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 :
The ideal op-amp has the following characteristics :
- The input current at the non-inverting and inverting input is zero ()
- The input offset voltage is zero ()
- The input impedance is infinite ()
- The output impedance is zero ()
- 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.
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.
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.
Below you can find a list of multiples op-amp circuits that are commonly used :