What is the quiescent current of Hitachi 2SK1058 and 2sj162?

[guymrob]

Hi everyone,

I have finished contructing a 50W amplifier based on the Hitachi 2SK1058 and 2SK162 L-MOSFET. I need to adjust the quiescent current. What is the optimium setting that results in low distortion as well as reasonable power dissipation. Thanks for your feedback.

[slowhands]

The simple answer is just set it for 100ma per device.

The bias current is not critical, between 100ma and 200ma per deivice will work OK for class AB operation. Lower runs cooler, which I prefer. Borbely recommends about 150ma for Hitachi lateral MOSFETs, while Pass recommends about 100 ma for IR vertical MOSFETs

The optimal way to set this might be the slick method detailed in the Nelson Pass article on the Citation MF12, where you set it based on minimizing crossover distortion using only a scope, oscillator and power resistor.
http://www.passdiy.com/pdf/citation.pdf

Even though that MF12 implementation used vertical MOSFETs, and you are using laterals, the procedure is identical. The bias voltage will of course be quite different, perhaps 1 volt versus about 7-8 volts for the vertical MOSFETs. Both result in the same bias current and minimal distortion.

[Charles Hansen]

Here's a little more information:

1) 100 mA per device is a good starting point. Below that and you be in the area of a positive tempco, which isn't ideal.

2) There won't really be an optimum value that will minimize crossover distortion. There is an optimum bias for bipolars (besides full-on class-A), but not FETs.

3) Performance (sonic and measured) with FETs will basically increase without limit as the bias is increased. Unfortunately, so does the heat.

I would start at 100 mA, and go up from there. If you can rest your hand on the heatsink for a few seconds before it becomes uncomfortably hot, that is around 125 F. I wouldn't go much hotter than that or else you will risk danger of burns and also shortened component life.

[Idefixes]

According with slowhands and guymrob 100Ma in AB class is hr good way. In 5Ch Zenquito (SK1058/SJ162 push-pull) the front center CH and the rear Ch have 220mA/push-pull. The 2 front Ch have 1.12A/push-pull to have A Class up to 20W





Marc

[Nick Walker]

However in one of Slone's books he does explicityly say that he thinks the optimal bias current for these devices is 40mA. There is no explanation given though.

[Charles Hansen]

Here's one explanation -- some people like the sound of high-order harmonic distortion. Or to be a little gentler about it, they consider a warm amplifier to be more of a problem than high-order harmonic distortion.

[sam9]

"However in one of Slone's books he does explicityly say that he thinks the optimal bias current for these devices is 40mA. There is no explanation given though."

He changed his figures between books. In the earlier one he wrote 50mA between the source and output rail per device. Is the 100mA figure quoted here for each device or is it for each pair?

In a later book he uses the figure of 40mA to multiply by the sum of both of the source resistors to get a voltage figure, voltages being a little more convenient to measure.

Having built only bipolar amps so far, but contemplating a MOSFET effort entually, I'm curious to know if this is just a question of how the figure is stated or if there is actually a 2:1 difference in recomended bias measurement.

[Charles Hansen]

Originally posted by sam9
Is the 100mA figure quoted here for each device or is it for each pair?

The 100 mA figure suggested by slowhand (and by myself as a minimum) is per device. Spend some time with the data sheets and you will see that this is the nominal zero tempco point.

[PMA]

In case that you want to prevent the effect of Gm decrease (higher order harmonics as said) then bias them at least at 100mA, I would say 200 mA.

[Ultima Thule]

PMA,

I'm not sure I understand the rellationship between Gm and high order harmonics, could you (or anybody else) elaborate on this?

Thanks,

Michael