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Old 23rd April 2008, 05:19 PM   #1
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Default a problem - no bias current

guys - i have a really big problem (in my point of view)...

i draw in the simulator a schematic and there it is working perfectly.... but when i made a pcb board and everything i found out that i can't bias the amplifier..... i have no bias current flowing threw the output devices....
so i started measuring few spots and i found out that i have some current flowing threw the bias circuitry but i do not have any current flowing threw the driver transistors....

can somebody please help me and tell me what is going on and what should i look to solve the problem....the pcb seems to be o.k. (but still it can be faulty and i can't find the problem there)

i am working arround this problem for two days now and i am really crazy and sad now because of it....

the VAS, driver, and bias circuitry with some voltages looks like this....
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Old 23rd April 2008, 05:22 PM   #2
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i forgot to tell that amplifier is producing sound and it is not oscilating - no problem with that ....
but there is no bias current going in the output devices.....
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Old 23rd April 2008, 05:46 PM   #3
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Hi,

Not enough bias voltage, Vcd needs to be around 2.9V for standing current.

To turn your output stage on you need at least 4 diode voltage drops,
plus the expected voltage drop across the emitter resistors at the
nominal bias current.

See : http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

/sreten.
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Old 23rd April 2008, 06:17 PM   #4
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i tought that this is the reason but what is bothering me is that in simulator it is working with those values i have used (and that are on the sch in the first post of this thread) ....
how can it be that in simulator it is working with used values of resistors and in real life the values are a lot bigger ????
can someone explain this to me????

(i know that simulator is a simulator and real life is a real life) but i am very scared not to burn anything....


is it really that high i have to go - 4 diodes and a resistor of lets say 68R???
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Old 23rd April 2008, 06:31 PM   #5
sreten is online now sreten  United Kingdom
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Hi,

Vab should not be zero. If you look at the loop for the drivers with
Vcd = ~ 2.0V, Vab should be ~ 0.6V, which is nowhere enough for
a standing current in the output stage. Vab needs to be at least
two diode drops. Use one transistor as an amplified diode for bias.

/sreten.
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Old 23rd April 2008, 06:36 PM   #6
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o.k. - i will ad here one diode and than two diodes to see what will happen
i hope everything will be o.k. - i was not sure what is happening since in the sim everything was working fine with two transistor diodes and that resistor... and in real life this is not so....

oh man!


o.k. - thank You
very much...
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Old 23rd April 2008, 07:52 PM   #7
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o.k. - i put three diodes (two 1n4148 and a MJE15030 connected as a diode) and a resistor (68R in series with 56R).....

this gave Vcd arround 4V...... Vab was still at zero V..... should i go even higher than that??? how high i can go??
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Old 23rd April 2008, 08:11 PM   #8
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now it is looking like this ....

still no bias current over the output emitter resistors....


can anybody help please..
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Old 23rd April 2008, 08:25 PM   #9
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Voltage on opened base-emitter must be around 0.6V. Thus Vab must be 4 1.2 = 2.8V. Check mounting, first transistors in follower, nominal of 1 and 10 Ohm resistors
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Old 23rd April 2008, 08:28 PM   #10
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thank you for your answer - in other words - schematic like this should work.... o.k. i will check the resistor connection (i have done that but i will do it again).... also i will check Vbe of the driver transistors and than i will come back...

thank You for the reply again...
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