# Constant Current Biasing of BJT

#### salil87

Joined Nov 4, 2011
18
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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#### hgmjr

Joined Jan 28, 2005
9,027
The circuit does not appear to have a defined input. What input are you referring to?

hgmjr

#### salil87

Joined Nov 4, 2011
18
The circuit does not appear to have a defined input. What input are you referring to?

hgmjr
hi hgmjr
The diagram is just for reference. Input can be any small signal at the Base. Output is Vc.

Thanks
Salil

Joined Dec 26, 2010
2,148
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.

#### hgmjr

Joined Jan 28, 2005
9,027
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

#### salil87

Joined Nov 4, 2011
18
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

#### Ron H

Joined Apr 14, 2005
7,014
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
That works. A resistor in series with the capacitor will reduce the gain (at frequencies where Xc<<R) and also reduce harmonic distortion.