need a small signal PNP operating at low Voltage - diyAudio
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Old 14th March 2015, 08:12 PM   #1
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Default need a small signal PNP operating at low Voltage

I'm looking for a PNP with low Cob capacitance and high linear hfe at very low voltage (Vce is about 1.2V )
Operating current is 5mA, so the mmbt5087 is almost in saturation at that low voltage/high current condition and something like 2SA1015,BC858 has too high capacitance (7-8pF). All the 120V low noise types like 2sa970/992 will not work at that conditions also (saturation and too high Cob)

low sat. Voltage types like BC337 (BC817) will work, but with >10pF

I found a MMBTH81, but it's an RF type.
would prefer some Audio-GenPurpose types
Any suggestions?
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Old 15th March 2015, 12:04 PM   #2
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It appears that you WANT an RF type to get low capacitance at low voltages and reasonable current. The challenge is to find high hfe
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Old 15th March 2015, 07:25 PM   #3
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yes, looks like this.
high hfe - Ok, 150 will be enough, but should be linear in this range without geting in quasi saturation.
I could only find the 2n5771 (expensive!) with same capacitance but even less hfe than mmbth81.
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Old 16th March 2015, 02:09 AM   #4
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Why don't you give the spec you want. what is the Vce MAX, beta, fT. Then go from there. Don't just tell what is not good enough. Tell what you are shooting for.

What is the purpose of the transistor?

Last edited by Alan0354; 16th March 2015 at 02:11 AM.
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Old 16th March 2015, 08:35 AM   #5
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i thought i've already wrote every needed information.

Vce max is not important, but you will not find any BJT with high Vce-Max operating at low Vce without quasi-saturation.
operating Vce =1.2-1.3V , Ic 5mA
hfe = > 150 at giving Vce,Ic and linear in the range 2-10mA, and should not fall at higher temp.
Cob <4pF at operating Vce (so at 0.7V reverse Voltage)

This PNPs will be used in the differential current mirror (like in the Bob Cordell's Poweramp)
I'll have 5mA current for each BJT. Low Cob and high linear hfe is very important for minimizing LTP load and increasing the pole frequency.
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Old 16th March 2015, 09:27 AM   #6
AndrewT is offline AndrewT  Scotland
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The bc860 is stated as typical Cob of 4.5pF and fT min of 100MHz.
It is available in both b and c grades.
It is the smd version of the excellent bc560c
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Old 16th March 2015, 09:52 AM   #7
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Cob and fT are always given in DS in "perfect conditions" for the particular BJT
BC860 will have >8pF at below 1V Vcb.
Regarding hFE linearity - best datasheet really explaining the things with low Vce and high current / high temperature is ONSemi's DS for BC846-850 (NPN)
http://www.onsemi.com/pub_link/Colla...C846AWT1-D.PDF
hFE at 1V and 5V Vce are giving for all grades (a,b,c) and different temperatures.
You can see the gradeC hFE for 1V Vce starting to fall at few mAmps, especially at higher temp. (page 9)
For 5V Vce it works up to 6-7mA.
This is quasi-saturation!

Last edited by dimashek; 16th March 2015 at 09:56 AM.
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Old 16th March 2015, 10:19 AM   #8
AndrewT is offline AndrewT  Scotland
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Keantoken has written a bit about this low Vce requirement.
I think he recommended some of the BCxxx as the best available.
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Old 16th March 2015, 10:40 AM   #9
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Thank you very much
Yes, he recomended BC327/337 for best hFE linearity at low Vce (that makes sence when i look to the curves in DS)
Unfortunately Cob is quite high.

I can also recomend the OnSemi's 50C02CH / 50A02CH being very linear at low Vce, even with Ic >10mA
This are my favorite BJTs for active device in cascoded VAS at relatively high current

Last edited by dimashek; 16th March 2015 at 10:54 AM.
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Old 16th March 2015, 10:44 AM   #10
AndrewT is offline AndrewT  Scotland
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My memory cannot help.
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