R (a)The biased diode in the feedback path provides(parallel) (b)The output clamps at E volts when the input reacher E R c)Transfer characteristic IGURE 5.13 An operational amplifier limiting circuit. Source: T.E. Bogart, Jr, Electronic Devices and Circuits, 3rd ed, Columbus, Ohio: Macmillan/Merrill, 1993, P 693. With permission.) are in many respects functionally equivalent to biased diodes. Figure 5. 14 shows two operational amplifier lipping circuits using Zener diodes. The Zener diode conducts like a conventional diode when it is forward biased, so it is necessary to connect a reversed diode in series with it to prevent shorting of R, When the reverse voltage across the Zener diode reaches V the diode breaks down and conducts heavily, while maintaining an entially constant voltage, Vo across it. Under those conditions, the total voltage across R, ie, vo equals V plus the forward drop, Vp across the conventional diode Figure 5.15 shows double-ended limiting circuits, in which both positive and negative peaks of the output waveform are clipped. Figure 5. 15(a)shows the conventional parallel clipping circuit and(b)shows how double ended limiting is accomplished in an operational amplifier circuit. In each circuit, note that no more than one diode is forward biased at any given time and that both diodes are reverse biased for -E< v.< E, the linear region. Figure 5. 16 shows a double-ended limiting circuit using back-to-back Zener diodes. Operation is similar that shown in Fig. 5. 14, but no conventional diode is required. Note that diode D, is conducting in a forware direction when D, conducts in its reverse breakdown(Zener)region, while D, is forward biased when D conducting in its reverse breakdown region. Neither diode conducts when -(Vn+0.7)< vo<(Va +0.7), which is the region of linear amplifier operation. Precision Rectifying Circuits A rectifier is a device that allows current to pass through it in one direction only. a diode can serve as a rectifier because it permits generous current flow in only one direction-the direction of forward bias Rectification is the same as limiting at the 0-V level: all of the waveform below (or above)the zero-axis is eliminated. However, a diode rectifier has certain intervals of nonconduction and produces resulting "gaps"at the zero-crossing points of the output voltage, due to the fact that the input must overcome the diode drop(0.7 V for silicon) before e 2000 by CRC Press LLC
© 2000 by CRC Press LLC are in many respects functionally equivalent to biased diodes. Figure 5.14 shows two operational amplifier clipping circuits using Zener diodes. The Zener diode conducts like a conventional diode when it is forward biased, so it is necessary to connect a reversed diode in series with it to prevent shorting of Rf . When the reverse voltage across the Zener diode reaches VZ, the diode breaks down and conducts heavily, while maintaining an essentially constant voltage, VZ, across it. Under those conditions, the total voltage across Rf , i.e., vo, equals VZ plus the forward drop, VD, across the conventional diode. Figure 5.15 shows double-ended limiting circuits, in which both positive and negative peaks of the output waveform are clipped. Figure 5.15(a) shows the conventional parallel clipping circuit and (b) shows how doubleended limiting is accomplished in an operational amplifier circuit. In each circuit, note that no more than one diode is forward biased at any given time and that both diodes are reverse biased for –E1 < vo < E2, the linear region. Figure 5.16 shows a double-ended limiting circuit using back-to-back Zener diodes. Operation is similar to that shown in Fig. 5.14, but no conventional diode is required. Note that diode D1 is conducting in a forward direction when D2 conducts in its reverse breakdown (Zener) region, while D2 is forward biased when D1 is conducting in its reverse breakdown region. Neither diode conducts when –(VZ2 + 0.7) < vo < (VZ1 + 0.7), which is the region of linear amplifier operation. Precision Rectifying Circuits A rectifier is a device that allows current to pass through it in one direction only. A diode can serve as a rectifier because it permits generous current flow in only one direction—the direction of forward bias. Rectification is the same as limiting at the 0-V level: all of the waveform below (or above) the zero-axis is eliminated. However, a diode rectifier has certain intervals of nonconduction and produces resulting “gaps” at the zero-crossing points of the output voltage, due to the fact that the input must overcome the diode drop (0.7 V for silicon) before FIGURE 5.13 An operational amplifier limiting circuit. (Source: T.F. Bogart, Jr., Electronic Devices and Circuits, 3rd ed., Columbus, Ohio:Macmillan/Merrill, 1993, p. 693. With permission.)
v (a) Positive lim v。 R IGURE 5 14 Operational amplifier limiting circuits using Zener diodes. Source: T.R. Bogart, Jr, Electronic Devices and Circuits, 3rd ed, Columbus, Ohio: Macmillan/Merrill, 1993, P 694. With permission.) conduction begins In power-supply applications, where input voltages are quite large, these gaps are of no concern. However, in many other applications, especially in instrumentation, the 0.7-V drop can be a significant portion of the total input voltage swing and can seriously affect circuit performance. For example, most struments rectify ac inputs so they can be measured by a device that responds to dc levels. It is obvious that nall ac signals could not be measured if it were always necessary for them to reach 0.7 V before rectifica could begin. For these applications, precision rectifiers are Figure 5. 17 shows one way to obtain precision rectification using an operational amplifier and a diode. The circuit is essentially a noninverting voltage follower(whose output follows, or duplicates, its input)when the diode is forward biased. When vin is positive, the output of the amplifier, v, is positive, the diode is forward biased, and a low-resistance path is established between v, and v, as necessary for a voltage follower. The load voltage, Vp then follows the positive variations of vin =v*. Note that even a very small positive value of vin will cause this result, because of the large differential gain of the amplifier. That is, the large gain and the action of the feedback cause the usual result that w* a v. Note also that the drop across the diode does not appear in vu nd the dio biased. This ef vely c the feedback loop, so v no longer follows vin. The amplifier itself, now operating open-loop, is quickly drig to its maximum negative output, thus holding the diode well into reverse bias Another precision rectifier circuit is shown in Fig. 5. 18. In this circuit, the load voltage is an amplified and inverted version of the negative variations in the input signal, and is 0 when the input is positive. Also in contrast with the previous circuit, the amplifier in this rectifier is not driven to one of its output extremes. When vn is negative, the amplifier output, v, is positive, so diode d, is reverse biased and diode D, is forward biased. D, is open and D, connects the amplifier output through R, to v. Thus, the circuit behaves like an ordinary inverting amplifier with gain -R/R The load voltage is an amplified and inverted(positive)version of the negative variations When vin becomes positive, v, is negative, D, is forward biased, and D2 is reverse biased. D, shorts the output v, to v, which is held at virtual ground, so v, is 0 e 2000 by CRC Press LLC
© 2000 by CRC Press LLC conduction begins. In power-supply applications, where input voltages are quite large, these gaps are of no concern. However, in many other applications, especially in instrumentation, the 0.7-V drop can be a significant portion of the total input voltage swing and can seriously affect circuit performance. For example, most ac instruments rectify ac inputs so they can be measured by a device that responds to dc levels. It is obvious that small ac signals could not be measured if it were always necessary for them to reach 0.7 V before rectification could begin. For these applications, precision rectifiers are necessary. Figure 5.17 shows one way to obtain precision rectification using an operational amplifier and a diode. The circuit is essentially a noninverting voltage follower (whose output follows, or duplicates, its input) when the diode is forward biased. When vin is positive, the output of the amplifier, vo , is positive, the diode is forward biased, and a low-resistance path is established between vo and v–, as necessary for a voltage follower. The load voltage, vL, then follows the positive variations of vin = v+. Note that even a very small positive value of vin will cause this result, because of the large differential gain of the amplifier. That is, the large gain and the action of the feedback cause the usual result that v+ ' v –. Note also that the drop across the diode does not appear in vL. When the input goes negative, vo becomes negative, and the diode is reverse biased. This effectively opens the feedback loop, so vL no longer follows vin. The amplifier itself, now operating open-loop, is quickly driven to its maximum negative output, thus holding the diode well into reverse bias. Another precision rectifier circuit is shown in Fig. 5.18. In this circuit, the load voltage is an amplified and inverted version of the negative variations in the input signal, and is 0 when the input is positive.Also in contrast with the previous circuit, the amplifier in this rectifier is not driven to one of its output extremes. When vin is negative, the amplifier output, vo , is positive, so diode D1 is reverse biased and diode D2 is forward biased. D1 is open and D2 connects the amplifier output through Rf to v –. Thus, the circuit behaves like an ordinary inverting amplifier with gain –Rf /R1. The load voltage is an amplified and inverted (positive) version of the negative variations in vin. When vin becomes positive, vo is negative, D1 is forward biased, and D2 is reverse biased. D1 shorts the output vo to v–, which is held at virtual ground, so vL is 0. FIGURE 5.14 Operational amplifier limiting circuits using Zener diodes. (Source: T.F. Bogart, Jr., Electronic Devices and Circuits, 3rd ed., Columbus, Ohio: Macmillan/Merrill, 1993, p. 694. With permission.)
R -E E2中E E, a) Double-ended parallel clipper E E (b)Operational amplifier with double-ended clipping FIGURE 5.15 Double-ended clipping, or limiting. ( Source: T.F. Bogart, Jr, Electronic Devices and Circuits, 3rd ed, Colun bus, Ohio: Macmillan/Merrill, 1993, P 695. With permission. -AA Vn+0.7)↓ FIGURE 5.16 A double-ended limiting circuit using Zener diodes. Source: T F. Bogart, Jr, Electronic Devices and Circuits, 3rd ed, Columbus, Ohio: Macmillan/Merrill, 1993, P 695. With permission. Defining Terms Biased diode: A diode connected in series with a dc voltage source in order to establish a clipping level. Clipping occurs when the voltage across the combination is sufficient to forward bias the diode Limiter: A device or circuit that restricts voltage excursions to prescribed level(s). Also called a clipping circuit. Related Topics 5.1 Diodes and Rectifiers 27.1 Ideal and Practical Models e 2000 by CRC Press LLC
© 2000 by CRC Press LLC Defining Terms Biased diode: A diode connected in series with a dc voltage source in order to establish a clipping level. Clipping occurs when the voltage across the combination is sufficient to forward bias the diode. Limiter: A device or circuit that restricts voltage excursions to prescribed level(s). Also called a clipping circuit. Related Topics 5.1 Diodes and Rectifiers • 27.1 Ideal and Practical Models FIGURE 5.15 Double-ended clipping, or limiting. (Source: T.F. Bogart, Jr., Electronic Devices and Circuits, 3rd ed., Columbus, Ohio: Macmillan/Merrill, 1993, p. 695. With permission.) FIGURE 5.16 A double-ended limiting circuit using Zener diodes. (Source: T.F. Bogart, Jr., Electronic Devices and Circuits, 3rd ed., Columbus, Ohio: Macmillan/Merrill, 1993, p. 695. With permission.)