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Old 12th February 2013, 07:52 AM   #1
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Default 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?
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Old 12th February 2013, 08:01 AM   #2
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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.

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Old 12th February 2013, 04:25 PM   #3
Bonsai is offline Bonsai  Taiwan
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Has your amp got an output coil?
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Old 12th February 2013, 06:56 PM   #4
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Looks good actually.
At what output voltage are you measuring the bandwith?
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Old 13th February 2013, 10:53 AM   #5
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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.
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Old 13th February 2013, 11:41 AM   #6
JMFahey is offline JMFahey  Argentina
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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
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Old 13th February 2013, 01:11 PM   #7
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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
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Old 13th February 2013, 04:59 PM   #8
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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
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Old 13th February 2013, 05:03 PM   #9
JMFahey is offline JMFahey  Argentina
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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.
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Old 14th February 2013, 05:53 AM   #10
Bonsai is offline Bonsai  Taiwan
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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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