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ENTC 3320. Absolute Value. Half-Wave Rectifier. The inverting amplifier is converted into an ideal (linear precision) half-wave rectifier by adding two diodes. . - 0.6 V. 0 V.

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Entc 3320

ENTC 3320

Absolute Value

Half wave rectifier
Half-Wave Rectifier

  • The inverting amplifier is converted into an ideal (linear precision) half-wave rectifier by adding two diodes.

- 0.6 V

0 V

Entc 3320

  • When E, is positive, diode D1 conducts, causing the op amp’s output voltage, VOA,to go negative by one diode drop (-0.6 V).

    • This forces diode D2 to be reverse biased.

      • The circuit’s output voltage Vo equals zero because input current I flows through D1.

      • For all practical purposes, no current flows through Rf and therefore Vo = 0.

Entc 3320

Entc 3320

Entc 3320

  • This causes D2 to conduct.

    • The circuit then acts like an inverter, since Rf = Ri, and Vo =+E1.

      • Since the (—) input is at ground potential, diode D1 is reverse biased.

      • Input current is set by E/Ri and gain by ─Rf/Ri.

      • Remember that this gain equation applies only for negative inputs, and Vo can only be positive or zero.

Entc 3320

Entc 3320

  • The most important property of this linear half-wave rectifier will now be examined.

  • An ordinary silicon diode or even a hot-carrier diode requires a few tenths of volts to become forward biased.

    • Any signal voltage below this threshold voltage cannot be rectified.

    • However, by connecting the diode in the feedback loop of an op amp, the threshold voltage of the diode is essentially eliminated.

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  • For example. in Fig. 7-2(b) let rectifier will now be examined. E, be a low voltage of —0.1 V. E, and R, convert this low voltage to a current that is conducted through D2.

    • VOA goes to whatever voltage is required to supply the necessary diode drop plus the voltage drop across R~. Thus millivolts of input voltage can be rectified, since the diode’s forward bias is supplied automatically by the negative feedback action of the op amp.

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  • Finally, observe the waveshape of op amp output rectifier will now be examined. V~ in Fig. 7-3. When E~ crosses 0 V (going negative), V(~ jumps quickly from —0.6 V to +0.6 V as it switches from supplying the drop for D2 to supplying the drop for D1. This jump can be monitored by a differentiator to indicate the zero crossing. During the jump time the op amp operates open loop.

Entc 3320

Entc 3320

  • The output voltage rectifier will now be examined. Vo equals 0 V for all negative inputs.

    • Circuit operation is summarized by the plot of V~ and VOA versus E.

7 1 4 signal polarity separator
7-1.4 Signal Polarity Separator rectifier will now be examined.

  • The following circuit is an expansion of the previous circuits.

    • When E, is positive, diode D1 conducts and an output is obtained only on output V0,.

      • V0, is bound at 0 V.

    • When E, is negative. D2 conducts, V0. = —(—E,) = +E,. and V0~ is bound at 0 V.

Introduction waveshapes.

  • The precision full-wave rectifier transmits one polarity of the input signal and inverts the other.

    • Thus both half-cycles of an alternating voltage are transmitted but are converted to a single polarity of the circuirs output.

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  • For example, the absolute values of |+2 | and | shown below.2 | are +2.

    • The symbol | | means “absolute value of.”

      • In a precision rectifier circuit the output is either negative or positive, depending on how the diodes are installed.

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  • Three types of precision rectifiers will be presented. shown below.

    • The first is inexpensive because it uses two op amps. two diodes, and five equal resistors.

      • Unfortunately. it does not have high input resistance.

    • ~o a second type is given that does have high input resistance but requires resistors that are precisely proportioned but not all equal.

      • Neither type has a summing node at virtual ground potential.

Full wave precision rectifier with equal resistors
Full-wave precision rectifier with equal resistors. shown below.

  • Thefirst type of precision full-wave rectifier or absolute-value circuit is shown below.

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  • Figure resistance equal to 7-8(a) shows current directions and voltage polarities for po~iti~e input signals. Diode D~ conducts so that both amps A and B act as inverters, and V0 = +E,.

    • Figure 7-8(b) shows that for negative input voltages, diode D.~ conducts. Input rent I divides as shown, so that op amp B acts as an inverter. Thus output voltage V0 positive for either polarity of input E, and V0 is equal to the absolute value of E,