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Old 6th April 2005, 11:17 PM   #1
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Default First amplifier attempt

Here my first attempt at an amplifier schematic. Can you guys please check that the design will work and I haven't done anything stupid?

Also, I need help figuring out what components/values all of the transistors, diodes, and that feedback capacitor in the VAS should have.

I want to have an output power around maybe 100 watts.

And as a technical note, this is a Class-B output stage, right? I believe its a complementary feedback pair.

Finally, how should I go about designing a PCB layout? I understand that it is a crucial part of the design of an amp, but I don't know how to lay one out or which parts needed to be layed out in particular ways, etc.

Click the image to open in full size.

Thanks a million,
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Old 6th April 2005, 11:29 PM   #2
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Oh, and should I use MOSFETS as the followers in the Output stage?
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Old 7th April 2005, 01:23 AM   #3
fab is offline fab  Canada
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Hi Scott

I believe also in the virtue of the CFP.

Your input stage may not be correct with these resistors value.
The current source gives about 650 ua,m so 325 ua per leg and the 2k2 reisitor uses about 295 ua which gives only about 30 ua for the CFP transistors. And these calculations are theoritical since the vbe of the transistors may not be 0.65v. I would increase the tail current so the CFP transistor have more current than the input transistors.

The negative feedback resistors divider is missing.

The RC network is missing at the amp output.
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Old 7th April 2005, 04:36 AM   #4
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Hi Scott.

Here goes. Flip the front end over and use a 2SK389 dual FET then cascode it with npn bipolars and use a 2 transistor current source. Use a resistor as a load and a buffered Vas and your CFP output. Still simple but good supply rejection so the performance is much less sensitive to power supply vagaries. Could you label the components to ease discussion.
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Old 7th April 2005, 05:43 AM   #5
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circuit topology looks nice of course you have a few errors as some of the other members have pointed out
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Old 7th April 2005, 03:33 PM   #6
Leolabs is offline Leolabs  Malaysia
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Looks like LM12 inner to me
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Old 7th April 2005, 10:58 PM   #7
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2k2 at input is very low input Z and makes pole (with input cap) 'too close to' audio range. Use at least 10 times bigger.

I am wondering about your 2k2 as local feedback in VAS...
I don't think you will really need it, also it lowers PSSR.

Are transistros in mirror not swaped left with right?

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Old 7th April 2005, 11:35 PM   #8
ilimzn is offline ilimzn  Croatia
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LTP tail current reference diode string and current mirror VAS load should not be connected?!
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