Hi,
I just measured DC offset of an SE MOSFET power amp. Since I use vertical MOSFET (IRFP 240), it seems that it will vary as themperature changes. The output should be 1/2 Vcc, but due to themp rise, it may go to > 1/2 Vcc. Iq itself should be constant as I use BJT feedback control for CCS (taken from current sense R), while the R is a high quality (Dale non inductive). So I suspect the problem may be in the MOSFET itself.
I have some NTC, but not sure where I should put it to compensate bias. Could I use it in MOSFET side (e.g. to the ground - as R will be lower if hot, means lower voltage), to compensate (lower bias voltage) when it is hot? But how to keep it balance, not over-compensate?
Thanks,
Ervin L
I just measured DC offset of an SE MOSFET power amp. Since I use vertical MOSFET (IRFP 240), it seems that it will vary as themperature changes. The output should be 1/2 Vcc, but due to themp rise, it may go to > 1/2 Vcc. Iq itself should be constant as I use BJT feedback control for CCS (taken from current sense R), while the R is a high quality (Dale non inductive). So I suspect the problem may be in the MOSFET itself.
I have some NTC, but not sure where I should put it to compensate bias. Could I use it in MOSFET side (e.g. to the ground - as R will be lower if hot, means lower voltage), to compensate (lower bias voltage) when it is hot? But how to keep it balance, not over-compensate?
Thanks,
Ervin L
single ended running on dual polarity supplies can have near zero output offset.
But you must control that offset. Feedback or servo or some other compensation.
Single ended running on a unipolar supply must have an output offset. This is often near half the supply voltage. This is usually blocked with a DC blocking capacitor. The small changes in output offset before the DC blocking capacitor are of little consequence provided a margin is available to prevent clipping of the output signal.
But you must control that offset. Feedback or servo or some other compensation.
Single ended running on a unipolar supply must have an output offset. This is often near half the supply voltage. This is usually blocked with a DC blocking capacitor. The small changes in output offset before the DC blocking capacitor are of little consequence provided a margin is available to prevent clipping of the output signal.
If you only have one active device in a circuit loop (in this case your MOSFET) then you can't separately regulate voltage and current, as Ohm's Law insists on coupling them. You can regulate one, or the other, or some linear combination of the two but that is all. In effect you only have one knob to twiddle. To control both of them separately you need two active devices.
evinl, I'm no solid state expert, but I'll have a go...
my understanding is as follows...
Your circuit is... Single Ended IRFP240 MOSFET source follower output, sitting on a Constant Current Source. The power supply is single polarity, therefore you want the DC voltage at the output (MOSFET Source) to be at V supply / 2 , and it will be capacitor coupled from the ouput to the load.
Your problem is... as time passes, as the circuit temperature rises, the DC voltage at the output drifts (up).
The likely causes are...
1) temperature rise causing change of characteristic in (vertical type) IRFP240 MOSFET
2) heatsink not keeping temperature low enough for stable operation
3) Constant Current Source can not compensate (?not sure about this one?)
The likely solutions are...
A) use 'much larger' heatsink
B) use exactly the same type of IRFP240 MOSFET as the 'pass transistor' in the Constant Current Source, and attach it to the same heatsink as the output transistor [...for equal temperature drift?]
C) control the Constant Current Source with a temperature compensated device... I have seen this done with a TL431 (but I do not have a schematic)
Hope this helps,
G.
I suppose you are aware that it's not possible to get a qualified answer without the exact schematic of your amp?
The rise in offset is caused by the current through the mosfet increasing with temp. If you are using a ring of two current source, mount the current sense transistor on the heatsink next to the mosfet. This will cause a slight increase in current through the ccs with rising temp. and help counteract the rise in offset. The only other real option is to use a servo.
I use base schematic from Pavel Macura + Shaan modification, but I modify both. I use dual rail supply, use 0 / GND as signal pass and center reference (I use DoZ pre amp dual rail schematic). I modify the pre amp feedback to take from MOSFET output to get 0/center reference (as if I connect this to pre amp output, it will be > 0 Volt = Vgs required for MOSFET to work at Iq set up by the CCS).
I use BJT as CCS instead of the same MOSFET, but I can replace/swap with same MOSFET as I use BJT feedback to control the basis/gate voltage.
I use BD 139 for that instead of TL 431, but not in the same HS of MOSFET/CCS. Should I change BD 139 to TL431?
I don't add the capacitor in the basis of BJT CCS, should it source of problem (e.g. oscilating)?
I am also not sure where to put the NTC (or LM35) as compensation, as the MOSFET and BJT CCS does not have the same thermal effect.
Thanks,
Ervin L
Yes. And I add pre amp Doz direct couple, and use dual supply so that there is no need of Elco in the signal (input-output). Only 1 coupling in input. But I have to fight to stabilize DC offset. Anything wrong?
I also tend to replace BJT with JFET but it doesn't work yet.
Ervin L
Hi,
I just rearrange the SE to Push Pull Complementary, it seems that DC offset is more stabil. I don't know why, but may be since I am using complementary MOSFET (240/9240) rather than MOSFET-BJT in previous design (Pavel Macura based SE).
Anyway, I still like the sound signature of SE, may be it has more even / 2nd harmonics. In Push Pull, the sound is so clean, without any audible harmonic, so all depends on the source.
About the power in PP, does it still matter with Iq adjustment, as MOSFET will always On anyway?
Ervin L
I just rearrange the SE to Push Pull Complementary, it seems that DC offset is more stabil. I don't know why, but may be since I am using complementary MOSFET (240/9240) rather than MOSFET-BJT in previous design (Pavel Macura based SE).
Anyway, I still like the sound signature of SE, may be it has more even / 2nd harmonics. In Push Pull, the sound is so clean, without any audible harmonic, so all depends on the source.
About the power in PP, does it still matter with Iq adjustment, as MOSFET will always On anyway?
Ervin L
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