EXPERIMENT NO. 7Precision Circuits Date: _________________PS
No.:__________Batch No.________
ID No. ________________Name:
__________________________________________
Aim: To study the various precision circuits using op-amps.
Equipment & Components: Analog Electronics Trainer kit, DSO
& Function Generator (Analog Discovery kit), Digital Multi
Meter, 741 ICs, Diodes, Zener diodes, Resistors, Capacitors and
Connecting wires.Theory: Introduction:The use of op-amps can
improve the performance of a wide variety of signal processing
circuits. In rectifier circuits, the cut-in voltage drop that
occurs with an ordinary semiconductor diode can be eliminated to
give precision rectification. Waveforms can be limited and clamped
at precise levels when op-amps are employed in clipping and
clamping circuits. The error with peak detectors can also be
minimized by the use of op-amps.Precision Half-wave and Full-wave
RectifiersFigure 7.1 shows the circuit of a fast precision
rectifier which uses an inverting amplifier. When the input is
negative, the op-amp output is positive. is reverse biased and is
forward biased. = (
Figure 7.1 Fast precision rectifier and its transfer
characteristics.During the positive half-cycle of the input, the
op-amp output goes negative, causing to be reverse biased. Without
in the circuit, the op-amp output would be saturated to the
negative saturation voltage, However, the negative voltage at the
op-amp output forward biases . This tends to pull the op-amp
inverting input terminal in a negative direction which may cause
the output to go positive. So, the output settles at a voltage
which keeps the input voltage close to ground level. In this case,
that voltage is the forward voltage drop below the ground, say 0.7
V.Precision full-wave rectifier The left-side of the circuit in
Figure 7.2 is a precision half-wave rectifier as shown in Figure
7.1. but with the diodes reversed. The right-side is an inverting
summing amplifier. The input voltage is applied to terminal A of
the summing amplifier and to the input of the precision rectifier.
Note that = 2 and so the rectified voltage applied to the terminal
B of the summing amplifier is 2.During the positive half-cycle of
the input, the voltage at A is and that at B is 2. The output from
the summing circuit with is .During the negative half-cycle of the
input, and . Consequently, the output is(It is seen that the output
is full-wave rectified version of the input. With voltage gain of
the circuit is 1. A precision full-wave rectifier is also known as
the absolute value circuit. The transfer characteristic of the
rectifier is shown in figure 7.2(b).
(a) (b)Figure 7.2 (a) Precision full-wave rectifier circuit and
(b) its transfer characteristicsPositive and Negative ClippersA
positive clipper , a circuit that removes the positive parts of the
input signal, can be formed by an op-amp with a rectifier diode as
shown in the Fig. 7.3(a). In this circuit, the op-amp is basically
used as a voltage follower with a diode in the feedback path. The
clipping level is determined by the reference voltage Vref. Figure
7.3 (a) Positive clipper circuit and (b) input and output waveforms
and transfer characteristics (Vref=1 V)During the positive half
cycle of the input, the diode conducts unly until Vin=Vref. This
happens because when Vin
