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 davidel94 12th February 2013 07:52 AM

audio amplifier bandwidth

hi I am simulating an amplifier but there is a problem:
with no load the bandwidth is 0Hz-480kHz while with 4 ohm load it is 0Hz-200kHz.
what does it mean?

 jan.didden 12th February 2013 08:01 AM

Transistors have a gain that drops with Ic. If you have no load, you don't see that drop. With load, you see a lower gain at the frequency you are looking at. That manifest itself as a faster roll off.

jan

 Bonsai 12th February 2013 04:25 PM

Has your amp got an output coil?

 TANDBERGEREN 12th February 2013 06:56 PM

Looks good actually.
At what output voltage are you measuring the bandwith?

 davidel94 13th February 2013 10:53 AM

yes it has the output zobel network and then a coil of 2uH in parallel with a 2ohm resistor. the input voltage is 1V while the output is 27.3dB.

 JMFahey 13th February 2013 11:41 AM

Quote:
 hi I am simulating an amplifier but there is a problem: with no load the bandwidth is 0Hz-480kHz while with 4 ohm load it is 0Hz-200kHz.
That's not a problem :eek:

 magnoman 13th February 2013 01:11 PM

If you are simulating using ac analysis then the response is linear about some initial operating point, thus the effects like changing beta (with Ic) and voltage level shouldnt show up.
But as Bonsai points out the R/L is different for each run ( 2uH / 4 ohms is pretty close to 200Khz, but there must be some additional contributions).
Thanks
-Antonio

 davidel94 13th February 2013 04:59 PM

I repeated the simulation without the input filter, the output zobel network and inductor, now there are no variations on the bandwidth. thanks a lot to everybody

 JMFahey 13th February 2013 05:03 PM

That's fine ... in the simulation World.
Now if you are ever going to build that amp and connect it to anything, please do add all the auxiliary stuff back.

 Bonsai 14th February 2013 05:53 AM

You should include an input filter and I always recommend an output inductor /Zobel network.

Try running your sims without the the input filter and output inductor/Zobel network with a capacitive load to get a measure of how load tolerant your design is. Start at about 100pF and work your way up to say 2uF in half decade steps. You can use the .step param command.

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