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    Working with diodes and operational amplifiers

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    D1, D2, D3, D4 together forms a bridge rectifier circuit, which will make the AC voltage output of the transformer to DC (actually, a pulsating DC). They must be identical PN junction diodes.
    If we label the transformer connected points as A and B, (upper A, lower B) and assume an AC waveform, the following process will occur.

    Suppose, the positive peak of the AC Voltage appears at A, the diode D1 will be forward biased (D2 reverse biased) and will conduct the positive part to point D. At the same time, the lower part is at -ve peak, so the diode D3 will conduct (D4 reverse biased) to C. So we will get a rectified output (with C negative and D positive) for this half cycle.
    Now, during the second half cycle, A becomes negative and B becomes positive. The diode D4 is now forward biased and it will give the negative voltage to point C while the diode D2 is forward biased and will conduct the positive voltage to D. Now also, we have a positive voltage at D and negative at C.
    Or, at all times of the AC waveform, we will get a a positive voltage at D and negative at C. So this is called a full wave rectifier. The diodes D1 to D4 constitute a bridge rectifier.

    From the symbol, it is clear that ZD1 is a zener diode. Zener diodes are used for voltage regulation and are always REVERSE biased when voltage regulation is required. So the function of ZD1 is to regulate the output voltage. Output will be equal to the reverse breakdown voltage of the diode, which is called the zener voltage.

    Again from the symbol, as it is emitting light, the type of diode is An LED (Light Emitting Diode). Its purpose is to indicate the presence of an output voltage. It will light up when the voltage is present at the output.

    (The small boxes are current limiting resistors; they prevent the current in the circuit from shooting to a high value, there by protecting the zener diode and LED)


    a) Inverting Amplifier:

    Note that the input is connected to the inverting terminal of the OPAMP. The extra resistor RB is a current bias-compensation resistor. It reduces the current bias by eliminating non-zero current at the inputs.

    b) Non-inverting Amplifier

    Here the input is connected to the non inverting terminal of the opamp.

    c) Summing amplifier

    Output signal of the amplifier is
    d) Difference amplifier

    Differential amplifier amplifies the difference between two input signals (-) and (+). This amplifier is also referred to as a differential-input single-ended output amplifier.

    e) Voltage follower

    Output voltage just follows the input (or the output voltage is the same as the input.)

    Additional info is available from this site: http://en.wikipedia.org/wiki/Operational_amplifier_applications

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