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Could someone please explain the importance or non importance in slew rate when selecting an amp. What are effects, what is considered a fast or slow slew rate? Thanks inmates for your help.
Follow Ups:
You can count on "way too slow" to matter, and "way too fast" to perhaps indicate that the amp *could* be unstable (BUT that doens't have to be, it's just possible).To give you an idea, a 20kHz sine wave at maximum amplitude 'X' in volts (1.4 * RMS value) has a slew rate of
2*pi*20000*X. in volts/second. Divide by 1,000,000 for volts/microsecond.
If your amp delivers 'Y' watts into 8 ohms, that means that the RMS voltage is sqrt( 8 * Y) volts. An easier way to calculate 'X' above is to figure out, then, sqrt(16*y) (that accounts for the 1.4 factor), and put that into the 2*pi*20000*X equation above.
ANY amp with power output Y should ALWAYS have a higher number than what you get out of that equation. Another spec to look for is "full power bandwidth", it will usually tell you the maximum frequency you can get through the amp without slope (slew-rate) limiting.
btw, sqrt() means "square root".
What IS slew rate? It's the rate at which the amp can change its output voltage, in calculus terms, it's the minmax of the absolute value of the derivitive of the output voltage. (I.e. the minimum value that the maximum absolute value of the derivitive can take)
JJ
Thanks for this very theoretically well founded statement. As i can see your calculations go well for use in feedback less systems, to allow minimum the desired frequencies, as sine waves to get through.But what happens when you have a global feedback system, like an op-amp, would the slew rate of the amplifier not affect the feedback factor at higher frequencies?
The output continously tries to track the input signal, but the slew rate would probably be more or less proportional to the propagation delay in the op amp. This means that the output would track an input signal, with a delay of XX microseconds where XX gets higher for a lower slew rate amplifier.
I think the sonic properties of slew rate is VERY much: 'It depends'.
The simple answer is: Use a VERY high slew rate but Unity Gain stable op amp like the LM6172 (National) or AD825 (Analog Devices) and use a good bypassing scheme. Then you are sure to get the best performance.
Lars Clausen
My comments hold regardless of feedback.But setting a 20kHz bandwidth is, as I said before, a very EASY spec.
Anything should meet it.
JJ
The slew rate of the output drivers generally determines the bandwidth of the amp, however, this is usually specified into a particular specified load, like 8 ohms pure resistive. Slew rate can be affected by adding complex inductive/capacitive loads. I'm not sure there is really any industry standards, like there are for frequency response, so the conditions of this measurement may vary significantly from amp to amp. In any case, higher slew-rate is always better, because it prevents amplitude roll-off and phase shift at the highest audio frequencies. Comparing amps using slew-rate may be problematic, because it may not be apples-to-apples.A fast slew-rate might be 100V/Usec and a slow one might be 1V/Usec.
Actually, both jj and you are correct. If the output devices are too slow, they will limit the slew rate, all else being equal. The Ft of the output devices is also important, but not 100%. A more modern input and drive stage can drive 'slow' devices better than a more primitive circuit, because the Gm of the input stage can be reduced, and this will lower the compensation cap's value.
Slew rate is the same as power bandwidth this is how powerfull the amp is at high freq. As jj's example calculation shows the max rate of change of a sine function is a function of it's freq and of its amplitude. When the slew rate of the amp is small the max amplitude at a high freq is small.
That depends greatly on the design of the amp.
JJ
The slew rate of a bi-polar transistor three stage audio power amplifier is usually determined in the second stage. A three stage amp usually consists of a differental input stage, a gain stage where the output voltage is developed, followed by a buffer stage. The stability of the amplifier is usually established by the miller capacitance of the voltage gain stage. This capacitance is being charged by the currents in the differential input stage. It is the current limitations of the diff stage in charging the miller capacitance that determines the slew rate of the amplifier. The output stage normally has no voltage gain and only has to supply enough current to charge any capacitance on the output. This capacitance is not that great and can easilly be charged by the currents in the output stage. Driving a 4 or 8 ohm load requires more current that a few hundred picofarads of capacitance.
Slew-rate is obviously measured at the output drivers and is a function of their drive, biasing and power supply. You are correct, predrivers can and will affect slew-rate.
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