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?
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?
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.
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.
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/Collateral/BC846AWT1-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!
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/Collateral/BC846AWT1-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!
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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
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
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I'll give it a try. The A-grade could work here. The capacitance is bit too high
It's not the distortion figures that i'm after. The loop stability is a major concern.
I don't what a nonlinear Cob to play a significant role in the loop stability.
Yes, i'm pretty shure
here is a part of the schematics
https://dl.dropboxusercontent.com/u/25364971/IPS.png
It's a Bob Cordell's IPS, slightly modified.
CFP(shiklai) LTP is cascoded (not shown) . OUT1,OUT2 are connections to VAS
C21-R56 compensation introduce a pole at about 20kHz.
If the current mirror (Q19,20) capacitance is too high, the pole frequency will drop AND it will vary (stability issue, phase distortion) if the BJT is not properly chosen.
My Vsupply is already 64Volts for frontEnd.
Q19,20 will always operate at low Vce (one Vbe voltage drop of Q11,13), no matter how high the supply voltage. You can put diodes (or LEDs) in the Q11,13 Emitters to increase Q19,20 Vce, but this will lower the linearity and pole frequency of follower stage.
Q19,20 will always operate at low Vce (one Vbe voltage drop of Q11,13), no matter how high the supply voltage. You can put diodes (or LEDs) in the Q11,13 Emitters to increase Q19,20 Vce, but this will lower the linearity and pole frequency of follower stage.
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