Operational Amplifiers: Inverting Vs Non-Inverting Topologies

2023-02-28 | By Dandeniyage Dasith Rajev De Silva

License: None

An Op Amp is in essence a voltage amplifying device. It uses components like inductors, capacitors and resisters as feedback components to create the gain in voltage. An Op Amp has three terminals: an inverting, non-inverting, and output terminal. Some ideal Op Amp characteristics include infinite resistance at both terminals; i.e., no current flows into the terminals; both terminals are held at the same voltage, an output resistance of 0, an infinite open loop gain, infinite bandwidth and a zero offset.

Before we continue, we need to understand a key concept:

Negative Feedback
Granted Op Amps can provide very very high gains; this characteristic also means that the amplifier is notoriously hard to control and is very unstable. Some of this intense gain can be spread across a resistor connected from the output terminal back to the inverting terminal. This will allow the final gain to be controlled – this process is named Negative Feedback and can help produce a much more stable and controllable Op Amp gain.

In more words: negative feedback is the use of a component to feed part of the output current back to the input terminal, specifically the “inverting input” – and usually this component is a resistor. This creates a closed loop circuit that trades a bit of output gain for the ability to better control the op amp.

This blog post aims to highlight the differences between two very useful Op Amp topologies: the Inverting Amplifier Topology and the Non – Inverting Amplifier Topology.

The Inverting Amplifier Topology

In this topology, the inverting terminal receives negative feedback from the output through the resistor Rf.

Now assuming this Op Amp is ideal, we know that the inverting and non-inverting terminals are at the same voltage (virtual short). Since the non-inverting input of the Op Amp is held at 0V by being directly connected to ground, it means that the inverting terminal of the Op Amp must also be held at 0V. 

So, we can apply Kirchhoff’s Current Law to the inverting terminal of the amplifier as such:

We define gain as a ratio of the output Voltage to the input Voltage. So, simplifying the above equation to find that ratio we get:

Along with this inverted/negative gain, the output signal (assuming its sinusoidal) will be 180 degrees out of phase with the input.

Non-Inverting Amplifier Topology

In this case the feedback resistor is connected to the non-inverting terminal, but the input is now connected to the non-inverting terminal. Again, we assume the Op amp is ideal and the two terminals are at the same voltage, in this case Vin since the non-inverting terminal is connected directly to the input.

Applying Kirchhoff’s Current Law to the Inverting Terminal of the Op Amp we get the nodal equation:

Simplifying the circuit as we did before to get the gain (Vout/Vin) we are left with:

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