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【正文】 in analog integrated circuits to generate a current without causing attenuation at a point in the signal path to which the current source is attached. The collector of a bipolar transistor, the drain of a field effect transistor, or the plate of a vacuum tube naturally behave as current sources (or sinks) when properly connected to an external source of energy (such as a power supply) because the output impedance of these devices is naturally high when used in the current source configuration. JFET and NFET current source A JFET can be made to act as a current source by tying its gate to its source. The current then flowing is the IDSS of the FET. These can be purchased with this connection already made and in this case the devices are called current regulator diodes or constant current diodes or current limiting diodes (CLD). An enhancement mode N channel MOSFET can be used in the circuits listed below. Simple transistor current source Figure 3 shows a typical constant current source (CCS). DZ1 is a zener diode which, when reverse biased (as shown in the circuit) has a constant voltage drop across it irrespective of the current flowing through it. Thus, as long as the zener current (IZ) is above a certain level (called holding current), the voltage across the zener diode (VZ) will be constant. Resistor R1 supplies the zener current and the base current (IB) of NPN transistor (Q1). The constant zener voltage is applied across the base of Q1 and emitter resistor R2. The operation of the circuit is as follows: Voltage across R2 (VR2) is given by VZ VBE, where VBE is the baseemitter drop of Q1. The emitter current of Q1 which is also the current through R2 is given by22)( 22 R VVRVII BEERER ????. Figure 3 Since VZ is constant and VBE is also (approximately) constant for a given temperature, it follows that VR2 is constant and hence IE is also constant. Due to transistor action, emitter current IE is very nearly equal to the collector current IC of the transistor (which in turn, is the current through the load). Thus, the load current is constant (neglecting the output resistance of the transistor due to the Early effect) and the circuit operates as a constant current source. As long as the temperature remains constant (or doesn39。s gain. R2 allows the load current to be set at any desirable value and is calculated by 2R2 RBEEI VV ?? or 2RZ IVR ?? , since VBE is typically V for a silicon device. (IR2 is also the emitter current and is assumed to be the same as the collector or required load current, provided hFE is sufficiently large). Resistance R1 at resistor R1 is calculated asBZZS IKI VVR *1 ??? ,where, K = to 2 (so that R1 is low enough to ensure adequate IB), (m in)2)(FERECB h IIII ??? ,and hFE(min) is the lowest acceptable current gain for the particular transistor type being used. A more mon current source in integrated circ
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