200 Watt Lateral Mosfet Amplifier

I can't see where the resistors are but if they are gate resistors, the value is OK.

Groundrule is as low as possible to avoid (MOSFET-) oscillations but not too large because the output stage will get slow.

The value is easy to check if you have an oscilloscope. Usually it the N-channel which oscillates, also when the current is over a certain value. This type of oscillation can be real nasty and you can also hear them. It's important to get rid of these oscillations.
I think that I had same problems that macgyver have.

If you build that amplifier, it doesnt work if there is MJE340 (Q3) as voltage gain stage and resistors R3 and R8 are 470ohm. Voltage gets too high at base of the MJE340 and output is driven to - supply rail. I put 75ohm resistors to R3 and R8 and amplifier worked fine.

R3 and R8 are at emitters of current mirror.

I think that R3 and R8 should be 470ohm when you use IRF610 as voltage gain stage (Q3).

It would be nice if someone could confirm my conclusions.
I have checked the schematics (my first answer wasn't :bullseye: exactly) and I suspect something is wrong in the interfacing between the first and the second stage. At DC Q3 must have the same current as the Q10 which is 10 mA. R3, R8 and R16 don't match.

Anthony Holton, have you any explaination of this?

Whitout any further investigation I suggest R3, R8 to be 100 ohm.
peranders said:

You can download the pdf file of this project, much clearer...

Yes, I constructed that amplifier based on that pdf file and the whole problem is that amplifier doesn't work if it's build based on that pdf construction manual.

That was the point of the topic starter too.
Maybe Anthony accidentially left wrong resistor values in that pdf file.

If you compare blurry picture to pdf file, you notice that resistor values can't be same, even if you can't see exact values in the blurry picture.
carlmart said:
Why don't you ask Anthony himself about that?

Make a search on this Forum and mail him.


I heard somebody already asked, but Anthony is maybe busy or something and didn't answer. And I havent seen Anthony writing here a while.

My another channel will be ready soon, let's see how it works compared to another channel, witch is build using different components (R3, R8, Q3).
Greetings Gentlemen

The 470 Ohm resistor values were set for
+-70 volt rails. For lower voltage rails values between 100 to 220 can be used.
Experiment it changes the sound quality...

For irf610 as the VAS use a source resistor value between 10 and 47 ohms
and R3 and R8 220 Ohms .

If you use the zoom function in Acrobat reader your can get a fuzzy free image of the schematic. Download the PDF from my web site at this link http://www.aussieamplifiers.com/download.htm


Anthony Holton

The Saint said:
Greetings Gentlemen

If you use the zoom function in Acrobat reader your can get a fuzzy free image of the schematic. Download the PDF from my web site at this link http://www.aussieamplifiers.com/download.htm


Anthony Holton


Yes, I know how to use zoom funktion :D

The fuzzy schematic is http://www.aussieamplifiers.com/images/chpsch.jpg
I noticed that values of resistors in that picture are below 100ohm and I tried 75ohm succesfully.

I am using +-45V volt rails (Unloaded 63V).

This is my first DIY amplifier and I have learned a lot about amplifiers, because these problems made me think about what is the purpose of all those components.

Thanks for answering Anthony.
The Saint (Anthony), shouldn't you mention (in your documentation) about the workning point of the second stage? This can be essential in order to succeed, for the poor guys who don't know so much about amp design.

The current generator at the top must have the same current at the bottom transistor with a fixed "base-neg rail" voltage, taken from the diff stage.
Heatsink requirements


Does anyone have any guidelines on the heatsink requirements of this N-channel amp. I would like to build one (+-70V rails) using the 3 pairs of outputs, but need help selecting a heatsink (i will use a fan too).

I know that for BJT's, you can calc their dissipation as:

Vtr * Itr

(voltage across transistor * current though it). Does this apply also to mosfets? What calculation is used if not?


Mosfets are the same except that the turn on resistances is much higher, therefore the thermal losses are higher.
If you allow for IRFP240 devices, about 2 volt drop across them.
Lateral devices which have a turn on resistance of around 1.5 Ohms will drop 4 to 5 volts across them.

General rule of thumb with the N-channel is to select a heatsink with a rating of at least 0.5 degrees/watt or better.

With fan cooling, which I think in this case is a bit of an overkill. Unless you are going to be using this amplifier in PA applications, drop the fan cooling.


Anthony Holton
If you don't mind I must correct you guys a little bit.

Maximum power dissipation has little to do with Rdson. Rdson is only important IF:

1 The driver can drive 12-14-volts ABOVE the power supply voltage. This means that you must feed the driver with 15-20 volts more than the "power" voltage. My QRO amp used +-60 V "power" voltage and +-80 V driver voltage. I simply added 20 volts to the 60.

2 You really want to squeeze out everything from the amp.

Maximun power dissipation is given by

Pdiss = ((Vcc)^2)/(4*Rload), note resistive load!

This maximum power loss comes at one third(*) of full power (assuming that PS is stiff and you can get a voltage swing near the power supply voltage (Rdson important).

Those of you who want to read more search for the pdf "Power Operational Amplifiers" by Robert Louis Watson, found at Burr-Brown/Texas. Don't have the exact URL.

(*) I'm not sure of this, taken from the memory.