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IG81 11th November 2009 01:37 PM

Emitter follower related question
 
Hello,

When biasing an emitter-follower with a current source in the place of a typical emitter resistor, can the equivalent emitter resistor be calculated by V-emitter / I-emitter?

I need this value in order to calculate input and output impedances.

Thanks!

jan.didden 11th November 2009 01:50 PM

Quote:

Originally Posted by IG81 (Post 1977108)
Hello,

When biasing an emitter-follower with a current source in the place of a typical emitter resistor, can the equivalent emitter resistor be calculated by V-emitter / I-emitter?

I need this value in order to calculate input and output impedances.

Thanks!

The impedance from the current source can be calculated from delta-V/delta-I. That is much higher than Ve/Ie. So what you see from the base is that very high Isource impedance in parallel with the load, and most of the times it will be the load that dominates, meaning you can disregard the Isource.

The input impedance looking into the emitter is that Isource in parallel with whatever the impedance into the emitter itself is, and the latter will dominate, meaning you can disregard the Isource.

jd

IG81 11th November 2009 02:21 PM

Thanks for a quick answer janneman.

If I understand correctly, I can disregard the Isource impedance and simply use Rload in my calc.

How about for the base? I also have a current source in place of the base-to-ground resistor in the "voltage divider", how does that go?

ig

jan.didden 11th November 2009 02:29 PM

Quote:

Originally Posted by IG81 (Post 1977150)
Thanks for a quick answer janneman.

If I understand correctly, I can disregard the Isource impedance and simply use Rload in my calc.

How about for the base? I also have a current source in place of the base-to-ground resistor in the "voltage divider", how does that go?

ig

Well in any case the Isource is (very) high impedance. So the impedance of the base divider is the Isource in parallel with the resistor from base to supply (remember that for ac and signal, the supply is considered ground).

But do you have a circuit diagram? I suspect what you are trying to do at the base will not be thermally stable and might need individual adjustment for each device.

jd

IG81 11th November 2009 02:47 PM

1 Attachment(s)
Here's what it looks like, don't look at component values, just topography.

ig

jan.didden 11th November 2009 03:03 PM

Quote:

Originally Posted by IG81 (Post 1977178)
Here's what it looks like, don't look at component values, just topography.

ig

Can't find anything wrong here.

jd

IG81 11th November 2009 03:51 PM

Quote:

Originally Posted by janneman (Post 1977191)
Can't find anything wrong here.

jd

So for the input Z of the circuit, can I also "neglect" Isource and just use the +rail-to-base resistor || with Zin-base?

Thanks,

ig

jan.didden 11th November 2009 04:31 PM

Quote:

Originally Posted by IG81 (Post 1977241)
So for the input Z of the circuit, can I also "neglect" Isource and just use the +rail-to-base resistor || with Zin-base?

Thanks,

ig

Well, depends on the resistor values. If the Isource has 150k dynamic impedance and the other R is also 150k, clearly you can't ignore Isource: the parallel value is 75k.
If the R is 10k, the parallel value is about 9.3k. Are you familiar with calculating parallel impedances?

jd

IG81 11th November 2009 05:00 PM

Quote:

Originally Posted by janneman (Post 1977299)
Well, depends on the resistor values. If the Isource has 150k dynamic impedance and the other R is also 150k, clearly you can't ignore Isource: the parallel value is 75k.
If the R is 10k, the parallel value is about 9.3k. Are you familiar with calculating parallel impedances?

jd

Yeah, it's more how to translate a current source into an impedance that I was looking into here. I think I can see what you mean by "dynamic impedance" that should not be neglected if the lower Z it can take is not big enough to make the parallel impedance be close enough to the resistor above.

az

jan.didden 11th November 2009 05:22 PM

Quote:

Originally Posted by IG81 (Post 1977331)
Yeah, it's more how to translate a current source into an impedance that I was looking into here. I think I can see what you mean by "dynamic impedance" that should not be neglected if the lower Z it can take is not big enough to make the parallel impedance be close enough to the resistor above.

az

An Isource can be considered as an impedance but you can't measure it by its voltage over its current. You measure it by varying the voltage a bit and see how the current varies, then the impedance (the dynamic impedance) is delta-V/delta-I. Ideally, the current doesn't vary with voltage and then the dynamic impedance is infinite...

jd


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