# Constant Current Biasing of BJT

Discussion in 'General Electronics Chat' started by salil87, Nov 12, 2011.

1. ### salil87 Thread Starter New Member

Nov 4, 2011
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0
Hi
As shown in the diagram, a BJT is biased using a constant current source. But I am not getting how it would work. If Ie(Emitter Current) does not change (due to the constant current source) then how will the input affect the output? Some Confusion. A Little Help needed.

Thanks
Salil

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2. ### hgmjr Moderator

Jan 28, 2005
9,030
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The circuit does not appear to have a defined input. What input are you referring to?

hgmjr

3. ### salil87 Thread Starter New Member

Nov 4, 2011
18
0
hi hgmjr
The diagram is just for reference. Input can be any small signal at the Base. Output is Vc.

Thanks
Salil

Dec 26, 2010
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300
This seems to be simply an illustration of a biasing method. A signal path could be established by some components not shown in the diagram, for instance a capacitor connected to the emitter.

Alternatively, a constant current output from the collector may be all that is required, for instance to establish a bias or reference voltage for something else.

5. ### hgmjr Moderator

Jan 28, 2005
9,030
214
My impression of the circuit is that it is intend to illustrate a simple BJT stage with a constant current source in the emitter circuit. If it is part of a BJT problem then a lot depends on the question asked about the circuit.

If the question is what is the effect of the output at Vc with an input signal applied to the base then for small signal changes on the input I would expect the voltage at Vc to remain unchanged.

hgmjr

6. ### salil87 Thread Starter New Member

Nov 4, 2011
18
0
Yup Got it(i think). A Capacitor should be connected across the current source which would allow the changes in the emitter current due to the input signal. This would allow the input signal to change the output. Correct me If I am wrong(i hope not ).

Thanks
Salil

7. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
That works. A resistor in series with the capacitor will reduce the gain (at frequencies where Xc<<R) and also reduce harmonic distortion.