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Old 29th June 2006, 02:30 AM   #1
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Question how to decide these resistance value.??

I read some books , they all said R22 and R23 are degeneration resistor.
But I do not understand what is it mean ....
And how to decide the R22 and R23 resistance ?????
Now , i use R22 = R23 = 33ohm.
Because the CCS is about 6 mA , most books said let the R22 and R23 drop about 50mV to 100mV, so i chose 33ohm to get 100mV.

But in VA stage , the Q10 get a 8mA CCS , and now R29=75ohm to drop about 600mV.

Now , i get a question , the collector of Q8 and Q7 will get different Voltage Bias.
The collector of Q7 will get 100mV + 600mV = 700mV depend on -Vss.
But the collector of Q8 will get 600mV(R29) + 600mV (Q10 Vbe) = 1200mV depend on -Vss.
Will it effect the differential input ???

Or we have to design let the both collector get the same voltage bias???

The CCS of VA stage only get about 8mA .
But BDV66 and BDV67 need 250mA Ib , Is 8mA not enought ???
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Old 29th June 2006, 07:28 AM   #2
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I think it is really secondary issue, but for perfect balance in differential stage you should equalize collector voltages. This means about 180 ohms of R23 and R22.

Quote:
But BDV66 and BDV67 need 250mA Ib
This seems strange to me. Are you sure? current gain (Hfe) sould be at least severeal thounsend.



P.S. If I could suggest something- give a 1k resistor at base of Q6 and a 1u cap across diodes. Without it amplifier will behave ugly at positive cliping, because tail current may modulate and even latch for some time.
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Old 29th June 2006, 07:52 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
250mA Ib, is this the absolute maximum cuurent when saturated?
or is it the Ic that should be set up as a quiescent bias?

The voltage across the emitter degenerating resisitors can be in the range from a few tens of mV to 600mV. You make the choice and then test for stability and sound quality.
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Old 29th June 2006, 10:15 AM   #4
sajti is offline sajti  Hungary
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Location: Vác, Hungary
Default Re: how to decide these resistance value.??

Quote:
Originally posted by titanchen68
I read some books , they all said R22 and R23 are degeneration resistor.
But I do not understand what is it mean ....
And how to decide the R22 and R23 resistance ?????
Now , i use R22 = R23 = 33ohm.
Because the CCS is about 6 mA , most books said let the R22 and R23 drop about 50mV to 100mV, so i chose 33ohm to get 100mV.

But in VA stage , the Q10 get a 8mA CCS , and now R29=75ohm to drop about 600mV.

Now , i get a question , the collector of Q8 and Q7 will get different Voltage Bias.
The collector of Q7 will get 100mV + 600mV = 700mV depend on -Vss.
But the collector of Q8 will get 600mV(R29) + 600mV (Q10 Vbe) = 1200mV depend on -Vss.
Will it effect the differential input ???

Or we have to design let the both collector get the same voltage bias???

The CCS of VA stage only get about 8mA .
But BDV66 and BDV67 need 250mA Ib , Is 8mA not enought ???

Don't care with this DC values! 600mV difference on the input stage collector voltage is no problem. Your amplifier has no any problem with it.

sajti
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Old 29th June 2006, 11:08 AM   #5
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Thanks All ....
could you tell me what does degeneration resistor purpose ???
It boring me along time , i think about it , but didn't get answer.

In BDV66's spec , It only said 250mA Base current .
does it mean if i want to get maximum Ic current , I have to feed 250mA Ib ???
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Old 29th June 2006, 12:50 PM   #6
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Degeneration resistors make a transistor less senstive to Vbe variations between each other and give local current feedback, so that transconductance is lower but more linear.

250mA base current in datasheet means problably, that under some conditions you can inject to base as much as 250mA and it still be alive. Normally you calculate base current by collector current divided by hfe (beta).
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