电子信息专业英语演讲.doc

in the mid 1940s a team of scientists working for Bell Telephone Labs in Murray Hill, New Jersey, were working to discover a device to replace the present vacuum tube technology. Vacuum tubes were the only technology available at the time to amplify signals or serve as switching devices in electronics. The problem was that they were expensive, consumed a lot of power, gave off too much heat, and were unreliable, causing a great deal of maintenance. The scientists that were responsible for the 1947 invention of the transistor were: John Bardeen, Walter Brattain, and William Shockley. Bardeen, with a Ph.D. in mathematics and physics from Princeton University, was a specialist in the electron conducting properties of semiconductors. Brattain, Ph.D., was an expert in the nature of the atomic structure of solids at their surface level and solid-state physics. Shockley, Ph.D., was the director of transistor research for Bell Labs. Transistors are active components and are found everywhere in electronic circuits. They are used as amplifiers and switching devices. As amplifiers, they are used in high and low frequency stages, oscillators, modulators, detectors and in any circuit needing to perform a function. In digital circuits they are used as switches. There is a large number of manufacturers around the world who produce semiconductors (transistors are members of this family of components), so there are literally thousands of different types. There are low, medium and high power transistors, for working with high and low frequencies, for working with very high current and/or high voltages. Several different transistors are shown on 4.1. The most common type of transistor is called bipolar and these are divided into NPN and PNP types. Their construction-material is most commonly silicon (their marking has the letter B) or germanium (their marking has the letter A). Original transistor were made from germanium, but they were very temperature-sensitive. Silicon transistors are much more temperature-tolerant and much cheaper to manufacture.3Fig. 4.2: Transistor symbols: a - bipolar, b - FET, c -MOSFET, d - dual gate MOSFET, e - inductive channel MOSFET, f - single connection transistor The second letter in transistors marking describes its primary use:C - low and medium power LF transistor,D - high power LF transistor,F - low power HF transistor,G - other transistors,L - high power HF transistors,P - photo transistor,S - switch transistor,U - high voltage transistor.Here are few examples:AC540 - germanium core, LF, low power,AF125 - germanium core, HF, low power,BC107 - silicon, LF, low power (0.3W),BD675 - silicon, LF, high power (40W),BF199 - silicon, HF (to 550 MHz),BU208 - silicon (for voltages up to 700V),BSY54 - silicon, switching transistor.There is a possibility of a third letter (R and Q - microwave transistors, or X - switch transistor), but these letters vary from manufacturer to manufacturer.The number following the letter is of no importance to users.junction transiston A junction transistor consists of a thin piece of one type of semiconductor material between two thicker layers of the opposite type. For example, if the middle layer is p-type, the outside layers must be n-type. Such a transistor is an NPN transistor. One of the outside layers is called the emitter, and the other is known as the collector. The middle layer is the base. The places where the emitter joins the base and the base joins the collector are called junctions. The layers of an NPN transistor must have the proper voltage connected across them. The voltage of the base must be more positive than that of the emitter. The voltage of the collector, in turn, must be more positive than that of the base. The voltages are supplied by a battery or some other source of direct current. The emitter supplies electrons. The base pulls these electrons from the emitter because it has a more positive voltage than does the emitter. This movement of electrons creates a flow of electricity through the transistor. The current passes from the emitter to the collector through the base. Changes in the voltage connected to the base modify the flow of the current by changing the number of electrons in the base. In this way, small changes in the base voltage can cause large changes in the current flowing out of the collector. Manufacturers also make PNP junction transistors. In these devices, the emitter and collector are both a p-type semiconductor material and the base is n-type. A PNP junction transistor works on the same principle as an NPN transistor. But it differs in one respect. The main flow of current in a PNP transistor is controlled by altering the number of holes rather than the number of electrons in the base. Also, this type of transistor works properly only if the negative and positive connections to it are the reverse of those of the NPN transistor.NPN BJT with forward-biased EB junction and reverse-biased BC junction An NPN transistor can be considered as two diodes with a shared anode. In typical operation, the base-emitter junction is forward biased and the basecollector junction is reverse biased. In an NPN transistor, for example, when a positive voltage is applied to the baseemitter junction, the equilibrium between thermally generated carriers and the repelling electric field of the depletion region becomes unbalanced, allowing thermally excited electrons to inject into the base region. These electrons wander (or diffuse) through the base from the region of high concentration near the emitter towards the region of low concentration near the collector. The electrons in the base are called minority carriers because the base is doped p-type which would make holes the majority carrier in the base.To minimize the percentage of carriers that recombine before reaching the collectorbase junction, the transistors base region must be thin enough that carriers can diffuse across it in much less time than the semiconductors minority carrier lifetime. In particular, the thickness of the base must be much less than the diffusion length of the electrons. The collectorbase junction is reverse-biased, and so little electron injection occurs from the collector to the base, but electrons that diffuse through the base towards the collector are swept into the collector by the electric field in the depletion region of the collectorbase junction. The thin shared base and asymmetric collectoremitter doping is what differentiates a bipolar transistor from two separate and oppositely biased diodes connected in series应用4 In the Diode tutorials we saw that simple diodes are made up from two pieces of semiconductor material, either silicon or germanium to form a simple PN-junction and we also learnt about their properties and characteristics. If we now join together two individual signal diodes back-to-back, this will give us two PN-junctions connected together in series that share a common P or N terminal. The fusion of these two diodes produces a three layer, two junction, three terminal device forming the basis of a Bipolar Junction Transistor, or BJT for short.Transistors are three terminal active devices made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small signal voltage. The transistors ability to change between these two states enables it to have two basic functions: switching (digital electronics) or amplification (analogue electronics). Then bipolar transistors have the ability to operate within three different regions:1. Active Region - the transistor operates as an amplifier and Ic = .Ib 2. Saturation - the transistor is fully-ON operating as a switch and Ic = I(saturation) 3. Cut-off - the transistor is fully-OFF operating as a switch and Ic = 0Typical Bipolar TransistorThe word Transistor is an acronym, and is a combination of the words Transfer Varistor used to describe their mode of operation way back in their early days of development. 应用集成放大Bipolar Transistor ConfigurationsAs the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output. Each method of connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement.1. Common Base Configuration - has Voltage Gain but no Current Gain. 2. Common Emitter Configuration - has both Current and Voltage Gain. The Common Base (CB) ConfigurationAs its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to both the input signal AND the output signal with the input signal being applied between the base and the emitter terminals. The corresponding output signal is taken from between the base and the collector terminals as shown with the base terminal grounded or connected to a fixed reference voltage point. The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a current gain for this type of circuit of 1 (unity) or less, in other words the common base configuration attenuates the input signal.The Common Base Transistor CircuitThis type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Vout are in-phase. This type of transistor arrangement is not very common due to its unusually high voltage gain characteristics. Its output characteristics represent that of a forward biased diode while the input characteristics represent that of an illuminated photo-diode. Also this type of bipolar transistor configuration has a high ratio of output to input resistance or more importantly load resistance (RL) to input resistance (Rin) giving it a value of Resistance Gain. Then the voltage gain (Av) for a common base configuration is therefore given as:Common Base Voltage GainWhere: Ic/Ie is the current gain, alpha () and RL/Rin is the resistance gain.The common base circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or radio frequency (Rf) amplifiers due to its very good high frequency response.The Common Emitter (CE) ConfigurationIn the Common Emitter or grounded emitter configuration, the input signal is applied between the base, while the output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the normal method of bipolar transistor connection. The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward-biased PN-junction, while the output impedance is HIGH as it is taken from a reverse-biased PN-junction.The Common Emitter Amplifier CircuitIn this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as Ie = Ic + Ib. Also, as the load resistance (RL) is connected in series with the collector, the current gain of the common emitter transistor configuration is quite large as it is the ratio of Ic/Ib and is given the Greek symbol of Beta, (). As the emitter current for a common emitter configuration is defined as Ie = Ic + Ib, the ratio of Ic/Ie is called Alpha, given the Greek symbol of . Note: that the value of Alpha will always be less than unity.Since the electrical relationship between these three currents, Ib, Ic and Ie is determined by the physical construction of the transistor itself, any small change in the base current (Ib), will result in a much larger change in the collector current (Ic). Then, small changes in current flowing in the base will thus control the current in the emitter-collector circuit. Typically, Beta has a value between 20 and 200 for most general purpose transistors.By combining the expressions for both Alpha, and Beta, the mathematical relationship between these parameters and therefore the current gain of the transistor can be given as:Where: Ic is the current flowing into the collector terminal, Ib is the current flowing into the base terminal and Ie is the current flowing out of the emitter terminal.Then to summarise, this type of bipolar transistor configuration has a greater input impedance, current and power gain than that of the common base configuration but its voltage gain is much lower. The common emitter configuration is an inverting amplifier circuit resulting in the output signal being 180o out-of-phase with the input voltage signal.The Common Collector (CC) ConfigurationIn the Common Collector or grounded collector configuration, the collector is now common through the supply. The input signal is connected directly to the base, while the output is taken from the emitter load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. The emitter follower configuration is very useful for impedance matching applications because of the very high input impedance, in the region of hundreds of thousands of Ohms while having a relatively low output impedance.The Common Collector Transistor CircuitThe common emitter configuration has a current gain approximately equal to the value of the transistor itself. In the common collector configuration the load resistance is situated in series with the emitter so its current is equal to that of the emitter current. As the emitter current is the combination of the collector AND the base current combined, the load resistance in this type of transistor configuration also has both the collector current and the input current of the base flowing through it. Then the current gain of the circuit is given as:The Common Collector Current GainThis type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin and Vout are in-phase. It has a voltage gain that is always less than 1 (unity). The load resistance of the common collector transistor receives both the base and collector currents giving a large current gain (as with the common emitter configuration) therefore, providing good current amplification with very little voltage gain.电子开关While transistors have many uses, one of the less known uses by amateurs is the ability for bipolar transistors to turn things on and off. While there are limitations as to what we can switch on and off, transistor switches offer lower cost and substantial reliability over conventional mechanical relays. In this article, we will review the basic principles for transistor switches using common bipolar transistors. The most commonly used transistor switch is the PNP variety shown in Figure 1. The secret to making a transistor switch work properly is to get the transistor in a saturation state. For this to happen we need to know the maximum load current for the device to be turned on and the minimum HFE of the transistor. For example, if we have a load that requires 100MA of current and a transistor with a minimum HFE of 100, we can then calculate the minimum base current required to saturate the transistor as follows: Minimum base current = 100 MA / 100Minimum base current = 1 MAIn actual practice, it is best to calculate about 30% more current than we will need to guarantee our transistor switch is always saturated. In this case, we will use 1.3 MA. We must also select our supply voltage, so for this example we will use 12 volts. We can now calculate resistor R1 in the circuit as follows:Maximum Current Required = 100MASupply Voltage = 12 Volts While PNP transistors are normally used for a negative ground configuration, it is possible use a NPN transistor if a positive ground configuration is desired as indicated in Figure 2. The calculation of resistor values is identical to the PNP version. However, in the NPN transistor, R1 must be shorted to the positive end of the supply to turn the switch on.While our transistor switch can easily replace many mechanical relays, it does have a few drawbacks. The maximum design current must not be exceeded or the output voltage will be reduced. A short circuit of the output will overheat and destroy the transistor in many cases. Although the transistor is in saturation when turned on, about .3 volts is lost through the collector to the emitter of the transistor. We must also insure that the maximum power dissipation of the transistor is not exceeded. We can calculate the power dissipation by multiplying the current by .3 volts. In the case of 100 MA, the transistor must be able to withstand 30 milliwatts (.3 times .1). Transistor switches are used for a wide variety of applications. Many amateurs will notice that the circuit in Figure 1 is used as the PTT in many transmitter circuits. Transistor switches are commonly used to turn on transmitter circuits, LEDs, cooling fans and even relays. However, when using a transistor to turn on a relay coil, it is very important to use a 1N4001 diode reversed biased in parallel with the relay coil as in Figure 3. This is to prevent the kickback voltage in the reverse polarity from destroying the transistor. This reverse voltage occurs momentarily when the normal current stops flowing through the coil. It