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Old 14th February 2006, 01:49 PM   #11
mikee55 is offline mikee55  United Kingdom
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Angry Com on pic

No good at this am I
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Old 14th February 2006, 03:01 PM   #12
anatech is offline anatech  Canada
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Hi Lee1234,
The factory bias setting is 45mA per transistor. A little light maybe for those mosfets, but heatsink size and mounting need to be considered as well. It probably sounded "smoother" with higher bias. You can't apply this experience to all amplifiers though.

-Chris
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Old 14th February 2006, 04:20 PM   #13
Eva is offline Eva  Spain
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MOSFETs are not linear devices. Their "gain" (transconductance actually) is rougly proportional to the drain current squared and is very low until the current has increased to 1A or so. That's why MOSFET amplifiers ask for a high bias, as it linearizes them.

Bipolar transistors do have a much more linear current gain and superposition (high bias) is not desirable, except to compensate for their somewhat lower gain at very low current levels, particularly at high frequencies.
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Old 14th February 2006, 05:05 PM   #14
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It seems that everybody here , talk about the linearity of high or low bias , in the proverbial resistive load.

But the real advantaged of the high bias , both with mosfets or bipolars , came from the fact , that high bias , the open loop output impedance become lower and the performance of the amp become more unilateral with a real reactive speaker load and specially , the EMF almost "sees" a short at the output , with less chances of intermodulation with the input signal...

Sometimes , better than PC simulations , is nice , to make some real tests at the workshop ...
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Old 14th February 2006, 05:11 PM   #15
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Is more important, how is bias stabil, when is floating rail voltage... Sometime I had one well known japanese amp on measuring....Bias, which was setuped in accordance with service manual ( it was class AB amp ) on correct value by AC 230 V, fall by 210 V ( which represent normal " floating " at rails by driwing with musical signal - - 10 % ) at zero = was at pure class B.... And it is feature of commonly used one transistor's biasing circuit, which you can see at mostly of amps....
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Old 14th February 2006, 09:54 PM   #16
Eva is offline Eva  Spain
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Tube dude:

When the bias is set too high in a class-AB bipolar output stage, the output impedance jumps abruptly between the usual value and half that value, depending on whether both sets of output devices are or not conducting at the same time. That jump is a major source of intermodulation. The optimum bias for these designs should just provide a smooth transition between each bank of output devices, compensating for the gain loss when Ic is only a few mA.



Upupa Epops:

I have measured it and I'm fully aware of that issue.

First: relevant changes in bias with small changes in supply voltage should be taken into account (this should only happen in poor amateur designs, despite any marketing).

Second: when an amplifier is playing, the dies of the output devices are somewhat hotter than when it's idle. They may be, say, 20șC or 30șC hotter than the heatsink, but their temperature drops very quickly when you disconnect the load and the input signal in order to measure bias current. After a few seconds, the dies are already at the same temperature as the heatsink and your bias measurement has been fooled. To circumvent that, I leave the multimeter turned on and connected to the test points and I disconnect the input signal and the load very quickly while looking at the display. As expected, the multimeter shows a bias value that may be two or three times higher than expected (only 20% to 50% higher in clever designs with truly good compensation) and this value falls quickly to the expected value after a few seconds

Third: most amplifier circuits, both with MOS and bipolar output stages, suffer from current tails when they are asked to perform relatively quick zero-current crossing events. This happens because the gate or base drive circuit inserts charge into the bases/gates of one side faster than it extracts from the gates/bases of the other side. This in turn causes a dynamic increase in biasing only observed when playing significant amounts of high frequencies. Almost all bipolar CFP designs suffer from that phenomena, and also all MOS designs lacking gate buffers.


I have measured all that. In standard EF outputs, you may make the ground of either your oscilloscope or your amplifier floating. Then, you can just connect the oscilloscope ground to the output of the amplifier (before the inductor, if there is any) and each probe to the other end of one emitter resistor from each bank of output devices. As a result, you will see exactly how much current is being delivered to the load and cross-conducted. In order to get some RF isolation (desirable but not strictly required), I achieve common mode filtering by wrapping several turns of each probe wire around a ferrite toroid

Also, I'm currently working into a MOSFET amplifier that achieves stable bias not dependent on supply voltages or heatsink temperatures, and it's done without any thermal feedback. To achieve that, each output device is enclosed into a local current feedback loop with very high open loop gain. More about that may be read in the "Reinventing the N-channel wheel" thread : http://www.diyaudio.com/forums/showt...895#post843895
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Old 14th February 2006, 10:27 PM   #17
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Quote:
Originally posted by Eva
Tube dude:

When the bias is set too high in a class-AB bipolar output stage, the output impedance jumps abruptly between the usual value and half that value, depending on whether both sets of output devices are or not conducting at the same time. That jump is a major source of intermodulation. The optimum bias for these designs should just provide a smooth transition between each bank of output devices, compensating for the gain loss when Ic is only a few mA.

Mi amigo Eva

Yes, thats is true in the forward path of the amp, but now imagine how the amp behave , when is feed by the output side , when is the transducer that feeds some energy out of phase at the output ...

PS: esta noche devias estar a namorar...
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Old 14th February 2006, 10:46 PM   #18
Eva is offline Eva  Spain
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Nothing special happens. When the load is reactive, the zero-current crossing point just does not happen at 0 volts. Note that open loop output impedance of classic topologies with VAS is very high, since these circuits are current driven and their low output impedance is just an artifact of global feedback. Essentially they act as current servos, sinking or sourcing current from the output as required, until it reaches the preset voltage (following the input waveform). The zero-current crossing process is the same, despite at what voltage it happens, and current gain should be kept as constant as possible during that transition because low output impedance is just a side effect of current gain (and inversely proportional to it).
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Old 14th February 2006, 10:56 PM   #19
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Quote:
Originally posted by Eva
Note that open loop output impedance of classic topologies with VAS is very high, since these circuits are current driven and their low output impedance is just an artifact of global feedback.
Very high open loop output impedance??

Not always...see some Accuphase amps and John Curl designed Parasound 1500...one mosfet driver with near , or less than 0,5 Ohms output impedance , followed bay a bipolar output , with a current gain of ~ 100...we get a open loop output impedance (independent of the overall feedback ) of ~0,5 /100 =0,005 open loop output impedance...

That I call extremely low open loop output impedance , independent of the VAS output impedance and clever design...
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Old 14th February 2006, 11:22 PM   #20
Eva is offline Eva  Spain
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Could you explain how a non-saturated MOSFET may have 0.5 ohms of output impedance?

Maybe a class A source follower swinging betwem 4A and 5A at DC?

This is not practical neither real for AC operation, it's just conceptual over-simplification, because that MOSFET is going to have more than 1nF effective capacitance between gate and source, thus effectively shunting the input to the output. MOSFETs are evil devices, usually analysed in an oversimplified way. Their high input impedance gets actually very low at AC. Did you know that the gate may sink or source current without any Vgs change?

This is so true, that in the Mhz range you need more current to drive the gate than the current conducted by the device itself. It's just like a bipolar that runs out of gain when operated above its FT.
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