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?
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
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.
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.
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?
What is the purpose of the transistor?
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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.
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.
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
It is available in both b and c grades.
It is the smd version of the excellent bc560c
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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Keantoken has written a bit about this low Vce requirement.
I think he recommended some of the BCxxx as the best available.
I think he recommended some of the BCxxx as the best available.
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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What's wrong with the good old BC556 et al?
I did I lot of simulations on distortion, the best OPS is easily 20dB higher distortion than a reasonable IPS and VAS. The small signal part is normally not the bottle neck in the distortion of the amp.
I did I lot of simulations on distortion, the best OPS is easily 20dB higher distortion than a reasonable IPS and VAS. The small signal part is normally not the bottle neck in the distortion of the amp.
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.
You sure you are not making too much of the loop stability?
You have to be more specific in how you use the transistor.
You have to be more specific in how you use the transistor.
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.
Q11/13 have almost 60V Vce and 1.5mA Ic.
This will be 120V 2sa970 or similar. (BC856B is in my simulation only because I have no correct model for 2SA970)
They cannot be used for Q19,20
This will be 120V 2sa970 or similar. (BC856B is in my simulation only because I have no correct model for 2SA970)
They cannot be used for Q19,20
some "requirements" can be better dealt with by circuit changes
boost front end stage supply V and add some V biasing circuitry if you expect to improve on the Cordell Mosfet Amp circuit
a couple of LEDs worth of V across Q ce junctions makes a difference
boost front end stage supply V and add some V biasing circuitry if you expect to improve on the Cordell Mosfet Amp circuit
a couple of LEDs worth of V across Q ce junctions makes a difference
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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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