Mechatronics and Measurement Lecturer:Dung-An Wang Lecture 2
Lecture outline Reading:Ch3 of text Today s lecture Semiconductor 2
Diode 3
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Zener diode Voltage-regulator diodes. This family of diodes exhibits steep breakdown curves with well-defined breakdown voltages 5
Voltage-regulator Even when the current through the zener diode changes (deltaiz), the output voltage remains relatively constant (delta Vz is small) 6
Practice Design example 3.1 on p. 84 7
Light-emitting diodes (LEDs) 8
Photodiodes If photons excite carriers in a reverse-biased pn junction, a very small current proportional to the light intensity flows 9
Bipolar Junction Transistor n-type silicon in the emitter is more heavily doped than the collector, so the collector and emitter are not interchangeable. 10
Physics Base to emitter junction forward biased, electrons diffuse from the emitter n-type region to the base p- type region. Base to collector junction is reverse biased, there is a depletion region that prevent flow of electrons from the base region into the collector region. because base region is manufactured to be very thin and the emitter n-type region is more heavily doped than the base, most of the electrons from the emitter accelerate through the base region with enough momentum to cross the depletion region into the collector region without recombining with holes in the base region. 11
Beta: order of 100 12
3.4.2 Common Emitter Transistor Circuit Common emitter circuit 13
Common emitter circuit The power dissipated by the transistor ( ICVCE ) is smallest, for a given collector current, when it is fully saturated. If the transistor is not fully saturated, it gets hot faster and can fail. In full saturation,v CE is at its minimum, which is about 0.2 V for a BJT. So in saturation, the base to- emitter junction is forward biased ( V BE 0.7 V), there is a small drop from the collector to the emitter ( V CE 0.2 V). 14
EXAMPLE 3.4 15
3.4.3 Bipolar Transistor Switch 16
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3.4.6 Phototransistor and Optoisolator light emitted by the LED causes current to flow in the phototransistor circuit. This output circuit can have a different ground reference, and the supply voltage Vs can be chosen to establish a desired output voltage range. With no common ground, the optoisolator creates a state of electrical isolation between the input and output circuits by transmitting the signal optically rather than through an electrical connection. 20
Lab exercise 5 Develop an understanding of how transistor circuits function. 21
3.5 FIELD-EFFECT TRANSISTORS 22
With a positive Vgs larger than Vt, as Vds is increased from 0, we enter the active region, also called the ohmic region, of the MOSFET. In this region, as Vgs is further increased, the conduction channel grows correspondingly, and the MOSFET appears to function like a variable resistor whose resistance is controlled by Vgs. 23
However, when Vgs - Vt reaches Vdd, there is no longer an electric field at the drain end of the MOSFET. Therefore, the width of the n-channel shrinks to a minimum value close to the drain resulting in what is called pinch-off. This pinch-off limits a further increase in drain current, and the MOSFET is said to be in saturation. In saturation, the current is almost constant with further increases in Vds. The drain-to-source resistance, called R on, is minimal (usually less than 5 Ω) as it enters the saturation region. 24
3.5.3 Applications of MOSFETs Vg <= 0 for the MOSFET to be cutoff so that no current is delivered to the load. When Vg - Vt Vdd, the MOSFET enters saturation resulting in nearly full voltagevs across the load (because Ron is small). 25
Flyback diode 26
If the control signal Vg is zero, the MOSFET will be cutoff resulting in a huge drain to source impedance (in megaohms) essentially blocking the analog signal ( Vout 0 V). The pull-down resistor R is required to hold the V out terminal at ground in the off state. When the control signal Vg is larger than the largest value of the analog input signal Vin plus threshold voltage Vt, the drain to source channel will conduct with a low resistance, and the output signal will track the input ( Vout =Vin ). 27
DESIGN EXAMPLE 3.4 28