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Old 20th March 2007, 09:49 AM   #1
roender is offline roender  Romania
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Default Searching for high hfe, low noise and cob PNP BJT

I need a low power PNP BJT which will work at only 0.7V Vce and Ic 2.5mA.
It must have low cob, over 200 hfe at 1V Vce and very high ft and very low noise.
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Old 20th March 2007, 07:59 PM   #2
MikeB is offline MikeB  Germany
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Good old bc560c?
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Old 21st March 2007, 06:33 AM   #3
roender is offline roender  Romania
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Hi Mike,
I already use bc560c (you know were) but it has a big problem. It has very unstable hfe with temperature, so unstable that it can't keep bias stable when cold air is blow over it.

ps Did you receive my e-mail with PowerSymasym EGLE files?

Mihai
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Old 21st March 2007, 12:14 PM   #4
AMV8 is offline AMV8  United Kingdom
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Hi

You have probably already thought of this but here goes. All I do with the BC 550 and BC 560 is to press them against a common heat sink to keep the temperature low and constant.

My "special " transistor for preamps and low noise is the lm394 ( in a can ) which does may meet all your needs but it is excellant.

Don
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Old 21st March 2007, 07:15 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
all silicon transistors have a similar Ic vs Vbe characteristic that varies markedly with junction temperature.

I think hFE has the same variation with temp.

These variations are normally reduced by using an LTP to balance out changes.
Even better balance is achieved by thermally coupling the devices and better still with a dual device.
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Old 21st March 2007, 07:54 PM   #6
Tim__x is offline Tim__x  Canada
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Perhaps if we could see your circuit we could suggest modifications to make the bias less device dependant.
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Old 21st March 2007, 08:30 PM   #7
roender is offline roender  Romania
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Ok, this is my amplifier.
R12 and R13 is no longer in circuit
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Old 21st March 2007, 09:19 PM   #8
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i think if you connect q8 and q9 that will resolve your problem
i think the bias stability will rise if you connect together q16 + q14; q7 + q6 + q15

i think you could even connect transistors in first differential stage, j1 + j2, q21 + q18
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Old 22nd March 2007, 07:05 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi Roender,
what is D1? a Zener or a 1mA CCS?
your output stage currents seem astray.
70mA through R5 generates 1.54V.
The bias across the emitter resistors will be about (1.54-1.3)=0.24V this generates Iq=2.4A.
Reduce Ir5 to about 61mA and the bias reduces to about 420mA , Vre~=21mV. Iq=2*[(22r*0.07A)-1.3V]/(0r1*2)
Note how sensitive Iq is to small changes in Ir5.
How will you adjust Vbias? 4diodes will not achieve accuracy. Are any on the output heatsink for temperature compensation?
You have cascoded the second LTP and achieved good HF response then killed that response by fiiting a Miller comp cap. Why have you fitted C14 to the collector of q10 rather than the collector of Q16?
Why have you fitted R12 & R13?

Tomahauk,
connect those transistors?
Do you mean match their parameters?
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Old 22nd March 2007, 07:46 AM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
your choice of values for C19 & C21 produce RC time constants near 220mS.
If you adopt the philosophy of setting the PSU RC to half an octave above this (330mS) then you are forced to adopt +-40mF/channel for 8ohm speakers and +-80mF/ch for 4ohm speakers.

Is C19 in series with a DC blocking cap in your source?
The combined RC of the input could usefully be reduced to 80mS and still retain good bass response. Then C21 can be reduced to 220uF and that allows smoothing to be reduced by half.

The input filters should set the frequency response of you amp. All the internal circuitry should exceed the bandwidth of the filters by a margin, that you select.
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