AoE x-Chapters - 2x.11 BJT bandwidth, fT

The 2N3904 looks like Mom's meatloaf or the Cleveland Brown's aspirations as a team, in transistor form. It's not fancy or nothin' but when filet mignon isn't available or too expensive it won't do ya wrong

P 135, I've seen lots of transistors ocillate with a well-bypassed collector. That's not always a fix. A base resistor apparently is.

Wire bonds are evil.

More interesting stuff, thanks.

You might want to add another sentence to the discussion of high-level injection effects on beta--the origin of the base current is recombination, which goes as the product of the electron and hole densities. HLE is when injected carrier density >~= doping density, resulting in enhanced recombination and so lower beta.

A Hartley oscillator is a Colpitts made by tapping the tank inductor instead of the capacitor, and there's no tapped element in your circuit. The emitter-follower oscillator is a negative-resistance oscillator, which is why adding base resistance fixes it.

You might mention Carl Battjes' fT doubler, in which each side of a diff pair has two transistors with their bases wired in series and their collectors in parallel. (Battjes has a chapter in one of Jim Williams's books, entitled "Who wakes the bugler?".)

Cheers

Phil Hobbs

Yes, I am not at all convinced that a high impedance at the collector is necessary for an emitter follower to oscillate. If there is a capacitive load on the output (emitter) then I think that it can make the input impedance have a negative real part.

I found this document interesting, though I cannot claim to have understood it fully yet:

An emitter follower with well bypassed base and collector, and just a resistive pulldown on the emitter, will often oscillate.

That guy does a lot of impressive-looking math and ignores the leads and wire bonds.

Well, with some inductive feedback allowed between emitter or collector current and the base wiring, you'll get over-unity gain sometimes. It's usually voltage slew capacitor coupled that gets 'bypassed'. A resistor in the base lead is the R-L filter component you need, sometimes.

Either that, or careful wire placement.