The most useful way to learn op-amp circuits is to treat each configuration as a feedback pattern. The op-amp output moves until the feedback network makes the two inputs nearly equal, as long as the real device still has enough supply voltage, input range, output swing, bandwidth, and stability margin to do it.

Follower gain = 1 Non-inverting: 1 + Rf/Rg Inverting: -Rf/Rin

Voltage follower

A voltage follower connects the output directly back to the inverting input. The signal enters the non-inverting input. The result is a buffer: the output voltage follows the input voltage, but the op-amp can usually drive a lower impedance load than the original signal source could.

Voltage follower op-amp circuit + - Vin Vout Output feeds back directly to the inverting input.
Voltage follower. Useful when a signal has the right voltage but cannot comfortably drive the next stage. Check input range and output drive current.

Non-inverting amplifier

A non-inverting amplifier keeps the output in the same polarity as the input. The feedback divider sets the gain. It is common in sensor scaling because the signal source sees the op-amp input rather than a low-value resistor network. 

Gain = 1 + Rf / Rg
Non-inverting op-amp amplifier circuit - + Vin Vout Rg Rf 0 V feedback path
Non-inverting amplifier. Gain starts at 1 and increases as Rf becomes larger relative to Rg. It is often a good first choice for amplifying a sensor voltage.

Inverting amplifier

An inverting amplifier applies the signal through an input resistor to the inverting input. The non-inverting input is usually tied to a reference point. The output moves in the opposite direction to hold the inverting node near that reference. 

Gain = -Rf / Rin
Inverting op-amp amplifier circuit - + Vin Vout Rin Rf reference
Inverting amplifier. This configuration is excellent for controlled gain, summing signals, and filters, but the input resistor loads the signal source.

Active low-pass filter

An active filter uses an op-amp with resistors and capacitors to shape frequency response. A low-pass filter passes slower changes and reduces faster noise. This is useful before ADC inputs, in audio paths, and anywhere a signal needs bandwidth limiting.

The simple RC corner frequency is: 

fc = 1 / (2 x pi x R x C)

Real active filters can be first order, second order, or higher. Component tolerance, op-amp bandwidth, output drive, source impedance, and layout all matter more as the frequency rises or the accuracy requirement becomes tighter.

Current sense amplifier

Current sensing often starts with a small resistor, called a shunt, in series with the load. Current through the shunt creates a small voltage. An op-amp or dedicated current-sense amplifier can scale that small voltage so a microcontroller, comparator, or protection circuit can use it.

Low-side current sense resistor with amplifier V+ Load + - shunt Kelvin sense traces ADC The op-amp scales the small voltage across the shunt.
Current sensing. The shunt resistor must be low enough not to disturb the load, but large enough to create a measurable voltage. Layout and grounding are part of the measurement.

Comparator-style threshold

If the job is simply to decide whether one voltage is above another, a comparator is usually better than a general op-amp. It is designed to switch cleanly, recover from saturation, and interface with logic. Op-amps can sometimes be used this way in slow, forgiving circuits, but it should be a deliberate choice.

Configuration Use it when Check carefully
Voltage follower The voltage is right but the source is weak. Input range, output swing, capacitive loads.
Non-inverting amplifier You need positive gain without loading the source much. Gain bandwidth, resistor tolerance, output range.
Inverting amplifier You need controlled negative gain, summing, or filter structure. Input impedance, reference voltage, feedback stability.
Active filter You need frequency shaping and buffering together. Op-amp speed, component tolerance, layout, noise.
Current sense stage You need to measure load current from a small shunt voltage. Common-mode range, offset, shunt power, PCB routing.

Practical rule

Pick the configuration from the signal problem first, then pick the op-amp from the real limits: supply rails, input range, output range, load, accuracy, speed, noise, and stability.

Read: op-amp basics Back to Lab Notes