Inverting Operational Amplifier
The
most widely used of all the op-amp circuits, the circuit is shown in figure.
The output voltage V0 is fed back to the inverting input terminal
through the Rf - R1 Network where Rf is the
feedback resistor. Input signal Vi is applied to the inverting input
terminal through R1 and non- inverting input terminal of op-amp is
grounded.
Consider as Vd
= 0, node a is at ground potential and the current i1 through R1
is
also since op-amp draws
no current, all the current flowing through R1 must flow through Rf. The output
voltage
Hence, the gain o f the
inverting amplifier (also referred as closed loop(CL) gain) is,
Alternatively, the
nodal equation at the node ‘a’ in following figure is
Where va is the voltage at node ‘a’. Since node ‘a’ is
at virtual ground va = 0, Therefore, we get
Physical diagrammatical phenomena of Inverting
Amplifier: The basic circuit for the inverting op amp circuit is shown below.
It consists of a resistor from the input terminal to the inverting amplifier
input of the circuit, and another resistor connected from the output to the
inverting input of the op-amp. The non-inverting input is connected to ground.
The
negative sign indicates a phase shift of 180 degrees between vi and
v0 . In this inverting
amplifier circuit the non-inverting input of the operational amplifier is
connected to ground. As the gain of the op amp itself is very high and the
output from the amplifier is a matter of only a few volts, this means that the
difference between the two input terminals is exceedingly small and can be
ignored. As the non-inverting input of the operational amplifier is held at
ground potential this means that the inverting input must be virtually at earth
potential. As a result, this form of amplifier is often known as a virtual
earth amplifier.
Input impedance: With the value of R1,
the input resistor being lower than the feedback resistor Rf, care
has to be taken when designing he circuit to ensure the input resistance is not
too low for any given application. Often values of 10kΩ are used as this
provides a reasonable load resistance for many circuits. However the exact
requirements need to be determined for each application.
Range of values for R1 and R2: Rf must be higher.
Even though the input impedance of the integrated circuit itself circuit may be
high, using a high value of feedback resistor is not advisable as results may
become a little unpredictable. As a very broad rule of thumb values for Rf
of up to 100kΩ or a little more should be fine.
Summing amplifier or virtual earth mixer
The fact that the inverting input to the
amplifier is virtually at earth potential provides some significant
possibilities for audio mixer applications.
This form of virtual earth mixer or
summing amplifier adds several different signals in a linear or additive
fashion. It is not the form of multiplier mixer used in RF circuits where a
non-linear action is used.
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