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35W MOSFET Power Amp - Your Opinion
35W MOSFET Power Amp - Your Opinion
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Old 21st March 2005, 07:27 AM   #1
binary is offline binary  Brazil
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Thumbs up 35W MOSFET Power Amp - Your Opinion

Attached is the pdf for the amp I designed using IRF540 / IRF9540 Mosfets complementary out.
I would like to read your comments in order to improve this project where needed, as I am planning to assemble it soon.
Thank you.
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File Type: pdf power_mosfet.pdf (11.1 KB, 931 views)
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Old 21st March 2005, 02:58 PM   #2
MikeB is offline MikeB  Germany
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Have you made sims with this amp ?
Have you checked distortionlevels for higher freqs like 10 khz ?
Connecting mosfetgates directly to a lowcurrentvas can be problematic...
With these 2 gates you have ~4nF capacity-load to the vas.
Or is it intended for a subwoofer ?

At these voltagelevels the to220 case from the mosfets can
be too small. You might consider a IRFP-type. (irfp240/9240
these have even lower inputcapacitance)

Your vas looks like ~6ma, that makes 250mw dissipation for
Q4/Q8, that gives ~80° heat. This works, but i wouldn't feel
good with that.

But, it's a "simple" amp, why don't you just try it ?
Everything else is naked theory...

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Old 21st March 2005, 04:33 PM   #3
jcx is offline jcx  United States
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I would make R14,16 = R13,15 to get equal load on diff pair and speed up Q7

equalize dc bias point by adding a diode (or diode connected Q) in series w/R16 to improve Q8 current mirror operation

equailze Q4,7 collector V with a series R || C or a Zener in Q7 collector to get the same dc power dissapation and better balanced operation in Q4,7 (and diode+ R16)

more current (and pwr without going to power packages) in vas/mirror can easily be had by paralleling extra Qs and emitter Rs with Q4,7,8

C2,5 should be non-polar, C2 should be at least 10X higher value

tail current resistor can be bootstrapped with R from output to get nearly constant tail current; add ( R1 / R20 ) * R6 between output and top of R6

R4,5 drop a lot of V but reducing them costs loop gain; added gain can be had with a R between Q4,7 emitters - once you've equalized their current as in my 1st recommendation
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Old 21st March 2005, 05:50 PM   #4
jcx is offline jcx  United States
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"(and diode+ R16)" goes with end of the next recommendation, too late to edit
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Old 21st March 2005, 08:35 PM   #5
jleaman is offline jleaman  Belgium
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Looks exactly like my amp's : O )


Except now i have a newer version of the board design and schematic..
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Old 22nd March 2005, 07:58 AM   #6
binary is offline binary  Brazil
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Dear Friends,
Thanks for all your comments. It will be useful at the time to build the final version.
I designed this amp on CircuitMaker 2000. I intend to use it to hear CDs, DVD-Audio and / or to plug my steel guitar on it. My intention is to get Hi Fi sound, but I can build an identical separate unit for subwoofer.

By the way, I have an idea to improve this circuitry that is to install a dynamic variable bias system on this amp, to work like the Super A amplifier, by modulating the base of Q6 with signal rectified output signal.

Now I am at the office, when I get home I will do these changes and submit a new design for you, including the above idea.

PS: jleaman, this circuit really was not based on you project.

Thanks for attention of all.
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Old 22nd March 2005, 08:00 AM   #7
jleaman is offline jleaman  Belgium
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It look very simular to mine. : O )
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Old 22nd March 2005, 09:11 AM   #8
traderbam is offline traderbam  Europe
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I don't entirely agree with all the previous comments, so you'll have to make your mind up which you adopt. Mine are:

1) C5 is the wrong way around (the base of Q3 will be slightly negative due to base current flow through R11)

2) If C6 is intended to filter out HF then it needs to be connected between Q3 base and ground and needs to be 300pF to 500pF.

3) Q6 is acting like a battery so it is prudent to place a wide-band capacitor between its collector and emitter, such as a 50uF tantalum.

4) The zobel network, R10/C4, is on the wrong side of L1. The zobel is there to provide a defined output load to the amp at HF. L1 does the opposite as its reactance increases with frequency.

It may be a little unstable in the flesh so check the performance carefully.
Have fun,
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Old 22nd March 2005, 09:30 AM   #9
Upupa Epops is offline Upupa Epops  Czech Republic
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This amp is too simply for good function of Mosfets. As exercise is it OK, but parametres will be not " top class ". Belive me, I had build many Mosfet amp.
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Old 22nd March 2005, 01:13 PM   #10
ilimzn is offline ilimzn  Croatia
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I would have to agree with some of the mods proposed. You are 1 resistor and 1 diode away from having a two stage differential amp with current mirror, so why not make one - it will work better.

Q4 and Q7 will form a differential VAS is you make R14 = R13 and connect a resistor between their emitters (this will have to be quite small, on the order of 10 ohms).

Making R16=R15, and connecting a diode in series with R16 (or better still a diode connected 2SA2240) will give you a proper current mirror.

The various minor things mantioned in the thred should be done too (C6 connects to the other side of R12, R10 to the other side of L1, C2 is the wrong way around).
I would strongly advise a constant current source in the tail of the first stage, as the VAS current will also depend on it, and hence the voltage across Q6, which is the bioas voltage. Otherwise, you will get bias variable with supply voltage. not a major issue but cheap to fix.

Three other things I would like to add:

1) This amp has two Cdom, the capacitance betwen C and B of Q4 and Q7 (normally one would find a small cap here, you will probably have to add them in the real amp). But beware: because C of Q7 sees nearly no voltage swing, a capacitor between C and B of Q7 will have little effect, unless you make C of Q7 swing about the same as C of Q4. You can do that by inserting a resistor between C of Q7 and where it now connects to Q8 + R16. The resistor will also take some of the power dissipation of of Q7. The actual value is largely a compromise and you will have to play with it in your simulator. You can use te AC analysis seto to a very high upper limit frequency to see how the gain curve of the amp looks like. Cdom compensation will give you a 6db/oct falling gain slope that should stay th same right down to the point gain = 0dB, this way the amp will be unconditionally stable.

2) A Vbe multiplier used for biasing MOSFETs (Q6 in your schematic) will invariably overcompensate bias voltage with temperature the way you are using it. The standard way of dealing with this is to insert a resistor in the emitter circuit or Q6. the non-standard way, which i have found to work far better, is to use the same element for temperature sensing, as the one making the heat - therefore, use a Vbe multiplier for BJT outputs, Vgs multiplier for MOS outputs. In other words, make Q6 a MOSFET. A small one like 2N7000 or BS170 will work, but I like something in TO220 best (IRF510, 610, 520) - even though it requires mounting hardware, it offers little thermal resistance to the heatsink and reacts fast and accurate. Vgs multipliers are considerably 'worse' than Vbe, because of lower MOSFET gain. Because of this you need a low ESR capacitor between it's D and S lines. I have to disagree with one of the previous posters - tantalums, even though evolved from what they used to be, REALLY should be avoided. They are the most unreliable capacitor. Use a regular or low ESR electrolytic with a non-inductive foil cap on the order of 10-47nF in parallel. unusually, as this kind of cap should not be used in audio, ceramic caps can work good here as well.
Another possibility is to bypass the top resistor (between D and G) with a 10-47n non-inductive ceramic cap.
A Vgs multiplier has the advantage that you can use relatively large resistors for R17, R18, VR1. Also, because you need to compensate for 2 x Vgs (output transistors) + Ibias x (R8 + R9), R17 and R18+VR1 will be nearly equal.
A Vgs multiplier also has the advantage that it tracks the thermal characteristics of the output transistors precisely, which means you can use smaller R8 and R9, i.e. the values you can use are less crytical so you can fine-tune them better.

3) Driving MOSFETs directly from a Vas may be asking for serious trouble, in the form of parasitic oscillations of the output MOSFETs. Your Vas is fairly low current so you may get away with it, but I would advise you to put series resistors in te gates of the output transistors (often even 10 ohms is enough). The added cost is negligible and it avoids a potential problem with little or no degradation of the amps characteristic

Finally, I really don't want to disclose all my secrets but I may be able to point out the way. Consider this question carefully:
Given that unmatched output transistors result in a less linear output stage, and output stage nonlinearity is typically the largest contributor to distortion, why do people use IRF540/9540, IRFP240/9240 and other 'complementary' MOSFETs when the only three things that make them complementary are the way they look, the fact one is N and the other P channel, and that they both have the same maximum voltage?
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