Operational Amplifier (op-amp)

Operational amplifier or op-amp have multiples uses in electronics. They can be use as voltage amplifier, voltage comparator and many others applications.

Op-amp consists of two inputs (positive and negative inputs) and an output. They also required two voltage supplies (a positive (Vs+) and a negative (Vs-) supply) to work. The (+) input is called the non-inverting input and the (-) input is called the inverting input. The symbol for the op-amp is given below :

Ideal op-amp

The ideal op-amp has the following characteristics :

The input current at the non-inverting and inverting input is zero ( $I_{IN} = 0$ )
The input offset voltage is zero ( $V_{OS} = 0$ )
The input impedance is infinite ( $Z_{IN} = \infty$ )
The output impedance is zero ( $Z_{OUT} = 0$ )
The open-loop gain is infinite
Infinite bandwidth

These characteristics are not valid for a real op-amp.

The input current at the non-inverting and inverting input is non-zero. Generally in the range of pA or tens of uA depending on the technology and model used.
The input offset voltage is non-zero. Generally in the range of uV depending on the technology and model used.
The input impedance is not infinite. The impedance is finite but generally very high.
The output impedance is non-zero. It is generally small in a real op-amp.
The open-loop gain is finite but very high (>100 000)
The bandwidth of a real op-amp is not infinite and will vary depending on the model used.

The real op-amp also have others small imperfection not discuss here like the common-mode gain, power-supply rejection, temperature effects and much more. All this information can be found in the datasheet of your specific op-amp. For most circuits analysis, the ideal op-amp characteristics can be used to simplify the analysis. This is a valid approach when working at low frequencies (several kHz) since the real op-amp has almost the same characteristics as an ideal op-amp.

Open-loop amplifier

The op-amp is a DC-coupled high-gain differential voltage amplifier with a single-ended output. In theory, the gain is infinite but real op-amp have a gain of around 100 000 or more (which is still a high gain). The output will depend on the positive and negative inputs.

If the voltage at the non-inverting (+) input is greater than the inverting (-) input, the output will be Vs+
If the voltage at the non-inverting (+) input is smaller than the inverting (-) input, the output will be Vs-

The open-loop amplifier acts like a comparator. Often, op-amp are used with a positive or negative feedback.

Closed-loop amplifier

The op-amp is mostly used with negative feedback. This is done by applying a fraction of the output voltage to the inverting input. This will greatly reduces the gain of the op-amp. The feedback circuit will determined the gain and behavior of the circuit and can be adjusted as needed. You can find more information in the op-amp circuits section for different type of circuit with feedback like the inverting and non-inverting amplifier.

Op-amp circuits

Below you can find a list of multiples op-amp circuits that are commonly used :