Inverting Op-Amp Amplifier Circuit Diagram

The + input is grounded. With negative feedback through R2, the op-amp drives its output to whatever voltage keeps its two inputs equal, so the − input also sits at 0 V. That node is called a virtual ground: it is at ground potential without being connected to ground.

Since the − input is at 0 V, the input current is simply Vin / R1. Almost no current flows into the op-amp's input, so all of it continues through R2 to the output. For that current to flow away from a node at 0 V, the output has to go negative when Vin is positive: Vout = −Vin × R2 / R1.

With R1 = 1 kΩ and R2 = 10 kΩ the gain is −10, so a 100 mV input gives −1 V out. The source sees R1 to the virtual ground, so the input resistance is 1 kΩ. Scaling both resistors up, to 10 kΩ and 100 kΩ, keeps the gain and loads the source ten times less.

The output can only swing between the supply rails, and here it goes negative for a positive input. The circuit needs a split supply such as ±12 V, or on a single supply, the + input biased to mid-supply and the signal capacitor-coupled.

• Running it from a single supply with the + input grounded. The output cannot go below the negative rail, so every positive input gives 0 V out.
• Making R1 too small for the source. The source drives R1 into a virtual ground, so a weak source sags; 10 kΩ and 100 kΩ give the same gain with ten times the input resistance.
• Leaving out decoupling. Put 100 nF from each supply pin to ground, close to the op-amp.
• Expecting the full gain at high frequencies. The gain falls once the gain-bandwidth product runs out: an op-amp with about 1 MHz of gain-bandwidth holds a gain of 10 only up to roughly 100 kHz.