What are opamps?
Typical Parameters (Slew Rate)
Slew Rate (SR)
The slew rate is a parameter that describes the operating speed of an opamp.
It represents the rate that can change per unit time stipulated by the output voltage
For example, 1V/us indicates that the voltage can change by 1V in 1us.
Ideal opamps make it possible to faithfully output an output signal for any input signal. However, in reality slew rate limits do exist.
When supplying a rectangular pulse at the input with steep rise and fall, this indicates the possible degree of change in the output voltage per unit time.
The rise and fall slew rates are calculated by the following equations.
The slew rate is stipulated based on the slower of 'rise' and 'fall'.
In other words, it signifies the maximum value of the slope of the output signal.
For signals with steeper changes (slopes), the output will become distorted and cannot follow. And even when configuring an amplifier circuit, since the slew rate is the ratio of output change, no change will occur.
Opamps are used to amplify both AC and DC signals.
However, opamps have limited response speed, and therefore cannot handle all types of signals.
In the above diagram of a voltage follower circuit, the input and output voltage ranges are restricted by the DC input voltage.
In addition, AC signals with a frequency component are constrained by the slew rate and gain bandwidth product.
Here, we consider the relationship between the amplitude and frequency, or slew rate.
The opamp determines the maximum frequency that can be output.
Calculate the slew rate required to output the waveform shown at right.
The slew rate is the slope of the tangent of the sine wave, differentiating the above equation.
The slew rate is
Furthermore, since the amplitude of the sine wave becomes Vpp=2A (peak-to-peak), the equation can be modified as follows.
This frequency f is referred to as the full power bandwidth.
These are conditions where the amplification factor in the opamp has not been set, in other words the relationship of the frequency and amplitude (within the output voltage range) that can be output by the opamp in a voltage follower circuit.
Ex: The frequency capable of outputting a 1Vpp signal in an opamp where SR=1V/us is
When exceeding the frequency calculated above (with a constant amplitude), the waveform is limited by the slew rate and the sine wave will become distorted and become a triangular wave.