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Op-Amps, a short form of Operational Amplifiers,
are integrated circuits with two inputs “inverting” and “non-inverting”. They
were used to model basic mathematical operations
in analog computers such as addition, subtraction, integration and
differentiation etc. Hence are called operational amplifiers.
Inverting
i/p and non-inverting i/p have function exactly like their names. They come in
different IC packages and also in variety like single op-amp, dual op-amp and
more in a single IC. As the number of op-amps inside IC/Chip increase,
obviously the number of pins in the IC package will keep growing. Here we have
shown sample op-amp IC LM741 which has one operational amplifier into it as
shown in diagram. As mentioned in previous topic, ICs are denoted as square,
rectangle, triangle and circle in a circuit diagram, the op-amps are always
shown with triangle shape in circuit. Two inputs are shown at the left side if
triangle and output on right side at one of the vertex of triangle.
They
operate at two different supplies also i.e. V+ and V-. Means if we are operating any op-amp at say 12V, then we need three terminal power supply as
+12V, -12V and Gnd. Simplest way to get such supply is to connect the battery
in this fashion. There are many other Op-amps which do not need negative
supply, simply Gnd and +12V or as specified in its manual.
Recommended
supply voltage for LM741C is ±15V to ±18V
and for LM741 & LM741A
is ±15V to ±22V. These different voltage levels are based on different variants
of this IC. The variant with letter “C” at the
end has different range and for plane LM741 (without any letter at the end)
& with letter “A”, the maximum allowed
supply voltage is higher. For all the three variants, the minimum supply
voltage is ±10V. So, as a standard we
can take ±12V which is operating voltage of many other circuits and components.
So for
this IC, +12V is to be connected to pin-7 and -12V will be connected to pin-4. The
Gnd will be common between input and output voltages; no pin of this IC needs
to connect with Gnd of supply.
Inverting Input: Any signal applied on
inverting input pin, will be amplified and inverted at the output pin. Positive
cycle is converted to negative cycle and vice-e-versa.
Non-Inverting Input: Any signal applied
on the non-inverting input pin, will be amplified and presented at output pin
without any phase change i.e. positive and negative cycles of input will be
presented in same way as they appear at input.
If
the voltage at inverting input is lower than voltage at non-inverting input,
the output voltage of an op-amp is equal to negative supply voltage.
And if
the voltage at non-inverting input is higher than inverting input, the output
voltage is equal to positive supply voltage. Hence it acts as a comparator i.e.
compares the voltages at input pin to decide output voltage as HIGH or LOW.
Application:
Op-Amps can
be configured in many different ways to use in different operations. There are
many other operational configurations for high level circuitry, which is out of
scope now; hence we will see some basic operation modes to understand the versatility
of the op-amps. For different critical operations we may need to choose different
models of op-amps, every op-amp may not be efficient enough to handle very
precise operations like in measuring instruments with high precision etc.
Offset null pin (pin-1 & 5) are mostly not used in normal applications,
hence can be left open.
Voltage Follower:
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The simplest
application of an op-amp is a “Voltage Follower” circuit as shown here. As we
can see we do not see any external components. Only inverting input is
connected directly to output pin and the input voltage is given to
non-inverting input. In all these sample circuits, I have not shown the power
supply, but obviously they are basic need to power the internal circuitry of
IC, hence need to be connected to power supply as explained above. As the name
suggests its output voltage “Follows”
the input voltage i.e. Vout = Vin. So, you may be wondering “what is the use of
such circuit if it simply passes on the input voltage to output pins without
any value add?”. Yes, it has value add
in taking input voltage from sources which have very low current delivery
capacity like sensors. The op-amps have very high input resistance (Resistance between
input pin and ground) in the range of “Mega Ohms”, hence they do not overload
the output of sensors. If they are overloaded, the reading of sensors will not
be accurate. So, voltage followers are the very first stage of any measuring instrument
which is collecting data from a sensor connected to the instrument.
Inverting and Non-Inverting Amplifier:
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The next
basic implementations of op-amps are inverting and non-inverting amplifier as
shown in this diagram. Here we can see few resistors added to op-amps. “Rf” is
connected between output pin and inverting pin, hence denoted with letter ‘f’
to indicate feedback (giving feedback from output to input pin). The other
resistor is connected to inverting input “Ri”, where ‘i’ denotes input side. In
such condition gain of the circuit is simply ratio of “Rf” and “Ri” i.e. Gain =
Rf/Ri. If we keep value of both resistors equal the gain is “One” i.e. no
amplification of input voltage.
In the
inverting circuit, as shown the output voltage is amplified by the gain factor
but the output if “Negative” as input was given to inverting input.
For the non
inverting amplifier the input is provided to non-inverting input pin of op-amp
and the output voltage is positive and a is “1 + gain x Vin”. So based on the
need of output, we decide whether we should go with inverting amplifier or a
non inverting amplifier.
Voltage Adder:
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Voltage
adder works as its name suggests, it adds the input voltages at different input
resistors and provide output after amplifying the added value with the gain
factor. Again since the input here is provided to the inverting input pin,
hence the output will be inverted or on negative side. Hence the output voltage
is shown with Negative sign on it. In case we want only to add the voltages
without any amplification, we can use “Rf” equal to “Ri” to make gain=1. We can
have more number “Rin” connected in similar way to add more input voltages if
required till the output voltage is within the supply voltage range. The output can never be more than the supply voltage.
Differential Amplifier:
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Again as the
name specifies, such op-amp circuit amplifies the difference of input voltage at
non-inverting and inverting pin. Here “Rf” is shown at two places, that mean
both the resistance should have same value, and both “Rin” should have same
value. In such configuration the output voltage is gain time the difference of
two input voltages as the formula shown in image. Again if we simply want
difference of two voltages at output without any amplification, keep all
resistor values same to make Gain=1.
Adder – Subtractor:
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This is the
combination of adder and substractor circuit. We have two inputs at non-inverting pin and two at inverting pin. Value of all “Rin” is taken same
here, and also the two “Rf” should have same value. In this configuration the
voltages at the non-inverting inputs are added and then the voltages at the
inverting inputs are subtracted from it. This value is amplified by the gain
factor and presented at output. It can be seen as extension of differential
amplifier with multiple input rather than one input for each inverting and
non-inverting. Again if don’t need any amplification, and only want the
difference values to be presented at output pin, we can keep all “Ri” equal
to all “Rf”.
These are
few basic implementations of op-amps sufficient for this level. There are many
more like, DC amplifiers, Voltage Clamper, Active Filters, Zero and Span
circuit, Multivibrator, Integrator and Differentiator etc.
At this
level, that’s all about op-amps, we will see the use of op-amp when we will see
the internal block diagram of IC 555 and its operation. IC 555 is also a very versatile
IC for entry level electronic projects.
Keep visiting for more interesting topics.
Keep visiting for more interesting topics.
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