What transistors do you recommend for a good LTP input?

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EWorkshop1708

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#1
NPN or PNP doesn't matter.

What transistors would you recommend for a LTP Differential stage?

Have to be:

High voltage (160V or more)
Low Noise
Good Frequency response.

*Would it be overkill or non sensible to use high voltage TO-220 Packaged transistors (MJE15034/5 etc) as the LTP in an amp?
 
#5
I have had 2SC2547 and 2SA1085 from Renesas recommended to me as excellent low noise high voltage transistors for LTP use. However, they are a bit hard to come by and a fair bit more expensive than MPSA42/92's. Dr Leach also seems to recommend MPS8099/8599's

I would recommend using a cascode as MikeB suggests, and using BC550C/560C's.
 
#6
MikeB said:
160volts ? Are you kidding ?

And yes, mje1503x in LTP would be "nuts".
Try mpsa42/92, it has rating of 300volts.
Some also use mje340/350.

Otherwise try cascoding, this makes more sense as you can choose
any highqualitytransistor for amplification, and only need HV-Bjt for
cascodes.

Mike
Click to expand...

Thanks.

I figured 160V because incase I built an amp that has 70 or 80V rails. I imagine the voltage of the transistors in the LTP have to be as high as the output, or at least the PSU voltage, right?

As far as cascoding goes, how would you bias it to where the transistors 50/50 share the voltage? (I'm also gonna search)
 
#7
In normal circumstances the input stage only has to withstand the voltage of one supply rail plus half of the amplitude of the maximum input signal

In faulty circumstances like a open base connection, input connector transients when signal makes contact before ground does, shorted output devices causing severe output DC, etc..., the LTP transistrors may get exposed to the full rail to rail voltage. They can be easily protected by creating +-15V zener+resistor references and clamping the LTP bases to these references with small signal diodes [ie: 1N4148]

Then you can cascode a 45V device like BC550C/BC560C at 30V leaving 15V of margin in case the 15V clamps have to do their job

As a cascode device you may use something like BC546B/BC556B that are still small signal devices and are rated at 65V, so maximum supply rails are +-95V

For higher rails an alternative is 2N5401/2N5551 that feature high Ft and are rated at 150V. These may be also used as LTP devices directly without cascode

There are also a lot of good small signal high voltage transistors with excelent specs from Toshiba and Sanyo, but these are pretty hard to find

I don't recommend MPSA42/92 for an input stage since they suffer from low Ft thus reducing phase margin, and low gain thus causing large input offset voltages
 
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#9
I agree with Eva. In 99% of the lifetime the input LTP can see only the half of the rail to rail voltage. In case of failure, You have to plan some "budget", say 20-25%. So I recommend to use 90-100V transistors for LTPs.

sajti
 
#10
Eva said:

For higher rails an alternative is 2N5401/2N5551 that feature high Ft and are rated at 150V. These may be also used as LTP devices directly without cascode
Click to expand...

Thanks.

Those are just what I'm looking for, and also a good reference to whoever else sees this thread with the same question.

Those transistors will be suitable, and have plenty of overhead to not have to worry about breakdown voltage when designing or prototyping different amps.

I'll be ordering some 2N5401/2N5551 from Onsemi. 😀
 
#11
sajti said:
I agree with Eva. In 99% of the lifetime the input LTP can see only the half of the rail to rail voltage. In case of failure, You have to plan some "budget", say 20-25%. So I recommend to use 90-100V transistors for LTPs.

sajti
Click to expand...

Yea, I see how that works. 😎 The other half of the total rail voltage is usually dropped across the current source, or a resistor.

90V or so makes sense. I have a bunch of old complementary 60V transistors, and I doubted that they'd be enough (without cascode) for a 100W amp, since rails can be 50V or so.
 
#14
Eva said:
In normal circumstances the input stage only has to withstand the voltage of one supply rail plus half of the amplitude of the maximum input signal

In faulty circumstances like a open base connection, input connector transients when signal makes contact before ground does, shorted output devices causing severe output DC, etc..., the LTP transistrors may get exposed to the full rail to rail voltage. They can be easily protected by creating +-15V zener+resistor references and clamping the LTP bases to these references with small signal diodes [ie: 1N4148]

Then you can cascode a 45V device like BC550C/BC560C at 30V leaving 15V of margin in case the 15V clamps have to do their job

As a cascode device you may use something like BC546B/BC556B that are still small signal devices and are rated at 65V, so maximum supply rails are +-95V

For higher rails an alternative is 2N5401/2N5551 that feature high Ft and are rated at 150V. These may be also used as LTP devices directly without cascode

There are also a lot of good small signal high voltage transistors with excelent specs from Toshiba and Sanyo, but these are pretty hard to find

I don't recommend MPSA42/92 for an input stage since they suffer from low Ft thus reducing phase margin, and low gain thus causing large input offset voltages
Click to expand...
A really old thread, but Eva has called up a spike protection for the LTP front end.
Are there any downsides to this zener + diode solution? like noise? leakage current? anything?

The Leach has a pair of dual 20V Zeners creating +40V or -40V on the cascodes.
The +20V & -20V references are already fitted, just waiting for a pair of diodes to be added.

Any thoughts?
 
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