Non-inverting operational amplifier circuit
the use of an operational amplifier or op-amp in a non-inverting
amplifier circuit
Operational amplifiers can be used in two basic
configurations to create amplifier circuits. One is the inverting amplifier
where the output is the inverse or 180 degrees out of phase with the input, and
the other is the non-inverting amplifier where the output is in the same sense
or in phase with the input.
Both operational amplifier circuits are widely used and they
find applications in different areas. When an operational amplifier or op-amp is
used as a non-inverting amplifier it only requires a few additional components
to create a working amplifier circuit.
Basic circuit
The basic non-inverting operational amplifier circuit is shown below. In this
circuit the signal is applied to the non-inverting input of the op-amp. However
the feedback is taken from the output of the op-amp via a resistor to the
inverting input of the operational amplifier where another resistor is taken to
ground. It is the value of these two resistors that govern the gain of the
operational amplifier circuit.
Basic non-inverting operational amplifier circuit
The gain of the non-inverting circuit for the operational
amplifier is easy to determine. The calculation hinges around the fact that the
voltage at both inputs is the same. This arises from the fact that the gain of
the amplifier is exceedingly high. If the output of the circuit remains within
the supply rails of the amplifier, then the output voltage divided by the gain
means that there is virtually no difference between the two inputs.
As the input to the op-amp draws no current this means that
the current flowing in the resistors R1 and R2 is the same. The voltage at the
inverting input is formed from a potential divider consisting of R1 and R2, and
as the voltage at both inputs is the same, the voltage at the inverting input
must be the same as that at the non-inverting input. This means that Vin = Vout
x R1 / (R1 + R2)Hence the voltage gain of the circuit Av can be taken as:
Av = 1 + R2 / R1
As an example, an amplifier requiring a gain of eleven could
be built by making R2 47 k ohms and R1 4.7 k ohms.
Input impedance
It is often necessary to know the input impedance of a circuit. The input
impedance of this operational amplifier circuit is very high, and may typically
be well in excess of 10^7 ohms. For most circuit applications this can be
completely ignored. This is a significant difference to the inverting
configuration of an operational amplifier circuit which provided only a
relatively low impedance dependent upon the value of the input resistor.
AC coupling
In most cases it is possible to DC couple the circuit. However in this case it
is necessary to ensure that the non-inverting has a DC path to earth for the
very small input current that is needed. This can be achieved by inserting a
high value resistor, R3 in the diagram, to ground as shown below. The value of
this may typically be 100 k ohms or more. If this resistor is not inserted the
output of the operational amplifier will be driven into one of the voltage
rails.
Basic non-inverting operational amplifier circuit with
capacitor coupled input
When inserting a resistor in this manner it should be
remembered that the capacitor-resistor combination forms a high pass filter with
a cut-off frequency. The cut off point occurs at a frequency where the
capacitive reactance is equal to the resistance.
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