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Old 11th March 2015, 08:15 PM   #1
CSOLEE is offline CSOLEE  United States
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Default Simple Mosfet amp -HELP

Need help. Built the following amp using FQA28N15 and 36P15 Mosfets. Based on the VSSA amps. Wanted a nice small board 10x10cm, everything fits. Did not want to buy anything so used what I had. The LTspice simulation works fine but when I tried a prototype out, the gate resistors for the outputs smoked when I got the output bias voltage to about 7.2 volts. Seemed like the outputs turned on and took off, voltage across the output source resistors kept going up. Any ideas? Bad design? I know the original VSSA circuits used Laterals.
Spice schematic attached.
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Old 11th March 2015, 08:56 PM   #2
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How did you get 7.2V of bias voltage across those two diodes?
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Old 11th March 2015, 08:57 PM   #3
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Output FETs are the wrong polarity or are drawn wrong. N channel have the arrow pointing out. P channel, pointing in, like standard semiconductors.
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Old 11th March 2015, 08:58 PM   #4
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Correct schematic attached
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Old 12th March 2015, 05:30 AM   #5
CSOLEE is offline CSOLEE  United States
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Quote:
Originally Posted by CSOLEE View Post
Need help. Built the following amp using FQA28N15 and 36P15 Mosfets. Based on the VSSA amps. Wanted a nice small board 10x10cm, everything fits. Did not want to buy anything so used what I had. The LTspice simulation works fine but when I tried a prototype out, the gate resistors for the outputs smoked when I got the output bias voltage to about 7.2 volts. Seemed like the outputs turned on and took off, voltage across the output source resistors kept going up. Any ideas? Bad design? I know the original VSSA circuits used Laterals.
Spice schematic attached.
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File Type: png VSSA MOSFET AMP.PNG (68.1 KB, 169 views)
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Old 12th March 2015, 12:13 PM   #6
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Quote:
Originally Posted by JonSnell Electronic View Post
Output FETs are the wrong polarity or are drawn wrong. N channel have the arrow pointing out. P channel, pointing in, like standard semiconductors.
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Old 12th March 2015, 12:14 PM   #7
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Originally Posted by CSOLEE View Post
Correct schematic attached
Inventing the wheel
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Old 13th March 2015, 08:26 AM   #8
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Quote:
Originally Posted by JonSnell Electronic View Post
Output FETs are the wrong polarity or are drawn wrong. N channel have the arrow pointing out. P channel, pointing in, like standard semiconductors.
No they don't. He has drawn them correctly.
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Old 13th March 2015, 08:35 PM   #9
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Inventing the wheel
HA, HA, you are so funny the lazy cat.........................
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Old 13th March 2015, 09:39 PM   #10
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The bias circuit is very poor, you need a proper Vgs or Vbe multiplier or you will get cross over distortion.
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