First amplifier attempt

scott_jcp · 6th April 2005, 11:17 PM

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.

Thanks a million,
Scott

scott_jcp · 6th April 2005, 11:29 PM

Oh, and should I use MOSFETS as the followers in the Output stage?

fab · 7th April 2005, 01:23 AM

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.

amplifierguru · 7th April 2005, 04:36 AM

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.

mastertech · 7th April 2005, 05:43 AM

circuit topology looks nice of course you have a few errors as some of the other members have pointed out

Leolabs · 7th April 2005, 03:33 PM

Looks like LM12 inner to me

darkfenriz · 7th April 2005, 10:58 PM

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?

cheers

ilimzn · 7th April 2005, 11:35 PM

LTP tail current reference diode string and current mirror VAS load should not be connected?!

6th April 2005, 11:17 PM	#1
scott_jcp diyAudio Member Join Date: Feb 2005 Location: Richardson, TX	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. Thanks a million, Scott

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6th April 2005, 11:29 PM	#2
scott_jcp diyAudio Member Join Date: Feb 2005 Location: Richardson, TX	Oh, and should I use MOSFETS as the followers in the Output stage?

7th April 2005, 01:23 AM	#3
fab diyAudio Member Join Date: Mar 2004 Location: Canada	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.

7th April 2005, 04:36 AM	#4
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	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.

7th April 2005, 05:43 AM	#5
mastertech Banned Join Date: Mar 2005 Location: Australia	circuit topology looks nice of course you have a few errors as some of the other members have pointed out

7th April 2005, 03:33 PM	#6
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	Looks like LM12 inner to me

7th April 2005, 10:58 PM	#7
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	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? cheers

7th April 2005, 11:35 PM	#8
ilimzn diyAudio Member Join Date: Feb 2005 Location: Zagreb	LTP tail current reference diode string and current mirror VAS load should not be connected?!

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