Current source loading

[SteveS]

My question concerns common source amplifiers (or common source with degenerative feedback) and their output impedance. From what I have read the output impedance, with the drain loaded with a resistor, is essentially the value of the drain resistor (drain resistor in paralell with the impedance looking into the drain).

If this is so, then how do I calculate the output impedance of a common source amplifier when the drain is loaded with a current source or current mirror? 😕

Thanks,
SteveS

 

[Jocko Homo]

Why do you need to know? How will it help you? You're not modeling this thing, are you.

Jocko

 

[SteveS]

Jocko,

I working on a design that uses a dual-triode as a differential pair loaded by a current mirror and I want to make sure I have a low enough ouptut impedance to drive a power MOSFET gate.

SteveS

 

[Jocko Homo]

You aren't driving it directly from the diff pair, are you?

Looks like your main concern is how low the -3 dB point is driving that MOSFET.

Where does the JFET enter into this?

Jocko

 

[HarryHaller]

Common Source

Well as long as we are all in Texas. Since the mosfet gate input looks like a large capacitance, I would worry about having the current to get a decent slew rate. This kind of of a poor match for front end and output stage. A small mosfet follower to drive the output mosfet would be a great idea. Say a Zetex VP3110.....

H.H.

P.S. That Jocko guy don't do mosfets, I have been trying for years to get him to.....

 

[Jocko Homo]

I was hoping the low -3 dB point would be enough to discourage him from doing it direct.

After he added a buffer, then I would have brought up slew rate.

Actually, I did MOSFET stuff back in high school. Low transconductance stuff, not even good enough for line stage work. So there, Hairy, did it before you did.

Jocko

 

[HarryHaller]

Mosfet misfits

They had mosfets when you were in high school? I did'nt know they had been invented yet..... Let's go eat chinese food.
H.H.

 

[SteveS]

Hey guys, thanks for the info. on the buffer stage. Have either of you checked out the Hybrid from Generoso Cozza?


http://digilander.iol.it/essentialaudio/index.htm

SteveS

 

[Jocko Homo]

Yes, Hairy, RCA made them, 3Nxxx versions. The single-gate versions were usually called IGFETs. They had low transconductance, poor linearity, and useless for anything but ham radio stuff.

The dual-gate versions used the extra gate for AGC or LO injection. You can find them in the tuner section of most TVs.

Thought you knew everything, didn't you?

Anyway back to the subject at hand........

Let me guess......triode diff-amp w/current mirrors.......high impedance with no current drive trying to drive........maybe...........a P-channel MOSFET? Even more capacitance than the N-channel ones.

OK.....obvious question: why?


Jocko