- 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.
An operational amplifier or op amps is a linear device used for ideal DC amplification. It is fundamentally a voltage amplifying device that works with external feedback components like resistors or capacitors. An op amp has three 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.
Before understanding how an op amp works, it’s important to know its op amp characteristics. Here are the key characteristics explained one by one:
Open Loop Voltage Gain(A)
The open loop voltage gain without any feedback for an ideal op amp is infinite. But typical values of open loop voltage gain for a real op amp ranges from 20,000 to 2, 00,000. Let the input voltage be Vin. Let A be the open loop voltage gain. Then the output voltage is Vout = AVin. The value of a typically is in the range specified above but for an ideal op amp, it is infinite.
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. That is there no current flowing in the input circuit. However, an ideal op amp has certain current flowing in the input circuit of the magnitude of few pico-amps to a few milli-amps.
Output Impedance (Zout)
Output impedance is defined as the ratio of the output voltage to the input current. The output impedance of an ideal op amp is zero, however, real op amps have an output impedance of 10-20 kΩ. 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 that is it can amplify any signal from DC to the highest AC frequencies without any losses. So therefore, an ideal op amp is said to have infinite frequency response. In real op amps, the bandwidth is generally limited. The limit depends on the gain bandwidth (GB) product. GB is defined as the frequency where the amplifier gain becomes unity.
Offset Voltage(Vio)
In an ideal op amp, the offset voltage is zero, meaning the output voltage is zero if there’s no difference between the inverting and non-inverting terminals. If both terminals are grounded, the output voltage remains zero. However, real op amps do have some offset voltage.
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 ∞. That means it is able to reject all common mode signal. Also from the formula, we can see the AD is infinite for an ideal op amp and ACM is zero. Therefore the CMRR of an ideal op-amp is infinite. Therefore it will reject any signal which is common to both.
However, real omp have finite CMRR, and does not reject all common mode signals.
