Non- Inverting Amplifier
A voltage follower (also called a
unity-gain amplifier, a buffer amplifier,
and an isolation amplifier) is an op-amp circuit which has a voltage gain of one. Voltage followers are generally used
to isolate stages from each other. A voltage follower generally has high input impedance and low output
impedance.
If
the signal is applied to the non-inverting input terminal and feedback is given
as following figure, the circuit amplifies without inverting the input signal.
Such a circuit is called non-inverting amplifier. It may be noted that it is
also a negative feed-back system as output is being fed back to the inverting
input terminal.
As the
differential voltage vd at the input terminal of op-amp is zero, the
voltage node a in above figure is vi , same as the input voltage
applied to non-inverting terminal. Now Rf and R1 forms a potential divider.
Hence
as no
current flows into the op-amp
A non-inverting amplifier is an op-amp circuit configuration which produces an amplified output
signal. This output signal of non-inverting op amp is in-phase with the input
signal applied. In other words a non-inverting amplifier behaves like
a voltage follower circuit.
A voltage follower (also called a
unity-gain amplifier, a buffer amplifier,
and an isolation amplifier) is an op-amp circuit which has a voltage gain of one. Voltage followers are generally used
to isolate stages from each other. A voltage follower generally has high input impedance and low output
impedance.
The basic non-inverting amplifier circuit using an op-amp is
shown above. In this circuit the signal is applied to the non-inverting input
of the amplifier. However the feedback is taken from the output 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.
The gain of the non-inverting amplifier
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
Applications:
Op amp as a voltage follower
A non-inverting amplifier using an op amp forms an ideal
voltage follower. The very high gain of the op amp enables it to present a very
high impedance to the signal source whilst being able to accurately follow the
voltage waveform.
An op amp is configured in its non-inverting amplifier
format, linking the output directly to the inverting input and applying the
input signal to the non-inverting input.
From
the gain equation it is possible to see that the voltage gain of this circuit
is unity.
Av = 1 + R2 / R1.
R2
is zero and R1 is infinity,
So
the term R2/R1 is zero and this means that Av = 1 (Unity Gain)
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