Slew Rate

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The slew rate is an important characteristic for amplifiers and electronics of elements

Introduction

The slew rate is comparably with a speed, in this case e.g. the temporal change of a voltage or an electric current. This term is admits from classical physics, a body moves with for example with 5 meters per second. In electro-technology the slew rate developed, for example a voltage changes its level within a certain time, defined in the unit volt per second in analogy.

Comparison from the everyday life

Already with a simple comparison of the slew rate of two amplifiers, first statements can be met from this parameter. A car or a motorcycle with a maximum speed of 250 km/h is clearly faster than one with 120 km/h. Simplified said, the slew rate is the maximum speed of an amplifier. Alone the maximum speed does not say yet much about applications and qualities of a vehicle. With a vehicle with a maximum speed of 250 km/h no truck may be expected. Do not ask - with a Slew rate of 4000 V/µs a Hifi amplifier output stage to expect is utopian, the inaugurated electronics engineer thinks thereby immediately of video operation amplifier or similarly fast parts

Oszillograph eines guten 10 MHz Rechteck Signal

Figure 1 shows a 10 MHz square wave signal of a Tektronix AWG430 generator. Measured with a Tektronix TDS5054 oscilloscope with a rise time of approx.. 800 picoseconds. At the scope a multi-color mode is adjusted, in order to represent the intensity of the "electron beam" better. Time base 10 ns/DIV, vertically 200mV/DIV. 50 ohms.

The picture shows that also very good generators are not yet perfect. A settling time and a finite slew rate will be always present. The figure shows that even a very good generator is still not perfect. A remaining settling time and a finite slew rate. The desired "rights angles" there are not in nature and it will not also give, only in mathematics. For measuring the Slew rate the optimum rectangle and fastest oscilloscope should be used, which are available. In particular with very fast operation amplifiers this is particularly important. For a Hifi amplifier is strongly reduced the requirement to the speed of test equipment, because of the lower speed of audio devices.

Oszillograph eines guten 1 kHz Rechteck Signal

Figure 2 shows the same test tequipment, now however a 1 kHz square wave signal. Meanwhile this signal on the oscilloscope looks perfectly. A settling time are not to be recognized visible with this time base. For a audio amplifier is 1 kHz signal a good choice, the primary wave (1 kHz) and also still many rectangle harmonic waves (3, 5, 7, 9 kHz etc..) are appropriate still within the audio range and can be transferred by the amplifier. To put on a 10 MHz square wave signal to a hifi amplifier would be senseless, it is the developer would like to know whether the amplifier reacts to higher-frequency signals with oscillation at the output.

It is forbidden that with a Slew rate measurement a loudspeaker is attached, since the tweeter is destroyed by the high frequencies and levels. 

For a test a level is recommended, approx.. 5% to 10% under the maximum output voltage with normal load (e.g. 4 ohms).

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Figure 3 is the same signal as in fig. 2, only oscilloscopes shows the signal in another representation method. The multi-colors mode is switched off and the "digital electron beam" stands on green, and shows like the square wave signal on a similar oscilloscope would look. To see also here, a minimum oscilloscope noise. A analogue oscilloscope is just as well suitable for the measurement of the Slew Rate as a digital.

Oszillograph der ansteigenden Flanke des 1 kHz Rechteck Signal

Figure 4 shows the rising edge of 1 kHz square wave signal. The rise time usually measured between 10% and 90% of the amplitude. Comfortable way computes the oscilloscope the rise time with 1.578 nanoseconds. The Slew rate of this signal amounts to 709.8 megavolt per second. Converted into the usual Volt/µs results in: 709.8 V per microsecond. The analog scope user must count the division lines of the scaling and accomplish the calculation of the delta y by delta x. The true square wave signal is still faster, since that oscilloscopes adds approx. 800ps. Such a rate of rise of the testsignals is already more than sufficient for a Hifi amplifier. To see very beautifully the clean settling square wave signal with very small overshooting, this are from similar nature as in fig. 1.

Oszillograph der fallenden Flanke des 1 kHz Rechteck Signal

Figure 5 shows the falling edge of 1 kHz of square wave signal. The Slew rate became negative now, since the voltage sinks. The oscilloscope computes -862.5 V per microsecond. The falling edge is faster than the rising edge. With these small times is however tolerable. Ideally it would be natural if both were alike. To observe beautifully that the flanks are from more clearly very symmetrical nature. That speaks qualitatively for the measuring instruments.

 

A recommended attitude for hifi amplifiers is about 1 kHz, measured with different amplitudes and also loads. Of course the 4 ohms or 8 ohms, in addition, under capacitive load (e.g. > 10 nF parallel to the Ohm's load - it is to try). Under capacitive load weaknesses come particularly pronouncedly to the appearance, high overshooting because of missing bandwidth and high dynamic internal resistance with higher frequencies. Some bad amplifiers show however already under Ohm's load violent overshoot.

Comprising it doesn't matter if the rise reach  7 V/µs or 9 V/µs, determing is the the dimension and the variation of the slew rate under capactive and resistive loads.

In the case of a measurement of the rate of rise of an amplifier, it is naturally meaningful to represent and compare input port and output channel at the same time on the oscillograph.

The figures 6, 7 and 8 show the measurement of the Slew rate of an amplifier, which was adjusted to a gain of 20. The input voltage is the green channel at the oscilloscope, the output voltage is the violet channel. As test signal serves a steep square wave signal with 500 mVpp amplitude and 100 kHz. The testsignal supplies the AWG430 from the fig. 1-5. Note: the amplifier is a circuit from two one behind the other cascaded operation amplifiers, with overall feedback loop.

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Figure 6 , this attitude of the time base points to first overview that no overshooting arises. The rate of rise is not yet measurable with this attitude.

Figure 7 shows the rising flanks of the signals. The amplifier swings cleanly and fast in, only with minimum overshooting. The indicated Slew rate amounts to 693.4 V/µs.

Figure 8 shows the falling flanks of the signals. The amplifier swings cleanly and fast in, only with minimum overshooting. The indicated Slew rate amounts to -1075 V/µs.

Fig. 7 and 8 clarify, for the rate of rise exist no accurate value. It always is as a function of where in the course of the curve one measures exactly. Normally between 10% and 90%, a measurement between 20% and 80% would supply other results.

Some dimensions, 

(only dimension, please don't put it on a pair of gold scales!!!):

  • Precision operational amplifier for DC applications 0,1 V/µs to 2 V/µs

  • Operational amplifier for general purpose 2 V/µs to 300 V/µs

  • Operational amplifier for high speed and video 300 V/µs to 6000 V/µs

  • Special amplifier e.g. for pulse > ca. 6000 V/µs

  • Hifi amplifier 0,5 V/µs to 80 V/µs

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Fast Power Amplifier for Measurement Applications

Figure 6 shows an excellent rectangle answer (initially and output signal) fast achievement amplifiers for measuring purposes. The amplifier controls large solar modules to the characteristic curve measurement from the no-load operation into the short-circuit inside, during a transient lightning lighting of few milliseconds duration. Here for the demonstration of the amplifiers stability a strongly capacitive load is loaded with 2 kHz rectangles. These amplifiers development was part of my job.

So a excellente rectangle answer must look, also under high electric current and with at the same time strongly capacitive load. The measurement was scaled in such a way that the amplitudes receive the same vertically scaling from input and output signal. It is only very difficult to recognize, since both signals lie nearly one above the other. The Slew rate was not measured here

Two links on these pages to the Slew Rate:

What are Open Loop, Slew Rate and Bandwidth?        Slew Rate Measurement Forest and Meadows Amplifier

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