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Old 25th March 2008, 08:19 PM   #1
corrieb is offline corrieb  South Africa
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Default Basic Electronics Question

Hi all,

I hope that one of you can assist. If you look at the image attached you will find the beginning of a differential input stage. Well basically I can calculate/gestimate all the currents and voltages for all transistors except for one. The transistor in question is Q6. From the attached image you will see the results from my simulation marked in red(emitter currents and voltage) and blue(base currents and voltage).

Is there a scientific approach that will allow me to accurately predict the voltages and currents?

Thank you,

CorrieB
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Old 25th March 2008, 10:29 PM   #2
CBS240 is offline CBS240  United States
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Hi CorrieB

The problem is, in the ideal model of a transistor with the arrangement of Q's 3-4 as the differential and Q2 (source), Q5 (mirror), the current will be perfectly balanced. Real transistor models will very from device to device in the characteristics. There needs to be a feedback relationship between the current in Q6 and the input. A resistor from collector of Q6 to GND or to +12V will create a voltage reference (output) that reflects the current of Q6. This reference needs to be fed back to the input at some level in order to establish an operating point in Q6. IOW, as I see it, the voltage across R3, and thus the current in Q6 is essentially undefined.



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Old 25th March 2008, 10:35 PM   #3
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It is guaranteed to be less than 12V/150 ohms = 80 mA
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Old 26th March 2008, 08:33 PM   #4
corrieb is offline corrieb  South Africa
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Thanks for the replies.

When designing a VAS stage that is buffered what current levels for the buffer transistor can I use as a recommendation?

CorrieB
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