Uh.... The collector is clearly grounded (at least for AC). So it's not a common emitter. It's a common collector, aka., emitter follower.
A little resistance in series with the base kills oscillation, but adds noise. A ferrite bead could be the trick.
In IC designs I used emitter followers as beta helpers. Usually adding a bit of load current on the beta follower, i.e., increasing the collector current prevented oscillation.
Tom
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The original circuit is definitely an emitter follower, but you can analyse the oscillator as a common-emitter stage with CLC feedback, see post #6.
Given the trace inductances around the device, it’s a common collector at LF but transforms into a common emitter at HF. I mentioned over in the other thread that part of the issue here is you never really know what the parasitic elements are around a circuit given layout, you can only take a guess.
Here is Chessman and Sokal’s original article https://www.hifisystemcomponents.com/downloads/articles/Prevent-Emitter-Follower-Oscillation.pdf
Here is Chessman and Sokal’s original article https://www.hifisystemcomponents.com/downloads/articles/Prevent-Emitter-Follower-Oscillation.pdf
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re the beta helper collector current: this was a lesson I had the learn the hard way. I run them at 750uA to 1.5mA now.
As you have to shunt away the extra impedance coming from the inductor at high frequency, I don't think there is other option to keep input impedance high.
I might have found an optimal solution.
Zobel network has a very good property. If the network is balanced, the input impedance stay constant and it is resistive only.
Wiki explains it better than me. https://en.wikipedia.org/wiki/Zobel_network
If we face the constant-impedance node of the Zobel network towards the input of the emitter follower, the input only see a resistive signal source. There will be no oscillation. The tricky part is the components need to be selected in a way that they can overcome any internal capacitance from the transistor (ideally).
The condition to balance the zobel network in the circuit below: R1*R2*C3 = L1, also R1=R2
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Thankfully I learned from those who'd learned the hard way. I forget the current I ended up running. Probably 1/10th the main bias current so maybe 10-100 µA. But that was on an IC. Every design review included both an AC sim of the impedance of the bias node and a transient sim where current was dumped into or sucked out of the bias node to look for signs of instability.
Tom
The output of the emitter follower could also be inductive. You don't need a long trace/wire to get inductance. A previous EF stage would act like an inductor.
This is mentioned in the 1st paper in the post #15.
For example, let's say the ft is 30MHz, a 100 Ohm base resistor would be translated into a 0.53uH inductor. It is not far from the so-called worst case, 1uH.
This is mentioned in the 1st paper in the post #15.
For example, let's say the ft is 30MHz, a 100 Ohm base resistor would be translated into a 0.53uH inductor. It is not far from the so-called worst case, 1uH.
👍
Finally, some births are a little more difficult or simply take a bit longer ..!
And if we now also incorporate switching elements into our MixedSignalDesign, it will be exciting, or are on the move in terms of measurement technology.
I love instabilities and parasitic oscillations. It always brings you back down to earth.
HBt.
Don’t you have to consider the boundary conditions Marcel mentioned? I’m just thinking that beta will have dropped off significantly at fT. Just a question - I’d have to play around with the equations a bit after first reading the linked to papers posted by Mark.
The impedance transformation is the result of beta rolling off to unit at ft. Thus, it does consider that.
Cascodes love to sing because the intrinsic output resistance skyrockets. Doesn’t take much reactance anywhere to make it try to go higher than infinity (which is minus infinity).
The first edition of Bob Cordell's book suggests running the upstream transistor (emitter follower) at 2mA. On page 60, Bob writes
That's an interesting approach!
I haven't attempted to calculate it, but it might even be fairly robust against inductance changes:
A base stopper (series) resistor mostly helps at high frequencies/for small inductances, because the negative resistance gets less negative at high frequencies.
A shunt resistor mostly helps at low frequencies/for high inductances, because the negative conductance gets less negative at low frequencies.
You have both, but I'm not sure about the effect of C3.
A base stopper can help by causing Cbc to dominate the impedance seen by the base of the transistor, essentially making the source impedance capacitive. This is a similar effect to a capacitor across the VAS degeneration. In some old amplifiers, base stoppers with parallel capacitors were used but I don't see anyone doing this anymore. It's possibly worth revisiting.
The exact current needed will depend on the device parameters. I suspect it's about finding a good spot on the fT(IC) curve.
Tom
Now that I am home, I see the link I posted here is the same as the first paper in Mark's earlier post. Apologies.