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 non-inverting op-amp 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 of non-inverting amplifier
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 the non-inverting op-amp circuit
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.
