LTspice square wave and audio amp test


2008-10-28 8:55 am
Hi.supposing we want to check the transient response of an audio amp with a square wave at the input.

how we set up the Trise and Tfall parameters of the voltage source in ltspice?

should we place a low pass filter after the square wave source in order to suppress the high frequences generated from the fundamental or the amplifiers filter at the input is sufficient for the simulation?

the values i was using fot Trise and T fall until now were 1 nanosecond for both.i dont know if these values give realistic results.
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2006-09-08 2:04 pm
A purpose of a squarewave test is to excite all possible resonances in the DUT (amongst other things).

This may not be realistic, because it would be difficult to feed such an ideal signal to the amplifier in the real world, but it can give insight into possible malfunctions.
If you see a severe ringing at tens of MHz, you can suspect a local instability in small signal stage or something similar.
The resulting high slew rate can also trigger interesting behaviours from the AUT.
Again, this is not necessariliy realistic, but it can give clues to potential limitations.
In addition to what Elvee said, I use the squarewave test to check the maximum slewrate of the amp and if it is symmetrical.

I use a squarewave with very steep transitions to design an input lowpass filter that ensures that the amp is never going into slew-saturation (a sharp squarewave input resulting in a full-scale output is pretty much the worst case I can think of)

I also use squarewave inputs with longer transitions (in the range of 3µs, depending on the circuit and what I want to see) without an additional filter to check what the output of the amp does at a defined dV/dt transition (like 20V/µs). Especially to check if the crossover transition from one device conducting to the other one goes smooth (which depends very much on the used model of the outputtransistors in place)