Operational Amplifier Fundamentals Unit 4 – The Voltage Follower
Exercise 3 – Voltage Follower AC Characteristics
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to determine the effects of slew rate
and full-power bandwidth on a voltage follower. You will observe the effects on an oscilloscope.
EXERCISE DISCUSSION
- Slew rate (SR) specifies maximum rate-of-change in output voltage over a period of time.
Mathematically, SR is equivalent to the slope of a waveform. - Slew rate (SR) is defined as a change in output voltage divided by the time interval during
which the voltage change occurs. For this pair of parameters, SR is given in units of
volts/time. - The graph illustrates how slew is measured. The calculation determines the slope of the line.
SR is given in volts/microsecond (V/μs). - If, for example, output voltage changes by 10V in 10 μs, the slew rate of the circuit is 1 V/μs
- Ideal op amps have an infinite slew rate: the output would instantaneously follow any change
at the input. - An ideal square wave has instantaneous rise and fall times.
- The LF441 op amp used on the circuit board has a specified slew rate of 1 V/μs. Generally,
this means that the op amp output voltage cannot change faster than 1V in less than 1 μs. - Slew rate decreases rise and fall times of a square wave. For a slew rate of 1 V/μs, it would
take 10 μs for a 10V swing to occur. - If an op amp circuit has an SR of 5 V/μs and its square wave input has rise and fall times of 1
V/μs, the circuit can duplicate the input waveform. - When the circuit SR is equal to or greater than the rate of change in the input waveform, the
circuit output waveform tracks (reproduces) the input waveform. - When circuit SR is less than the rate of change of an input waveform, the output waveform
does not fully track (reproduce) the input waveform. The "squareness" of the waveform is
reduced. - For a given slew-rate-limited circuit, an output waveform becomes triangular and decreases
in amplitude as input frequency increases. - Slew rate limitations also affect sinusoidal waveforms. On a sinusoidal waveform, the slope
is maximum at the zero crossing points and zero at the peak points of the waveform. - The slope of a sinusoidal waveform always changes.
- Slope defines a rate of change (volts per unit of time). Therefore, slope is also frequency
dependent. If the frequency of a constant amplitude sinusoidal waveform increases (period
decreases), the slope of the waveform increases.