Current Source

7 Key excerpts on "Current Source"

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  eBook - PDF

  Current Sources and Voltage References

  A Design Reference for Electronics Engineers

  • Linden T. Harrison(Author)
  • 2005(Publication Date)
  • Newnes

    (Publisher)

  13 Chapter 2 An Introduction to Current Sources 2.1 An overview Current Sources are basic electronic building blocks that are used extensively in the architectures of analog ICs, as well as in OEM circuit board designs. In both cases, Current Sources are created by combining diodes, resistors, and transistors (BJTs or FETs). They can also be created at the circuit board level by using discretes, matched pairs, transistor arrays, or by combining op amps with precision voltage references. The various techniques for doing so will be reviewed in Part 1 of this book. Although a few dedicated monolithic Current Source ICs are commercially available, it is often nec-essary to create one’s own circuit to match the particular needs of the application. Although most forms of today’s instrumentation use either voltage or current refer-ences, the former are far more available. As a result, designers frequently use voltage references together with precision resistors, so that a stable reference voltage is con-verted into a precise current. Applications for Current Sources range from biasing and stabilization to reference and linearizing. For example, in the design of an op amp, the IC designer will use Current Sources to create active loads for the amplifier stages and to establish precise bias levels. By providing a constant current, this forces amplifier stages to stay at the Q-points within their active linear regions (see Figure 2.1). In a circuit board design, a Current Source may be used for linearly charging a capacitor with a constant current, as in a precision timing circuit or in a peak detector. In a med-ical instrument application, a sensor and a low-noise, front-end amplifier could be biased using precision Current Sources to assist in recovering very-low-level signals. The advantages of using these building blocks is their inherent constant current out-puts, which are mostly independent of changes in supply voltage, temperature, load resistance, or load voltage.

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  Analog BiCMOS Design

  Practices and Pitfalls

  • James C. Daly, Denis P. Galipeau(Authors)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  chapter 3 Current Sources Current Sources are the foundation of circuit design in microelectronics. Current Sources provide biasing for circuit operation. They serve as out- put drivers. They serve as load elements in amplifier input stages. Even logic gates can be modeled as a collection of variable Current Sources. Analysis of circuits in proceeding chapters will often show a resistance biasing the block under analysis. Current mirrors are used almost ex- clusively for this purpose in microelectronics. Current Sources offer the advantages of smaller size, higher accuracy and can be designed to pro- vide temperature coefficients of current as needed. However, resistors can and do serve well as Current Sources in some instances. Let us first consider the characteristics of an ideal DC Current Source as provided in circuit simulators such as SPICE.  Constant current of any value is provided at all times.  Infinite output impedance means there is no change in the source current value due to changes in the output node voltage.  The source has infinite compliance, and will provide the specified current regardless of the voltage across the source.  An ideal Current Source can either sink or source current. The polarity of the specified DC current and the nodal connection of the Current Source to the rest of the circuit determine how the source behaves. (Most simulators have an ideal Current Source with two nodes: positive and negative. Positive current flow in the ideal source is defined as flowing into the positive node and out of the negative node.) Unfortunately, physical constraints apply in the real world of semicon- ductors, and real Current Sources fall short of perfection.  Current provided by integrated circuit Current Sources are constant within some tolerance, and the value of current depends on limita- tions of device size, power dissipation and process Early voltages.

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  eBook - PDF

  Introduction to Electric Circuits

  • James A. Svoboda, Richard C. Dorf(Authors)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  An independent Current Source provides a current independent of the voltage across the source element and is independent of any other circuit variable. Thus, when we say a source is independent, we mean it is independent of any other voltage or current in the circuit. An independent source is a voltage or current generator not dependent on other circuit variables. Suppose the voltage source is a battery and v t ð Þ ¼ 9 volts The voltage of this battery is known to be 9 volts regardless of the circuit in which the battery is used. In contrast, the current of the voltage source is not known and depends on the circuit in which the source is used. The current could be 6 amps when the voltage source is connected to one circuit and 6 milliamps when the voltage source is connected to another circuit. Figure 2.5-1b shows the symbol that is used to represent a Current Source. The current of a Current Source is specified, but the voltage is determined by the rest of the circuit. A Current Source is described by specifying the function i(t), for example, i t ð Þ ¼ 6 sin 500t or i t ð Þ ¼ 0:25 or i t ð Þ ¼ t þ 8 A Current Source specified by i(t) ¼ 0.25 milliamps will have a current of 0.25 milliamps in any circuit in which it is used. The voltage across this Current Source will depend on the particular circuit. The preceding paragraphs have ignored some complexities to give a simple description of the way sources work. The voltage across a 9-volt battery may not actually be 9 volts. This voltage depends on the age of the battery, the temperature, variations in manufacturing, and the battery + (b) (a) + – v(t) i(t) – v(t) i(t) FIGURE 2.5-1 (a) Voltage source. (b) Current Source. 28 2. Circuit Elements current. It is useful to make a distinction between real sources, such as batteries, and the simple voltage and Current Sources described in the preceding paragraphs. It would be ideal if the real sources worked like these simple sources.

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  Dorf's Introduction to Electric Circuits

  • Richard C. Dorf, James A. Svoboda(Authors)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  An independent Current Source provides a current independent of the voltage across the source element and is independent of any other circuit variable. Thus, when we say a source is independent, we mean it is independent of any other voltage or current in the circuit. An independent source is a voltage or current generator not dependent on other circuit variables. Suppose the voltage source is a battery and v t 9 volts The voltage of this battery is known to be 9 volts regardless of the circuit in which the battery is used. In contrast, the current of the voltage source is not known and depends on the circuit in which the source is used. The current could be 6 amps when the voltage source is connected to one circuit and 6 milliamps when the voltage source is connected to another circuit. Figure 2.5-1b shows the symbol that is used to represent a Current Source. The current of a Current Source is specified, but the voltage is determined by the rest of the circuit. A Current Source is described by specifying the function i(t), for example, i t 6 sin 500t or i t 0 25 or i t t 8 A Current Source specified by i(t) 0.25 milliamps will have a current of 0.25 milliamps in any circuit in which it is used. The voltage across this Current Source will depend on the particular circuit. The preceding paragraphs have ignored some complexities to give a simple description of the way sources work. The voltage across a 9-volt battery may not actually be 9 volts. This voltage depends on the age of the battery, the temperature, variations in manufacturing, and the battery + (b) (a) + – v(t) i(t) – v(t) i(t) FIGURE 2.5-1 (a) Voltage source. (b) Current Source. 28 CHAPTER 2 Circuit Elements current. It is useful to make a distinction between real sources, such as batteries, and the simple voltage and Current Sources described in the preceding paragraphs. It would be ideal if the real sources worked like these simple sources. Indeed, the word ideal is used to make this distinction.

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  Electrical Trade Theory N1 Student's Book

  TVET FIRST

  • SA Chuturgoon(Author)
  • 2021(Publication Date)
  • Troupant

    (Publisher)

  Current is represented by the symbol I and is measured in amperes (A). Definition of ampere The ampere is a unit of electric current equal to a flow of one coulomb (C) per second, where the fixed numerical value of the elementary charge e is taken to be 1,602176634 × 10 –19 when given in coulomb. [Note: This is the new internationally accepted definition as of 20/05/2019.] The movement or flow of electrons in a specific direction is what we call electricity . Direction of current flow In the old days, it was believed that electrons flow from the positive terminal of a battery to the negative terminal. This is known as the conventional flow of current . Later, it was discovered that electrons actually flow from the negative terminal to the positive terminal of a battery. This is known as electron flow . Electricity is needed by domestic and industrial consumers for the following effects that current flow produces: • Lighting. • Heating. • Chemical effects. • Magnetic effects. 4.1.2 Potential difference The flow of current through a circuit depends on the application of a potential difference across any two points in the electric circuit. Definition of potential difference The potential difference between two points in a circuit is the work done when 1 coulomb of charge is moved from one point to the other. Simply put, potential difference is an electrical pressure that produces current flow in a closed electric circuit . It is also known as terminal voltage . Potential difference is represented by the symbol V or PD and is measured in joules per coulomb or volts.

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  N1 Electrical Trade Theory

  • Tommy Ferreira, Trevor Adams, Tommy Ferreira, Trevor Adams(Authors)
  • 2021(Publication Date)
  • Future Managers

    (Publisher)

  Without a good understanding of these basic concepts, definitions and laws, you will find it difficult to understand electrical trade theory or any other electrical subject. 4.1 Ohm’s law 4.1.1 Terminology Current Current refers to a flow of electrically charged particles past a certain point in a circuit in a given time. The particles can be negatively charged electrons (electron flow) or positively charged protons (conventional current flow), as seen in Figure 4.1. The common symbol for current is the uppercase letter ‘I’. The standard unit of measurement is the ampere (commonly referred to as ‘amps’), and the symbol used is a capital ‘A’. D i r e c t i o n o f c u r r e n t D ir e c t i o n o f c u r r e n t F l o w o f e l e c t r o n s F l o w o f e lectr o n s Figure 4.1: Circuit showing current flow and electron flow Potential difference (voltage) Just as a difference in water pressure must exist for water to flow between two points, a difference in electrical pressure must be present to cause electrical current to flow. The greater the pressure, the greater the flow will be in both cases. The difference in electrical pressure between two points is termed ‘potential difference’. The unit of measurement for voltage is volt, and the symbol used to represent volt is a capital ‘V’. Definition Volt – the unit of measurement of force used to produce an electric current 35 N1 Electrical Trade Theory Resistance Resistance is the opposition to the free flow of electrons within a circuit. The device or wire designed specifically to oppose the flow of current in a circuit is known as a resistor. If it is designed so that its value can be adjusted, it is known as a variable resistor or a rheostat. A variable resistor is made to be readily adjustable. A preset resistor is one that is set to a certain critical value by the manufacturer of a product.

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  Linear Circuit Theory

  Matrices in Computer Applications

  • Jiri Vlach(Author)
  • 2016(Publication Date)
  • Apple Academic Press

    (Publisher)

  If we recall the definition of the short circuit from our previous discussion, then we see that a voltage source with zero voltage is equivalent to a short circuit . FIGURE 1.3.1 Independent voltage source: (a) Symbol. (b) Its characteristic. 12 Basic Concepts 1.1 1.2 1.3 1.4 1.5 1.6 1.7 An ideal independent Current Source is an element for which we will use the symbol shown in Fig. 1.3.2a. Similarly as in the above case, there exists no possibility of changing the amount of the current delivered by some external influence. The direction of the current is indicated by an arrow. We will always denote an independent Current Source by the letter J , for similar reasons as discussed above for the independent voltage source. The current of an independent Current Source is given, but we can say nothing about the voltages at its terminals, because they depend on the network to which the source is connected. Since the current delivered by the independent Current Source is fixed, but any voltage difference can be measured between its terminals, we can draw a graph (characteristic) describing these properties. It is shown in Fig. 1.3.2b. If the amount of current delivered by the source changes from, say, 2 A to 4 A , the straight line will shift to the right. Should we apply, say, − 2 A without changing the direction of the arrow in the figure, the vertical line will shift to the left of the vertical axis. If we consider a Current Source which delivers zero current, then the vertical line will coincide with the vertical axis. In such case no current can flow through the element but arbitrary voltage can appear across its terminals; we conclude that a Current Source with zero current is equivalent to an open circuit . A battery is an example of a nonideal dc source, because the voltage at the terminals depends on the amount of current we draw from it. Nevertheless, in many cases we take it as an ideal source and denote it by the letter E .

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