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Operational Amplifier (Op-Amp) Basics The op-amp is basically a differential amplifier having a large voltage gain, very high input impedance

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  1. #1 2008-11-15,11:21 AM
    scorpion1988 غير متواجد حالياً عضو
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    Operational Amplifier (Op-Amp) Basics

    The op-amp is basically a differential amplifier having a large voltage gain, very high input impedance and low output impedance. The op-amp has a "inverting" or (-) input and "non inverting" or (+) input and a single output. The op-amp is usually powered by a dual polarity power supply in the range of +/- 5 volts to +/- 15 volts. A simple dual polarity power supply is shown in the figure below which can be assembled with two 9 volt batteries.

    Inverting Amplifier:

    The op-amp is connected using two resistors RA and RB such that the input signal is applied in series with RA and the output is connected back to the inverting input through RB. The non inverting input is connected to the ground reference or the center tap of the dual polarity power supply. In operation, as the input signal moves positive, the output will move negative and visa versa. The amount of voltage change at the output relative to the input depends on the ratio of the two resistors RA and RB. As the input moves in one direction, the output will move in the opposite direction, so that the voltage at the inverting input remains constant or zero volts in this case. If RA is 1K and RB is 10K and the input is +1 volt then there will be 1 mA of current flowing through RA and the output will have to move to -10 volts to supply the same current through RB and keep the voltage at the inverting input at zero. The voltage gain in this case would be RB/RA or 10K/1K = 10. Note that since the voltage at the inverting input is always zero, the input signal will see a input impedance equal to RA, or 1K in this case. For higher input impedances, both resistor values can be increased.

    Non inverting Amplifier:

    The non inverting amplifier is connected so that the input signal goes directly to the non inverting input (+) and the input resistor RA is grounded. In this configuration, the input impedance as seen by the signal is much greater since the input will be following the applied signal and not held constant by the feedback current. As the signal moves in either direction, the output will follow in phase to maintain the inverting input at the same voltage as the input (+). The voltage gain is always more than 1 and can be worked out from V gain = (1+ RB/RA).

    Voltage Follower:

    The voltage follower, also called a buffer, provides high input impedance, a low output impedance, and unity gain. As the input voltage changes, the output and inverting input will change by an equal amount.



    Figure 1: non inverting, inverting, voltage follower

    Though designs vary between products and manufacturers, all op-amps have basically the same internal structure, which consists of three stages:

    1. Differential amplifier

    Input stage — provides low noise amplification, high input impedance, usually a differential output

    2. Voltage amplifier

    Provides high voltage gain, a single-pole frequency roll-off, usually single-ended output

    3. Output amplifier

    Output stage — provides high current driving capability, low output impedance, current limiting and short circuit protection circuitry


    Summing Amplifier

    Figure 2:Summing amplifier circuit.
    A summing amplifier circuit is shown in Figure 2

    1. Show that the output signal of the amplifier is


    2. Build the circuit, and check your prediction experimentally for a gain of 10.

    3. Measure the input impedance of the amplifier by placing various resistors in series with the source. To measure the impedance of one terminal, drive it with a small signal through a resistor and ground the other. Explain your result.


    Integrator

    Figure 3:Integrator circuit.
    An integrator circuit is shown in Figure 3

    1. Show that the output signal of the amplifier is

    2. Build the circuit with k, F and use square and sinusoidal wave forms to test the predicted behavior. Also place an Mresistor in parallel with the capacitor. This resistor drains charge to avoid saturation due to very low frequency or DC signals.



    Differentiator

    Figure 4:Differentiator circuit.
    A differentiator circuit is shown in Figure 4

    1. Show that the output signal of the amplifier is


    2. Build the circuit with k, F and use triangle and sinusoidal wave forms to test the predicted behavior.




    Schmitt Trigger

    Figure 5: Schmitt trigger circuit. and are relative to ground, or some reference between and .
    A Schmitt trigger circuit is shown in Figure 5 The analysis is not difficult. It is, however, tedious. The , voltage divider sets the rough neighborhood of the trigger thresholds. controls the hysteresis of the switch (the difference between the ``turn on'' and ``turn off'' thresholds). The feedback resistor should be a factor 10-100 larger than the voltage divider resistors. Otherwise, it drags the thresholds apart.

    1. Predict the ``turn on'' and ``turn off'' thresholds for k, k, k, and k. Rather than finding a general expression, it's fine to consider this particular case. For the analysis, assume a maximal output voltage swing of V. This actually varies with each op amp, but should not be far from the truth.

    2. Build the circuit, using the resistance values given above. Measure the input thresholds of the trigger and compare with your predictions





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  2. #2 2009-01-23,08:02 PM
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