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  • jalves61 10:58 am on March 22, 2020 Permalink | Reply
    Tags: Analog Electronics,   

    Transistor IV curves and Modes of Operation/Biasing 

    In the field of electronics, the most important active device is without a doubt the transistor. A transistor acts as a ON/OFF switch or as an amplifier. It is important to understand the modes of operation for these devices, both voltage controlled (FET) and current controlled (BJT).

    For the MOSFET, the cutoff region is where no current flows through the inversion channel and functions as an open switch. The “Ohmic” or linear region, the drain-source current increases linearly with the drain-source voltage. In this region, the FET is acting as a closed switch or “ON” state. The “Saturation” region is where the drain-source current stays roughly constant despite the drain source voltage increasing. This region has the FET functioning as an amplifier.

    ivcurve

    The image above illustrates that for an enhancement mode FET, the gate-source voltage must be higher than a certain threshold voltage for the device to conduct. Before that happens, there is no channel for charge to flow. From there, the device enters the linear region until the drain-source voltage is high enough to be in saturation.

    DC biasing is an extremely important topic in electronics. For example, if a designer wishes for the transistor to operate as an amplifier, the FET must stay within the saturation region. To achieve this, a biasing circuit is implemented. Another condition which effects the operating point of the transistor is temperature, but this can be mitigated with a DC bias circuit as well (this is known as stabilization). “Stability factor” is a measure of how well the biasing circuit achieves this effect. Biasing a MOSFET changes its DC operating point or Q point and is usually implemented with a simple voltage divider circuit. This can be done with a single DC voltage supply.  The following voltage transfer curve shows that the MOSFET amplifies best in the saturation region with less distortion than the triode/ohmic region.

    output

     
  • jalves61 12:31 pm on March 12, 2020 Permalink | Reply
    Tags: Analog Electronics,   

    Advanced Electronics and Optoelectronics: The MESFET 

    One of the more common FET transistor typologies is the MESFET (Metal Semiconductor field effect transistor). This active device is the oldest FET device concept. The MESFET is similar in  structure to a JFET (Junction Field effect transistor) but includes a Schottky junction instead of a P-N junction.

    The MESFET’s channel depends on three parameters: the velocity of the charge carriers, the density of these charge carriers, and the geometric cross section the carriers flow through. The gate electrode is connected directly to the semiconductor material, creating a Schottky diode. The MESFET is generally constructed from the compound semiconductor GaAs (Gallium Arsenide) to provide higher electron mobility. As shown, the substrate is semi-insulating to decrease parasitic capacitance.

    MESFET

    The device works by limiting the electron flow from source to drain, similar to a JFET. The Schottky diode controls the resistance of the channel (size of depletion region). Varying the voltage across the Schottky gate changes the channel size. Similar to other FETs, there is a certain pinch off voltage that causes the current to be very small, making the MESFET a switch or variable resistor. MESFETs can be depletion mode or enhancement mode. The MESFET is often used in high frequency wireless communication devices such as cell phones or military radars.

    (All information and photos obtained from “High Speed Electronics and Optoelectronics Devices and Circuits” by Sheila Prasad)

     
  • mbenkerumass 6:00 am on January 31, 2020 Permalink | Reply
    Tags: Analog Electronics   

    Common Emitter Amplifier 

    cea1

    The common emitter amplifier accepts AC signal inputs and amplifies the entire AC input signal. For this circuit to work however, the common emitter amplifier requires biasing to operate between the minimum and maximum peak values on the input signal. It is also necessary to keep the transistor operating in active mode.

    In amplifier design, minimizing distortion is a major issue. The Q-point or quiescent operating point of an amplifier is the DC operating current or voltage at the transistor with no input signal supplied. The Q-point for a transistor is typically half of the supply.

     

    Voltage Divider Biasing

    To achieve correct biasing, R1 and R2 must be chosen to maintain the base voltage at the transistor at a constant level. The base voltage VB is a function of the supply voltage and the two resistors at the base of the transistor.

    cea2

    Once the amplifier is properly biased, the voltage gain calculation is shown below. An important note is that the gain of the circuit is different for low frequencies than it is for high frequencies and the gain is then a function of the load resistance and the internal resistance of the transistor. Coupling capacitors C1 and C2 are used to separate the AC input signal from the DC biasing voltage.

    cea3

     
  • mbenkerumass 6:00 am on January 29, 2020 Permalink | Reply
    Tags: Analog Electronics,   

    Diode Voltage Clipping Circuits 

    We’re already discussed the PN junction in a previous post. Let’s explore some of the applications of the PN diode.

    diode2

    It was already discussed that due to the nature of the PN junction, current is only allowed to flow in one direction. This results in two possible scenarios using a diode, depending on the direction it is facing with respect to the source.

    diode1

    Diode Clipping Circuits

    Diodes in a forward bias allow current to pass, but thereby reducing the voltage level. In a reverse bias, current is stopped but the voltage remains unaffected.

    Diode clipping can be done for either the positive or the negative voltage of a sinusoidal (or analog) input voltage. In order to clip both the positive and negative sides of the input voltage, two diodes are needed.

    Positive voltage clipping:

    d1

    Negative voltage clipping:

    d2

    Two Diodes:

    d3

    If 0.7 Volts is not the desired output clipping voltage, a bias voltage can be added in each situation above.

    d4d5

     
  • mbenkerumass 6:19 am on January 25, 2020 Permalink | Reply
    Tags: Analog Electronics   

    Common Collector BJT Amplifier 

    cca1

    The common collector amplified is often described as a voltage follower, because it has a gain of approximately 1. The internal resistance of a BJT is quite small and without a loading resistance on the collector, no gain is achieved.

    cca2

    cca3

     
  • mbenkerumass 6:00 am on January 22, 2020 Permalink | Reply
    Tags: Analog Electronics   

    Review of OP-AMPs 

    Operational Amplifiers perform a number of different tasks such as amplification, filtering and performing mathematical operations. This is a review of a number of concepts related to OP-AMPS.

    Open Loop and Closed Loop Gain

    An op-amp’s amplification (open loop gain) without any feedback in the system is usually very large (~100,000 x). The system without any feedback is referred to as an open loop system. A closed loop system provides feedback to the system.

    Capture1

    The feedback in a system is determined by the impedances of the feedback loop and the input of the system. Furthermore, the op-amp can function as a non-inverting op-amp or an inverting op-amp, producing a system of positive or negative feedback respectively.

    Capture2Capture3

    It must be said however that the input and feedback impedances must not be only real-values resistances. The following system may easily make use of a complex impedance, and other RC combinations to produce a filter modeled by the transfer function.

    Capture4

     

    Mathematical operations using OP-Amps

    Below are the following Op-Amps:

    • Summing Amplifier
    • Comparator Amplifier
    • Differentiator
    • Integrator
    • Low-Pass Filter example
    • Subtractor
    • Voltage Follower/Buffer

    sumop

    compopdiffopintegratorlowpsubopvfol

     
  • mbenkerumass 6:00 am on January 18, 2020 Permalink | Reply
    Tags: Analog Electronics   

    Common Source JFET Amplifier 

    The advantages of using a Field Effect Transistor were discussed previously. The design of an amplifier using a junction field effect transistor (or another FET such as MOSFET) is generally the same procedure as that done for a BJT transistor amplifier.

    csa1csa2

     
  • mbenkerumass 4:50 am on April 30, 2019 Permalink | Reply
    Tags: Analog Electronics, Multisim   

    Differential Amplifier 

    This lab demonstrates the rejection of common-mode noise while amplifying differential-mode signals. This is the final circuit in Multisim.

    fulldifferentialamplifiercircuit

    The circuit is comprised of one oscillator, one inverting amplifier, two weighted summers and one differential amplifier.

    This is a screen capture of the noise disconnected.

    differentialamplifieroutput

     

     
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