What's the magic of 2N5551 / 2N5401?

[sumeshak]

They work fine for input stages. You can get parts that are ”better”, but having them on hand is better than waiting 6 months or a year to get the “best” ones.

A42/92 are sometimes used too. They aren’t the best choice for input stage because of the lower gain, but can be used in places where 5551/5401 can’t. When you see someone using A42 input stages, one of three things is happening. 1) The voltage is too high for anything else, 2) The builder only wants to carry one universal TO-92 pair that will work anywhere and sometimes they need 200+ volts, or 3) They don’t know any better.

A very old thread, the use of 2n5401 and 2n5551 in the ips, in my case they work very well at ~+/-70V supply

Thank you!

 

[sumeshak]

Thank you!

After doing some reading I found that 2SC2240/A970, and 2SC1845/A992 are better devices, but hard to get original ones.

Thanks and regards,
Sumesh

 

[sajti]

After doing some reading I found that 2SC2240/A970, and 2SC1845/A992 are better devices, but hard to get original ones.

Thanks and regards,
Sumesh

Try to get KSC1845F/KSA992F. Both available from Mouser.

 

[wg_ski]

Normally available from Mouser. May or may not be at this very instant.

 

[sajti]

Normally available from Mouser. May or may not be at this very instant.

They had them yesterday.

 

[jxdking]

Try to get KSC1845F/KSA992F. Both available from Mouser.

From the datasheet, their early effect looks wild. They won’t work well in VAS.
For input stage, it only sees half of the total voltage. Thus, 65 VCE rated BC546/BC556 could be used for up to 150W amp.

 

[sajti]

I use KSC/KSA in input stages since long time, with very good results. I feel TO92 devices too weak for VAS, in most cases.

 

[Mark Johnson]

[Re: KSC1845F/KSA992F] ... From the datasheet, their early effect looks wild. They won’t work well in VAS.

I curve traced those a while ago. KSC1845E { 400 < Beta < 800 } and KSC992F { 300 < Beta < 600 } . Plots attached.

The software insists on plotting PNPs in the first quadrant instead of the third, sorry about that. Horizontal axis is abs(Vce) in volts, vertical axis is abs(Ic) in milliamps.

 

[chermann]

From the datasheet, their early effect looks wild. They won’t work well in VAS.
For input stage, it only sees half of the total voltage. Thus, 65 VCE rated BC546/BC556 could be used for up to 150W amp.

...and again...i learnt something... 😉

 

[knutn]

And in addition to the Early effect, I would not use KSA992 above 1 mA...

 

[Mark Johnson]

Using two data points from the measured Ic vs Vc curve, we can extrapolate to find the Early voltage

• Vearly = [ I1 * (V2 - V1) / (I2 - I1) ] - V1

For the plots in post #28, I get

• KSC1845E Early Voltage = 269V
• KSA992F Early Voltage = 82V

Among the PNP small signal transistors I've tinkered with, the ones with the highest (Beta * Vearly) product {a figure of merit championed by Bob Cordell and others}, were high voltage devices from Zetex / Diodes Inc. Vce_max ratings of 300V and higher. Only measuring TO-92 and E-LINE, not TO-126 or TO-225.

 

[jxdking]

• KSC1845E Early Voltage = 269V
• KSA992F Early Voltage = 82V

Thus, if the bias point is 20v 10ma, the output impedances are about 30K/10K.

Do you know the early voltage of 2n5551/2n5401 and MJE340/MJE350. They don’t include IV curves in the datasheet.

 

[Mark Johnson]

KSC1845E Early Voltage = 269V
KSA992F Early Voltage = 82V

Thus, if the bias point is 20v 10ma, the output impedances are about 30K/10K.

And the "mu" values (maximum possible voltage gain with an ideal constant current load: mu = gm * rout) at 10 mA, are

KSA992F: mu= (0.010 / 0.0260) * 10K = 3,800 volts per volt
KSC1845E: mu= (0.010 / 0.026) * 30K = 11,500 volts per volt

Don't remember the others except I do recall that the MJEs from Diodes Inc were noticeably better than Onsemi and ST. But Diodes Inc only sold them in surface mount DPAK.

edit - just for fun -
The voltage gain of a KSA992F PNP common emitter amplifier at 10mA and Vce=20V, when loaded by a NPN constant current source built with a KSC1845E, is (0.010 / 0.026) * (10K || 30K) = 2,885 volts per volt. Assuming that neither one has an emitter degeneration resistor. The arithmetic is delightfully symmetric, the gain is the same when the amplifier is NPN and the constant current source is PNP. I think a stage gain of 2885 is acceptable.

 

[rikkitikkitavi]

And in addition to the Early effect, I would not use KSA992 above 1 mA...

Why not above 1mA?

 

[knutn]

Because of current gain dropping.

 

[chermann]

KSA992
that is the reason why.... after 1mA you just get less then 500hfe....

 

[chermann]

manufactorer REctron
2N5401

fairchild-onsemi

 

[chermann]

BC546..family...onsemi

 

[cabirio]

KSA992
that is the reason why.... after 1mA you just get less then 500hfe....

View attachment 1438129

My measurements don't match the datasheet plot, I get a much gentler hFE drop:

In any case, even if there was that hFE drop between 1 and 10 mA, it may be completely irrelevant depending on the specific application.

 

[chermann]

thanks for the real measurements