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Old 27th November 2012, 02:03 PM   #261
catalin is offline catalin  Romania
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Quote:
Originally Posted by AndrewT View Post
22r & 20mA gives an output Vbe of 440mV.
Is that correct? This seems far too low. I would expect 550mVbe to 600mVbe, giving a driver bias current range of approximately 25mA to 27mA

10mA to 12mA of output bias must be excluding the driver current.
The total current bias through the emitter resistor must be: driver bias + driver base current + output bias, i.e. approximately 35mA to 40mA.
Is that correct?
Hello Andrew ,it is correct ,in this configuration and with these bjt's .2SA1943/5200 has 400mV at 5 uA (the cut-off current ) .
I have measured with voltmeter and I have 430 mvolts Vbe so it is correct .

And yes those 10-12mA are only for the 5200/1943 excluding the drivers .With the drivers I have through 0.22 Re almost 32mA-33mA .

Can somebody run an imd fft ?
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Old 27th November 2012, 02:06 PM   #262
AndrewT is offline AndrewT  Scotland
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if you have Ire 32-33mA then some current is passing thro' the outputs. They are not cut off.
What is the output Vbe when the total output bias is 32mA?
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Old 27th November 2012, 02:16 PM   #263
catalin is offline catalin  Romania
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Originally Posted by AndrewT View Post
if you have Ire 32-33mA then some current is passing thro' the outputs. They are not cut off.
What is the output Vbe when the total output bias is 32mA?
Andrew ,I said that the cut off current for 2sa1943/c5200 is 5uA for 400mV Vbe ;that's why for 10mA we have 440mV. Of course that if Re has 32-33mA the difference from the drivers(which has 21mA) is present in the outputs(10-12mA) .Or better said the sum from outputs(12mA) and drivers(21mA) is present in Re(33mA) .
I didn't said that the cut off current of the outputs is 10mA .I presented the cut off current to understand where the Vbe voltage starts to open the Bjt.For example the 2SA1302 is not the same .They have the cut off current at 100uA .
Regarding your question ,for those 32mA Vbe is 440mV ,measured and simulated also
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Old 27th November 2012, 02:19 PM   #264
AndrewT is offline AndrewT  Scotland
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I don't understand.
I hear you saying the output is cut-off and then saying it is not cut-off !
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Old 27th November 2012, 02:23 PM   #265
catalin is offline catalin  Romania
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I don't understand.
I hear you saying the output is cut-off and then saying it is not cut-off !
Where I am saying that the output has the cut-off current in idle mode ?
Please quote .
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Old 27th November 2012, 02:27 PM   #266
AndrewT is offline AndrewT  Scotland
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then why are you stating 400mVbe 430mVbe and 5uA of Ic?

Either the output is cut-off or it is not.
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Old 27th November 2012, 02:34 PM   #267
catalin is offline catalin  Romania
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then why are you stating 400mVbe 430mVbe and 5uA of Ic?

Either the output is cut-off or it is not.
We have a funny conversation

I said that 2sa1943 has for the cut-off current (which is 5uA) a Vbe of 400mV .This is not the idle state .
This can be seen in datasheet .
Now by increasing the current to 10mA we get 440mV on Vbe which is different by those 5uA (the cut off current presented in datasheet) .This is the idle state .
So for 5uA the current is at the cut off limit
For 10mA the current is greater than the cut off limit which is the idle bias state .

I stated the 400mV at 5uA to understand why 2sa1943 present 440mV at 10 mA. Because the cut off current is so low .
For example 2sa1302 will have a higher Vbe for 10mA because it has a higher cut off current .
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Last edited by catalin; 27th November 2012 at 02:45 PM.
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