New Lineup IDEA - Power Follower/Output stage

[homemodder]

Originally Posted by Struth

Hi Guys

Clearly I do not understand things the same way as you, homemodder. Rather, what I understand is that there is more to BJT performance than mere Ft. Gain flatness over the current range actually used is more important and does not require that this be coincident with the maximum Ft point of the device. In this view, I am joined by name designers whom I respect.

Have fun
Kevin O'Connor

Not more important but as important as is the cob and the early voltage of the device / Low saturation voltage comes to mind in second plane. Gain flatness with maximum Ft is not required but if you want high performance both can be achieved at the same time with clever device selection. Try upping the bias on the driver on any class AB amp and youll hear the difference, especially with the dismall onsemi devices. These should are really only be used with triple EF stages and very high power amps were other devices are unfortuneatly not suitable or exist.

The original Bds have a gain which is just as flat as any toshiba or MJL, as a bonus you get higher beta which is advantageous and you get much improved Ft which is also advantageous. Thirdly the cob is also lower meaning lower high frequency distortion.

[AndrewT]

Homemodder,
which device pair would make a good substitute for the BDs at similar voltage capability?
And which device pair would make a good substitute for the BDs at a higher voltage capability, maybe upto 150Vce0

[homemodder]

The BDs from fairchild are not far off the originals or mje243 and mje253.
Higher voltage youd have to use toshibas, parralel them if you have to, even then much better than the onsemi devices. Another current ex sanyo and now onsemi is 2sa1507 and complement which is comparable to the best toshiba pair 2sa1930 and compliment.

[Struth]

Hi Guys

Saturation voltage is important in a switching circuit when you want low losses, but seems irrelevant in an audio amp. I don't run my amps to clipping, nor do I need to extract that last 100mV of swing from the supply rails.

Jkuetemann, the mosfets in your sim are now obsolete according to Toshiba. However, the K1058-J162 may still be current. They are all close enough to each other in performance to be swappable for the circuit. It is disconcerting to see these obsoletions occur, especially when you have these parts in your inventory; should you use them? keep some as spares?

Fortunately with the plastic packages, the mosfet and BJT pinouts are compatible. So, if you lay out a board for the LU buffer, you can use either technology. Just make a provision for the extra drive or predrive stages, and for gate zeners and balancing caps. Then, if the original mosfets die and cannot be replaced, BJTs can be subbed, or vice versa.

Have fun
Kevin O'Connor

[bear]

Originally Posted by lineup

Thanks. I will add inductance instead.
That will keep the distortion low within the circuit,
and at the same time protect against capacitive looads.

What I have found is that the addition of the inductor makes certain ringing into a capacitive load. Can you hear that? I dunno... Thinking out loud now, I wonder if anyone has done actual distortion measurements with the ringing?

_-_-bear

[Bigun]

Originally Posted by Struth

Frankly, I think my hearing must be going because I can't hear anything over 1MHz.

[Bonsai]

Originally Posted by homemodder

No they dont outperform the BDs, please show proof, I will then show proof otherwise. The bd137-140 series has less than half the Cob of parts like the toshibas. They also are much faster, 150Mhz compared to 100 that toshibas can manage. The important factor is that the toshibas need to be biased at 100ma and higher to be able to even reach 100Mhz wheres the BDs do this at 30ma. Where does one usually bias drivers for class AB at, 100ma and up, I dont think so. Further the BDs display hfe near twice as high as any toshiba.

They are only inferior when it comes to VA and SOA, use two in paralell and till their rated Vceo they easily outperform any toshiba or any other make of driver.

I think ST might still be doing these, but not sure of the spec.

They were nice GP devices in their day, but a bit let down by the low Vceo rating.

[dadod]

Originally Posted by homemodder

No they dont outperform the BDs, please show proof, I will then show proof otherwise. The bd137-140 series has less than half the Cob of parts like the toshibas. They also are much faster, 150Mhz compared to 100 that toshibas can manage. The important factor is that the toshibas need to be biased at 100ma and higher to be able to even reach 100Mhz wheres the BDs do this at 30ma. Where does one usually bias drivers for class AB at, 100ma and up, I dont think so. Further the BDs display hfe near twice as high as any toshiba.

They are only inferior when it comes to VA and SOA, use two in paralell and till their rated Vceo they easily outperform any toshiba or any other make of driver.

I have Philips handbook fro 1974 and it states fT for BD 135/137/139 is 250 MHz anf fT for BD136/138/140 is 75 MHz nothing about Cob.
dado

[Bonsai]

I would no use thm for TIS (VAS) applications.

[homemodder]

The original philips datasheets state ft of 160Mhz for pnp and 190 for npn but please note that these were taken at vce of 5 volt where nowadays its mostly taken at 10v which makes things look a little better but I believe the ft of the npn part could well reach 250 Mhz at vce of 10v. Thats how good these parts are, a real pity about the max Vce but you could push a paralelled pair to +- 50 volts if you re adventurous. Max Vce is stated open base.

The rest of the info can be had looking at datasheets for bc638/9 ect. The bds are simply these in to126 package, this is what the philips parts are and why the spice parameters are identical for the parts. The onsemi ones however are nowhere close to the originals so I dont even consider them as being bd components.

I agree better vas transistors are available but at least they are much better than mje340/350 which are commonly still found even on commercial products.