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  • mbenkerumass 7:00 am on January 11, 2020 Permalink | Reply
    Tags: , Transistors   

    BJT vs. FET 

    Transistors are important components that are used in a variety of applications. Some types can be used for switching, some for amplification or both. Other transistors perform exclusive tasks, such as the phototransistor, which responds to light by producing a current.

    The main premise of a transistor is that by feeding a transistor a source voltage or current (depending on the type), the transistor allows for the passage of electrons. This process is accomplished through pnp or npn semiconductor structures. The following diagrams provide a general example of the function of a transistor:

    t4t2

    transistor

    t3

     

     

    Bipolar Junction Transistors (BJT) are controlled using a biasing current at the base pin. This means that they will also consume more current than other transistors such as the FET. One advantage of BJT transistors is that they offer greater output gain than an FET. However, BJT can be much larger in size than FET and for this reason, they are less popular, despite being easier to manufacture.

    bjt

    Field Effect Transistors (FET) are voltage-controlled. For this reason they essentially draw no current and therefore do not pose a substantial load to a circuit. FETs are not as useful for gain as BJT, however if the intent is not for amplification then this is not a problem. FETs can be manufactured very small and this is important in manufacturing integrated circuits that use many transistors. FETs and especially the MOSFET subtype are more expensive to manufacture, but remain more popular than the BJT.

    fet

    Some FET transistor types are even constructed on the nano-scale. The FinFET for example is about 10 nm, currently used by Intel, Samsung and others.

    FinFET size

    (2)

    http://www.learningaboutelectronics.com/Articles/Types-of-transistors.php

     
  • jalves61 7:36 pm on January 10, 2020 Permalink | Reply
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    Quarter Wave Transformer Matching – Using Theory of Multiple Reflections 

    There are two ways to derive an impedance value for a quarter wave transformer line. The transformer is an excellent tool to match a characteristic impedance to a purely resistive load where a large bandwidth is not required. It is much easier to find this relationship by examining it from an impedance viewpoint, however the theory of multiple reflections is an excellent topic because it illustrates the contribution of multiple impedance lines to the overall reflection coefficient.

    The following circuit with the matching transformer is shown below.

    reflection

    The addition of the matching transformer introduces discontinuity at the first port. Ideally, the addition of the transformer will match the load resistance to Zo, minimizing all reflection, as will be shown. the bottom figure provides a “step by step” analysis of each trip of the wave as it travels. When the wave first hits the Zo and Z1 junction, it sees Z1 as a “load” and does not yet see the actual load resistance. Depending on the impedance match, some of the wave will be reflected and some will be transmitted. The transmitted part of the wave then travels to the load and a portion is again reflected with reflection coefficient 3. As that portion of the wave travels back to the Z1 and Zo junction, the process repeats. This process continues infinitely and results in the following equation.

    gamme

    Using the definition of a geometric series and writing the reflection coefficient in terms of impedance, the equation reduces to

    reduce.PNG

    The reflection is seen to reduce to zero when Z1 (the impedance of the quarter wave section of transmission line) is set to q.PNG

     
  • jalves61 12:00 am on January 9, 2020 Permalink | Reply
    Tags: Antennas   

    Distributed Antenna Systems 

    Distributed antenna systems (DAS) provide a convenient, power efficient way to move RF signals within buildings (iDAS or indoor DAS) or in outdoor places such as stadiums or venues (oDAS or outdoor DAS). A DAS consists of two main parts: a signal source and a distribution system. The signal source can be an outdoor antenna or a local base station.

    DAS.jpg

    The principle idea is this: to replace a single, high power antenna with several power efficient antennas without losing any area coverage. There are many types of distribution systems. The main idea for each system is to propagate the signals in such as way that maximum power and signal coverage is utilized. In one architecture, a master unit is connected to the base station using RF coaxial cable. The frequency of this energy is then increased to the optical range and carried using fiber optic cables to each remote unit on each floor of the building. The energy is then reduced down to the RF frequency and passive splitters are used to distribute the signals to each antenna. Architectures can be purely passive, purely active or a hybrid combination of both. DAS is advantageous for increased coverage but can result in higher costs due to increased infrastructure required.

     
  • mbenkerumass 6:00 am on January 8, 2020 Permalink | Reply
    Tags: ,   

    Microstrip Zig-Zag Passband Filter 

    zzf1zzf2zzf3

     
  • mbenkerumass 9:00 am on January 7, 2020 Permalink | Reply
    Tags: ,   

    Planar Boundaries, Total Internal Reflection, Beamsplitters 

    Refraction is an important effect in ray optics. The refractive index of a material influences how rays react when entering or leaving a boundary. For instance, if the ray is exiting a medium of smaller refractive index and entering a medium with a higher refractive index, the angle will tend towards being perpendicular to the boundary line. The angle of refraction is also greater than the angle of incidence. This case is called external refraction (n1 < n2) and (θ1 > θ2). If the ray is exiting a medium of higher refractive index into a medium with a lower refractive index, the rays will tend towards being closer to parallel with the medium boundary. This case is referred to as internal refraction (n1 > n2) and (θ2 > θ1). Both of these situations are governed by Snell’s Law:

    n1*sin(θ1) = n2*sin(θ2)

    When the rays are paraxial, the relation between θ1 and θ2 is linear (n1*θ1 = n2*θ2).

    refraction

    The critical angle occurs when n1*sin(θ1) = n2*sin(pi/2) = n2. θ1 in this case is then equal to the critical angle. If θ1 is greater than the critical angle θC, refraction cannot occur and the situation is characterized by a phenomenon known as total internal reflection (TIR). Total internal reflection is the basis for many optical systems and devices. Systems with total internal reflection are understood to be highly efficient even under more rigorous approaches to optics such as electromagnetic optics.

    tir

     

    Prisms are common applications of refraction. A prism of apex angle α and refractive index n deflects a ray incident at an angle of θ:

    prism2

    This is taken by using Snell’s law twice along two planar boundaries.

    prism1

     

    A beamsplitter is an optical component that divides a ray into a reflected and refracted (or transmitted) ray. The proportions of reflected to refractive light is a problem dealt with in electromagnetic optics. Beamsplitters are also used to combine two rays.

    beamsplit

    Beam directors apply Snell’s law and the rules governing refraction to direct rays in different directions. Three methods of directing waves are the biprism, the Fresnel biprism and the axicon.

     

     

     
  • jalves61 12:00 am on January 6, 2020 Permalink | Reply
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    Sound – ADSR Envelope 

    In terms of sound and music, the ADSR envelope describes how sound changes over time. In terms of physics of wave, a general envelope outlines the extreme points (max and mins) of a wave through a smooth curve.

    It is obvious to the human ear that when a musical instrument is played, its volume (amplitude) changes over time. For example when a guitar string is plucked, the string vibrates and the initial amplitude is high. After a brief period, the sound amplitude decays. Different musical instruments will have different ADSR envelopes to describe their sound characteristics.

    The “Attack” phase of the sound refers to how quickly a sound reaches its maximum amplitude. This is the initial phase of the sound. For most instruments, this period is extremely short (almost instantaneous). The next phase is the “Decay” phase, or the time the note takes to drop to the sustain level. The “sustain” level is generally the longest portion of time, and this is when the amplitude envelope stays relatively constant. The “release” is the period of time the sound takes to go from the sustain level to zero amplitude and is generally short.

    ADSR

     
  • mbenkerumass 6:00 am on January 5, 2020 Permalink | Reply
    Tags: ,   

    Microstrip Stepped Impedance Low-Pass Filter 

    Passes Freqeuncies below 10 GHz:

     

    steplpfilter1steplpfilter2steplpfilter3

     
  • mbenkerumass 6:00 am on January 4, 2020 Permalink | Reply
    Tags: ,   

    The P-N Junction 

    A P-N junction is created in a single semiconductor crystal by doping one side as a p-type and one as an n-type. The region where the two types converge is known as the p-n junction.

    The extra electrons that were added to the n-type semiconductor move towards the p-type junction side while the holes added through p-type doping are positioned closer to the n-type junction.

    pnj2

    As electrons leave the n-type region, it becomes positively charged. This process is called diffusion. The depletion region is the area between the p and n-type sides. The state of equilibrium in the p-n junction is the state of the depletion region without any external electrical potential applied. As mentioned before in a previous paper, the Fermi level is the average between the conduction band and the valence band. By altering the levels of electron holes and electrons in the p-type and n-type sections, holes drift toward the the n-type side and electrons move towards the p-type side, which causes both sections to be closer to the Fermi level in their regions of the material.

    pnj

    When voltage is applied to the pn junction, electrons and electron holes from either side tend towards equilibrium. If the positive potential is applied to the p-type and it is more positive than the n-type area, holes will travel towards the negative voltage. Through diffusion, electrons or electron holes may jump through the depletion layer. For the reason however that electron holes (positive charge) may only move in the direction of the n-type region and electrons (negative charge) may only move in the opposite direction. The direction of electron flow, due to their negative charge is opposite the conventional direction of current flow. Since electrons are only moving from the n-type region to the p-type region, it can be understood that current will only move in the direction going from the side of the p-type region towards the n-type region.pnj1

     

     
  • jalves61 12:00 am on January 3, 2020 Permalink | Reply
    Tags: Probability   

    Random Variables, PDFs and CDFs 

    The concept of probability is very important in the field of electrical engineering, where outcomes can be nondeterministic. In a nondeterministic outcome, an experiment can be repeated multiple times and have different outcomes. For example, in a communication link messages may not be delivered the same way each time. Another example would be the failure of manufactured parts even within a scheduled lifetime.

    A random variable is defined as a function that maps a real number to an outcome within a sample space (a set that contains all possible outcomes of a random experiment). The “real number” is sometimes called an observation. The range includes all the possible observations and the domain includes all of the possible outcomes of the experiment. A single random variable produces a single observation. Random variables are notated by capital letters generally towards the end of the alphabet (eg. X(a), U(a), etc).

    RV.png

    A useful function for describing probability of random variables is the Cumulative Distribution function (CDF). This function is defined as the probability that the random variable is less than a certain value, x.

    CDF.PNG

    Setting x = infinity shows that the CDF should equal to one. This is equivalent to the probability of the sure event (which is always one because it is the probability of the entire sample space). Setting x = 0 gives a probability of 0 (probability of the null event is always zero). The value of the CDF must always be between these two values and must never decrease. The CDF can be discontinous (in the case of discrete random variables) as well as continuous (in the case of continuous random variables).

    Finding the probability between two values is easily obtained from the CDF.

    CDFprob.PNG

    Taking the derivative of the CDF leads to the PDF (probability density function).

    pdfcdf.PNG

    This shows that the PDF and CDF are inverse functions. Alternatively, the PDF can be defined as

    pdf.PNG

    This is because the area under the PDF is what determines probability. The total integration of the PDF must equal one.

     
  • mbenkerumass 9:00 am on January 2, 2020 Permalink | Reply
    Tags:   

    P-Type and N-Type Semiconductors 

    N-Type Semiconductors are created when doping a semiconductor with impurities that adds extra valence electrons to the outermost shell to share free electrons with neighboring atoms. Phosphorous, arsenic and antimony are examples of atoms with five valence electrons, also known as pentavalent impurities, adding an extra electron for each doped atom. This does not mean however that an N-type semiconductor is negatively changed, because there will exist a balancing positive charge in the nucleus of the doped atom. An N-type semiconductor is a better conductor than intrinsic semiconductor materials.

    P-Type Semiconductors are formed by adding group 3 elements, known as trivalent impurity atoms such as boron, aluminum and indium to the semiconductor structure. These atoms have only three electrons in the outermost shell, producing an extra electron hole, which attracts neighboring electrons.

    p-type and n-type semiconductors

    To recap, N-type semiconductors:

    • possess pentavalent elements as impurity atoms to add a donor electron to the material.
    • do not have a negative charge since atom nucleus charge offsets added electrons, meaning they are electrically neutral.

    P-type semiconductors:

    • possess trivalent impurity elements as impurity atoms to add an electron hole.
    • are also electrically neutral.

     

     
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