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.
and you know what i can answer it too.. my n-channel amps run just fine just the way they are no problems and makin them distort is hard and ect ect the amps runs verry strong i love it im glad that i bought and built one..
I just want to unsterstand it better, as I'm only a newbie, I thoght Anthony could give better explanatios, it's his project!
And I want to understand how it works, every resistor, the ones in this topic for example, I don't understand well.
I have checked the schematics (my first answer wasn't 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.
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.
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.
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.