- Op Amp Definition: An operational amplifier (op amp) is defined as a linear device used for voltage amplification with three terminals: inverting input, non-inverting input, and output
- Open Loop Voltage Gain: Ideal op amps have infinite open loop voltage gain, but real op amps typically range from 20,000 to 200,000
- Input Impedance: The input impedance of an ideal op amp is infinite, meaning no current flows into the input circuit.
- Output Impedance: Ideal op amps have zero output impedance, acting as perfect voltage sources, but real op amps have output impedances of 10-20 kΩ.
- Common Mode Rejection Ratio (CMRR): This ratio measures an op amp’s ability to reject common mode signals, with ideal op amps having an infinite CMRR.
Op amp characteristics are the open-loop gain, impedances, bandwidth, offset and CMRR of an operational amplifier or op amps. It is a voltage amplifying device used with external feedback parts such as resistors or capacitors. An op amp has three signal terminals: inverting input, non-inverting input and output.
As you can see from the diagram, op amp has three terminals for input and output and 2 for power supply.
The op amp characteristics below compare the ideal model with real parts:
Open Loop Voltage Gain(A)
The open loop voltage gain without any feedback for an ideal op amp is infinite. Classic general-purpose bipolar parts often sit from about 20,000 to 200,000. CMOS and precision parts can be higher. Let the input voltage be Vin. Let A be the open loop voltage gain. Then the output voltage is Vout = AVin. A is large but finite on a real part, and infinite only in the ideal model.
Input Impedance(Zin)
Input Impedance is defined as the input voltage by the input current. The input impedance of an ideal op amp is infinite, so no current flows into the input pins. A real op amp draws a bias current, typically picoamps on CMOS or FET-input parts and up to microamps on older bipolar parts, not milliamps in normal linear use.
Output Impedance (Zout)
Output impedance is the ratio of a test voltage at the output to the current that voltage produces, with the inputs set as the datasheet requires. The output impedance of an ideal op amp is zero. Many older bipolar op amps have an open-loop output resistance of tens of ohms, not 10-20 kΩ. Closed-loop Zout is much lower. An ideal op amp behaves like a perfect voltage source delivering current without any internal losses. The internal resistance reduce the voltage available to the load.
Bandwidth(BW)
An ideal op amp has an infinite bandwidth, so the model amplifies from DC to any AC frequency without roll-off. A real op amp is limited by its gain-bandwidth product. GB is the frequency where the open-loop gain has fallen to unity, on a dominant-pole part.
Offset Voltage(Vio)
In an ideal op amp, the offset voltage is zero, so the output is zero when the two inputs are equal. If both terminals are grounded, the output voltage remains zero. Real op amps need a small differential input, Vio, to bring the output to zero.
Common Mode Rejection Ratio(CMRR)
Common mode occurs when the same voltage is applied to both the inverting and non-inverting terminals of the op amp. Common mode rejection is the op amp’s ability to reject this common mode signal. This leads us to the common mode rejection ratio.
The common mode rejection ratio refers to the measure of the ability of the op amp to reject the common mode signal. Mathematically it is defined as
Where, AD is the differential gain of the op amp, ∞ for an ideal op amp.
ACM refers to the common mode gain of the op-amp.
The CMRR of an ideal op amp is ∞, so the model rejects a pure common-mode input. From the formula, AD is infinite for an ideal op amp and ACM is zero, so CMRR is infinite.
A real op amp has finite CMRR and does not reject all common mode signals.
