MOS单管小甲

老爷车 · 发表于 2008-1-2 21:07

您上面说的意思说，纳而讯先生用的是120V300W的灯泡，而我用的是220V的灯泡，这样要么用600W的，要么提高电源电压。对吗？

老爷车 · 发表于 2008-1-2 21:16

原帖由 millwood 于 2008-1-2 21:04 发表

do you have a resistor on the source of the mosfet? between the source of the mosfet to ground? R6 in my shcematic? It is there to help stabilize bias and dc working point.

在源极加一个小电阻稳定电流？

millwood · 发表于 2008-1-2 21:17

原帖由 老爷车 于 2008-1-2 21:07 发表
您上面说的意思说，纳而讯先生用的是120V300W的灯泡，而我用的是220V的灯泡，这样要么用600W的，要么提高电源电压。对吗？

using 22v 300w bulb will work. and doesn't make a difference for now - your goal is to get the amp to sound.

you just need to remember that your light bulb has a resistance of 160ohm, while Nelson's has a resistance of 40ohm so your amp may not have the ability to deliver as much current as his. In other words, your amp will clip earlier than his when driving the same load.

millwood · 发表于 2008-1-2 21:18

原帖由 老爷车 于 2008-1-2 21:16 发表

在源极加一个小电阻稳定电流？

yes. it is absolutely essential to do that for D-MOSFET.

老爷车 · 发表于 2008-1-2 21:23

原帖由 millwood 于 2008-1-2 21:17 发表

using 22v 300w bulb will work. and doesn't make a difference for now - your goal is to get the amp to sound.

you just need to remember that your light bulb has a resistance of 160ohm, whil ...

22V300W! 哈哈，只能用汽车灯泡了。
他的阻抗低，我的阻抗高，要达到同样目的，我只能用更高电压了。

millwood · 发表于 2008-1-2 21:25

your 2sk1058 is a lateral mosfet, some of the earliest mosfet and was designed specifically for audio. it has lower gain (gm), and lower input capacitance (Ciss) and most importantly it has a negative temperature coefficient starting at ~200ma: when the temperature of the die goes up, its internal resistance goes up limiting the current. This helps it stabilize its current - thus you do not need a source resistor there.

most of the industrial mosfets we use today are Vertical mosfet (or D-MOSFET). They have higher gain (gm), higher input capacitance as well as a negative temperature coefficient. However, that negative temperature coefficient starts to kick in at 20-30amp range. As such, you have to have a source resistor to help those DMOSFET stabilize thermally.

if you don't, you risk destroying your mosfet and end up with a circuit that only works when the temperature is right.

老爷车 · 发表于 2008-1-2 21:27

原帖由 millwood 于 2008-1-2 21:18 发表

yes. it is absolutely essential to do that for D-MOSFET.

用稳流管做负载不能达到同样目的吗？

老爷车 · 发表于 2008-1-2 21:40

提高电压至120V试下。

millwood · 发表于 2008-1-2 21:51

原帖由 老爷车 于 2008-1-2 21:27 发表

用稳流管做负载不能达到同样目的吗？

you can use a constant current source as a load (on the drain of the mosfet)

But you have to have a source resistor on the source of the mosfet.

they are two separate things.

millwood · 发表于 2008-1-2 21:52

原帖由 老爷车 于 2008-1-2 21:40 发表
提高电压至120V试下。

it works best with a transformer as its load.

Be prepared to turn a few mosfets before you get it right.

老爷车 · 发表于 2008-1-5 18:35

参照纳尔逊的电路，稍微作了些调整，现在基本能正常工作了。

millwood · 发表于 2008-1-5 21:08

原帖由 老爷车 于 2008-1-5 18:35 发表
参照纳尔逊的电路，稍微作了些调整，现在基本能正常工作了。

good steps in the right direction. But more room for improvement.

a) your biasing scheme isn't recommended. the negative feedback used in my earlier scheme and by Nelson helps stabilize biasing in case your supply voltage fluctuates. you will find that as your biasing scheme is highly suspect to fluctions in supply voltage and your amp, as a result, may not work if the supply voltage has the slightest fluctuation.

here is an example.

the first chart shows the two biasing circuits. the one I recommend is on the top and the one you used is on the bottom. they are running each at ~500ma from a 80v supply. reasonably good output.

the 2nd chart shows the same two circuits, except that we are running at 60v supply. the one on the top idles at about 320ma, and produces a nice output waveform. yours is almost completely shut down: idling at 135ua, and produces a highly distorted waveform.

I tend to think that you don't want that to happen to your sound.

b) use a transformer as a load.

you will notice that your mosfet idles at 80v*0.7a=56w. That is a lot of heat to dissipate. Why?

you have to competing factors here.

1) to drive a low impedance load like a 8ohm speaker, you will need to produce a lot of current. When you do that, you will need to have your output device idle at even higher current. That means lots of heat dissipated.
2) to coup with high supply voltage, you will need to idle at low current levels to reduce heat dissipation.

what you want to do is to add a "current amplifier" here so that your mosfet can idle at low current, yet the current amplifier can still deliver tons of current to a low impedance load, like your speaker.

there are many ways to construct a current amplifier. the easier is a step down transformer.

a 220v/22v (10:1 step down) power amplifier, for example, can amplify the output current (variations) 10x.

so if you replace the drain resistors with a 220/22v amplifier, your mosfet can idle at 300ma - 700ma peak to peak, and your output current can swing 10*(700ma-300ma)=4amp. that will produce (4/2)^2*8/2=16wrms on your load. Much more than what a resistor-loaded mosfet can do (in this case, about 3^2/8/2=0.5w).

c) you shoul never use a trimmer as R7/R8: if the wiper ever goes open, your mosfet will be biased very high and burned in a split second.