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1、Unit 12 Basic Integrated Circuits,Current sources Current sources are the foundation of circuit design in microelectronics. Current sources provide biasing for circuit operation. They serve as output drivers. They serve as load elements in amplifier input stages. Even logic gates can be modeled as a
2、 collection of variable current sources. Analysis of circuits will often show a resistance biasing the block under analysis. Current mirrors are used almost exclusively for this purpose in microelectronics,下一页,返回,Unit 12 Basic Integrated Circuits,Current sources offer the advantages of smaller size,
3、 higher accuracy and can be designed to provide 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 direct current (DC) source as provided in circuit simulators such as SPI
4、CE. Constant current of any value is provided at all times.,下一页,上一页,返回,Unit 12 Basic Integrated Circuits,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 curren
5、t 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
6、 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.) ,下一页,上一页,返回,Unit 12 Basic Integrated Circuits,Unfortunately, physical constraints apply in the real world of semiconductors, and real current
7、sources fall short of perfection. Current provided by integrated circuit current sources are constant within some tolerance, and the value of current depends on limitations of device size, power dissipation and process Early voltages. Source output impedance values typically fall between several hun
8、dreds of kilohms and several megohms. The voltage appearing across the source modifies current.,下一页,上一页,返回,Unit 12 Basic Integrated Circuits,Real supplies power circuits, and every circuit element has a voltage drop across it due to the internal resistance. The minimum compliance voltage a current s
9、ource can realize is limited by its internal resistance, and the maximum compliance voltage is limited by the circuits supply and process breakdown voltages.,下一页,上一页,返回,Unit 12 Basic Integrated Circuits,We can build current sources and current sinks, but the same element typically doesnt perform bot
10、h functions under normal operating conditions. It is a fact of life that we have physical constraints that forbid the existence of ideal elements. Fortunately, the design of integrated circuit current references that are “good enough” occurs every day.,下一页,上一页,返回,Unit 12 Basic Integrated Circuits,Le
11、t us start our discussion of current sources with the simplest example: the resistor. If we connect a resistor R between a voltage supply V and a load, the current supplied to the load is dependent on the value of V, the value of R and the impedance of the load: In reality, resistor R will have some
12、 temperature coefficient and possibly a voltage coefficient. The absolute tolerance of the resistor will have a manufacturing tolerance associated with it.,下一页,上一页,返回,Unit 12 Basic Integrated Circuits,A resistor may seem to be a poor choice for a current source, but it may be that a resistor is perf
13、ectly adequate. For example, if the requirement is for a minimum current to be provided to the load, the tolerances on V and on R load can be considered, and R sized appropriately to always provide the minimum current. As previously stated, current sources used in microelectronics are usually made w
14、ith transistors in a configuration known as the current mirror. In a current mirror, one transistor serves as a reference device.,下一页,上一页,返回,Unit 12 Basic Integrated Circuits,Current is supplied to the reference device and the reference device generates a value of V be in bipolar technology, or a va
15、lue of Vgs in MOS technology. These reference voltages are then provided to other transistors that serve to recreate, or mirror, the initial current. Let us begin our discussion of transistor based current sources with the bipolar case, and then recreate the discussions for MOS.,下一页,上一页,返回,Unit 12 B
16、asic Integrated Circuits,Current mirrors in bipolar technology The relationship between collector current and base emitter voltage in bipolar transistors operating in their linear region is defined by what is commonly known as the diode equation: Where Is is a process parameter called saturation current and VT is thermal voltage.,下一页,上一页,返回,Unit 12 Basic Integrated Circuits,If a particular value of Vbe is placed across the BJTs base emitter junction, a particular value of colle
