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ENTC 3320
Absolute Value
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Half-Wave Rectifier
The inverting amplifier is converted into an ideal (linear precision) half-wave rectifier by adding two diodes.
0 V- 0.6 V
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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.
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Note the load is modeled by a resistor RL and must always be resistive. • If the load is a capacitor, inductor, voltage, or
current source, then V0 will not equal zero.
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The negative input E, forces the op amp output VOA to go positive.
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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.
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Circuit operation is summarized by the following waveshapes.
Vo can only go positive in a linear response to negative inputs.
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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 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 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.
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The diodes can be reversed as shown below. • Now only positive input signals are
transmitted and inverted.
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The output voltage Vo equals 0 V for all negative inputs. • Circuit operation is
summarized by the plot of V~ and VOA versus E.
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7-1.4 Signal Polarity Separator
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.
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This circuit’s operation is summarized by these waveshapes.
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PRECISION RECTIFIERS: THE ABSOLUTE- VALUE
CIRCUIT
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Introduction
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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The precision full-wave rectifier can rectify input voltages with millivolt amplitudes.• This type of circuit is useful to prepare signals
for multiplication, averaging, or demodulation.
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The characteristics of an ideal precision rectifier are shown below.
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The precision rectifier is also called an absolute-value circuit. • The absolute value of a number (or voltage) is
equal to its magnitude regardless of sign.
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For example, the absolute values of |+2 | and |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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Types of Precision Full-Wave Rectifiers
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Three types of precision rectifiers will be presented. • 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.
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Full-wave precision rectifier with equal resistors.
The first type of precision full-wave rectifier or absolute-value circuit is shown below.
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This circuit uses equal resistors and has an input resistance equal to R.
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Figure 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,
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