] Doug Mercer | university:courses:electronics:text:chapter-4 [06 Apr 2022 14:49] (current) – [4.1.1 The Active Voltage to Current Converter] Doug Mercer |
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| ======4: Op Amp applications - Advanced topics====== | ======Chapter 4: Op Amp applications - Advanced topics====== |
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| In this chapter we explore a number of example op amp configuration that are presented to illustrate certain advanced applications for operational amplifiers. Many of these more advanced uses for op amps will probably make more sense after the reader has studied the material on Bipolar Junction and Field Effect transistors in later chapters. The reader can skip this material for now and circle back after gaining an understanding of how transistors work. | In this chapter we explore a number of example op amp configuration that are presented to illustrate certain advanced applications for operational amplifiers. Many of these more advanced uses for op amps will probably make more sense after the reader has studied the material on Bipolar Junction and Field Effect transistors in later chapters. The reader can skip this material for now and circle back after gaining an understanding of how transistors work. |
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| This configuration is also often referred to as an Active Cascode. To understand the concept of the cascode or common gate (base) amplifier the reader is directed to study the section in Chapter 9 on the Cascode (9.3). | This configuration is also often referred to as an Active Cascode. To understand the concept of the cascode or common gate (base) amplifier the reader is directed to study the section in Chapter 9 on the Cascode (9.3). |
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| | An instructive application for this circuit technique can be found in this article on how to [[adi>media/en/technical-documentation/technical-articles/d61_en-convert.pdf|convert 1V to 5V signal to 4mA to 20mA output.]] |
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| ====4.1.1 The Active Voltage to Current Converter==== | ====4.1.1 The Active Voltage to Current Converter==== |
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| Looking at equation 2, the output impedance, Z<sub>OUT</sub> will be infinity if R<sub>1</sub>=R<sub>3</sub>, R<sub>2</sub>=R<sub>4</sub> and R<sub>S</sub>=R<sub>S'</sub> because the bottom term in the ratio will be exactly zero. To the extent that the resistors do not match the output impedance could be either positive of negative. For example if R<sub>3</sub> were slightly larger than R<sub>1</sub> the bottom term would be negative. | Looking at equation 2, the output impedance, Z<sub>OUT</sub> will be infinity if R<sub>1</sub>=R<sub>3</sub>, R<sub>2</sub>=R<sub>4</sub> and R<sub>S</sub>=R<sub>S'</sub> because the bottom term in the ratio will be exactly zero. To the extent that the resistors do not match the output impedance could be either positive of negative. For example if R<sub>3</sub> were slightly larger than R<sub>1</sub> the bottom term would be negative. |
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| | More details on the Howland current source can be found in [[adi>media/en/analog-dialogue/volume-52/number-4/a-large-current-source-with-high-accuracy-and-fast-settling.pdf|A large current source with high |
| | accuracy and fast settling]] and [[adi>media/en/analog-dialogue/volume-47/number-2/articles/choose-resistors-to-minimize-errors.pdf|Choose resistors to minimize errors in grounded-load current source]]. |
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| =====4.2 Precision Current to Voltage Converter===== | =====4.2 Precision Current to Voltage Converter===== |
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| Two configurations are shown in figure 4.2. Version (a) produces an output voltage V<sub>OUT</sub> from input current I<sub>IN</sub>. The output voltage is produced with respect to ground due to the virtual ground at the - input terminal of the amplifier and is negative (V<sub>OUT</sub> = -I<sub>IN</sub>R<sub>1</sub>) for currents flowing into the virtual ground (from the +V as shown) and positive for currents flowing out of the virtual ground (flowing toward -V). Version (b) produces an output voltage V<sub>OUT</sub> from input current I<sub>IN</sub> as well but now it is produced with respect to some negative node potential, -V. Unlike version (a) which accepts both sourcing and sinking currents, version (b) will only operate for currents flowing into the virtual ground at the - input terminal. There is an advantage to using a MOSFET, M<sub>1</sub>, over a bipolar transistor. In the MOSFET all the current in the drain also flows in the source (no current in gate) whereas in the case of a BJT the emitter current is increased due to the base current. The current in R<sub>1</sub> will thus be slightly larger than I<sub>IN</sub> and the voltage V<sub>OUT</sub> will not be exactly equal to I<sub>IN</sub>*R<sub>1</sub>. | Two configurations are shown in figure 4.2. Version (a) produces an output voltage V<sub>OUT</sub> from input current I<sub>IN</sub>. The output voltage is produced with respect to ground due to the virtual ground at the - input terminal of the amplifier and is negative (V<sub>OUT</sub> = -I<sub>IN</sub>R<sub>1</sub>) for currents flowing into the virtual ground (from the +V as shown) and positive for currents flowing out of the virtual ground (flowing toward -V). Version (b) produces an output voltage V<sub>OUT</sub> from input current I<sub>IN</sub> as well but now it is produced with respect to some negative node potential, -V. Unlike version (a) which accepts both sourcing and sinking currents, version (b) will only operate for currents flowing into the virtual ground at the - input terminal. There is an advantage to using a MOSFET, M<sub>1</sub>, over a bipolar transistor. In the MOSFET all the current in the drain also flows in the source (no current in gate) whereas in the case of a BJT the emitter current is increased due to the base current. The current in R<sub>1</sub> will thus be slightly larger than I<sub>IN</sub> and the voltage V<sub>OUT</sub> will not be exactly equal to I<sub>IN</sub>*R<sub>1</sub>. |
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| =====4.3 Precision Current Mirror===== | =====4.3 Precision Current Mirror===== |
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| Operational amplifiers are not designed to be used as comparators, so this section has been, intentionally, a little discouraging. Nevertheless there are some cases where the use of an op amp as a comparator is a useful engineering decision-what is important is to make it a considered decision, and ensure that the op amp chosen will perform as expected. To do this it is necessary to read the manufacturer's data sheet carefully, to consider the effects of non-ideal op amp performance, and to calculate the effects of op amp parameters on the overall circuit. Since the op amp is being used in a non-standard manner some experimentation may also be necessary, since the amplifier used for the experiment will not necessarily be typical and the results of experiments should always be interpreted somewhat pessimistically. | Operational amplifiers are not designed to be used as comparators, so this section has been, intentionally, a little discouraging. Nevertheless there are some cases where the use of an op amp as a comparator is a useful engineering decision-what is important is to make it a considered decision, and ensure that the op amp chosen will perform as expected. To do this it is necessary to read the manufacturer's data sheet carefully, to consider the effects of non-ideal op amp performance, and to calculate the effects of op amp parameters on the overall circuit. Since the op amp is being used in a non-standard manner some experimentation may also be necessary, since the amplifier used for the experiment will not necessarily be typical and the results of experiments should always be interpreted somewhat pessimistically. |
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| | **ADALM1000 Lab Activity [[university:courses:alm1k:alm-lab-comp|Comparators]]** |
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| =====4.9 Operational amplifier Schmitt trigger===== | =====4.9 Operational amplifier Schmitt trigger===== |
| **Return to [[university:courses:electronics:text:chapter-3|Previous Chapter]]** | **Return to [[university:courses:electronics:text:chapter-3|Previous Chapter]]** |
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