Reagarding Tietze/Schenk, I currently have 2nd Edition (1971) and 12th Edition (2002). The latter is very good on transistor basics and analog circuit / amplifier design, both chip-level and discrete, these topics cover almost half of the book (1500 pages total).
Syn08 has linked to the paper I also was referring to, the R//C circuit being on page 9. The problem is intrinsic base resistance which always seems to leave an inductive term in the output, and the driving stage has harder conditions. No free lunch...
The derivation in the T/S is similar, there are also some tables for various transformed impedances, and, very valuable, a diagram which shows the trouble region (with complex conjugate poles) in a normalized form (somewhat dependent on beta), formulas to check wether your design falls into that region and tips how to get out of it (one being that one has to select the shortest path to preserve maximum bandwidth). This diagram etc. is also available for FETs.
- Klaus
Syn08 has linked to the paper I also was referring to, the R//C circuit being on page 9. The problem is intrinsic base resistance which always seems to leave an inductive term in the output, and the driving stage has harder conditions. No free lunch...
The derivation in the T/S is similar, there are also some tables for various transformed impedances, and, very valuable, a diagram which shows the trouble region (with complex conjugate poles) in a normalized form (somewhat dependent on beta), formulas to check wether your design falls into that region and tips how to get out of it (one being that one has to select the shortest path to preserve maximum bandwidth). This diagram etc. is also available for FETs.
- Klaus
I suppose that belaboring the point is in some ways what this thread is all about. In this I think case the problem has more to do with language than math.GRollins said:
In an emitter follower there is degeneration, the active BJT actually increases or decreases current from the power supply into the load to reduce the error difference between the input signal and the output signal. From a mathematical perspective this fits a working definition of feedback, from language there is no external feedback network, all error canceling connections happen to be present intrinsic to the design. Is this feedback?
Take a more complex circuit; A D to A converts a bit stream to an analog signal. Use a digital inverter to run a second DAC so that the output is 180 degrees out of phase with the first circuit. Now using an analog stage, invert one DAC output and sum it to the other DAC. Some of the internal DAC errors are canceled. I think a mathmodel would show the cancellation as being the same as selective feedback. But there is no connection or network from an output stage to a previous stage, so is this feedback?
For my personal design process, any circuit that uses the difference between an output signal and an input signal to correct the output to more closely match the input signal is easiest to visualize as feedback.
Of course what just said leaves all feedforward schemes lying bloody in the ditch on the side of the road.
I am not challenging your definition, there is no real right or wrong here, instead I'd appreciate a look into the thought process of other designers. If I am too far off topic, feel free to ignore this post.
I still don't think you're viewing my post in context. Remember that my post was before the schematic. My understanding of the circuit under discussion at that point was that the signal went across the circuit from equivalent points in a balanced circuit, thus leaving the positive signal to cancel the negative signal. No feedback. No feedforward. There's not a term for it that I'm aware of, so I'll coin one: Feedacross.
If I recall correctly, there's a place in the ML-2 schematic that does something similar to what I'm describing. After the complementary differential at the input, but before the folded cascode, there are (I think) two resistors that connect opposing phases. There's no "back" to it, nor is there any "forward." The signal goes across the circuit. For that matter, Nelson uses something along the same lines to control volume in at least two of his preamps and the horizontal resistor in a "vertical H" biasing scheme for a complementary differential might be said to fall into the same class. Neither back, nor forward...across.
Grey
If I recall correctly, there's a place in the ML-2 schematic that does something similar to what I'm describing. After the complementary differential at the input, but before the folded cascode, there are (I think) two resistors that connect opposing phases. There's no "back" to it, nor is there any "forward." The signal goes across the circuit. For that matter, Nelson uses something along the same lines to control volume in at least two of his preamps and the horizontal resistor in a "vertical H" biasing scheme for a complementary differential might be said to fall into the same class. Neither back, nor forward...across.
Grey
Charles Hansen said:
Charles:
Get rid of feedback in the cascode Wilson mirror rises noise to -105 dB! (with the 240R I am around -126 dB - 20kHz bandwidth) and I am also unable to compare high order harmonics, the best I can do is 50/100R.
I am a little disappointed by this hint, unless I misinterpreted your advice.
Schematic
KSTR said:
This would be the non-feedback analysis approach.
As much as some of the guys around are going to hate it, the emitter/source follower can be analyzed as a series-shunt feedback circuit, and stability can be evaluated using the Bode or Hurwitz criteria. The results are perfectly equivalent with the gyrator approach above.
This feedback analysis approach is sometimes a textbook example of series-shunt feedback circuit. If somebody is interested, I can dig for details in my library.
To paraphrase John: We need no stinkin' feedback
Anything can ultimately analyzed and explained without the feedback theory, but wouldn't be a pity to give up such a wonderful instrument?
john curl said:Now that I have everyone's attention, I'll tell my story about computers.
It all started back in 1963, when I dropped out of school after 3 years of engineering/physics and got a job at Lockheed Aircraft running what, at the time was one of the biggest computers in the world, the IBM 7094. It was a challenging job, as the programs were always acting up and wasted valuable computer time. Once I asked the programmer what his program was doing and he told me wing vibration analysis that required solving a 50th order matrix. I was impressed! After all, when many here were in 'knee-pants' I already had studied linear algebra, somewhat extensively, because of my physics background. I used to solve 3rd order matrices at the bus stop, just for something to pass the time. 4th order was daunting, but 50th order, wow! A lifetime of effort, seemingly. So began my love of computer simulation.
Then, in 1966, many years later, I finished school and was soon snapped up by Friden Calculator to work under 3 experienced engineers, and I would have techs that worked under me.
One, the senior engineer, a grizzled old guy, of maybe 45, carried his slide rule at his hip, like any good engineer at the time. He was the boss, and he was skeptical of the scheme of using the new and powerful ECAP program to validate our circuits. He was used to using his handy slide rule, and that was good enough for him.
Another engineer, perhaps in his early 30's was very excited about computer simulation. After all it was designed by IBM for the US military, and had been extensively used by them already, it must be near perfect! Well I was hired, partially BECAUSE I was so excited about using computer simulation. We were even written up in 'Electronics' magazine in 1966 for being pioneers into the new world of computer simulation!
Well, I became in charge of making the simulations. With the help of the other engineers, and a curve tracer, I made the models for the active devices. It was all DC analysis, so it was easier than it might be in audio, today.
Anyway, I ran the computer myself, after checking that technicians punched cards matched the circuit we were simulating, etc. We were especially interested in what was called 'worst case analysis'. The computer generated a so called 'sensitivity matrix' that showed the sensitivity of a change of any component or voltage, on the final output.
Worst case analysis would plug in the percentage variations that we chose and tried to find the worst case effect on the output. These were discrete digital germanium circuits, and it was a GO/ NO GO situation. The circuit either worked (worst case) or it didn't.
Well guess what? The computer gave us NO GO answers to previously worked out designs, and in 1967, yes 41 years ago, the head engineer whipped out his trusty slide rule and PROVED our results were incorrect! Were we embarrassed, and I thought that we might be fired. So much for trusting computers completely. We double checked our work, of course, but we knew then that if in doubt, for some reason, the computer program would tend to give us a more than worst case result. Probably a good thing.
Today, when I find exclusive use of computer emulation, I remember when I too got into trouble over it. So there, fellow engineers, and quit treating me like an inexperienced kid!
During the war...
Attachments
Hi PMA, interesting simulation. The tendency to oscillation in followers is one reason that I developed transconductance amps back in the early '70's for the Grateful Dead.
Borko, please remember: Experience can promote greatness, but inexperience can promote failure.
Personally, I get a real kick out of looking at WW2 footage and even the accomplishments of the engineers (on both sides) at the time. I actually can learn new things, but when I was young, I was too inexperienced to realize this.
Borko, please remember: Experience can promote greatness, but inexperience can promote failure.
Personally, I get a real kick out of looking at WW2 footage and even the accomplishments of the engineers (on both sides) at the time. I actually can learn new things, but when I was young, I was too inexperienced to realize this.
Re: Re: Re: Re: Neutralization
Charles,
Sounds like quite a journey.
One way to run open loop triple BJT sans OP network is to configure
it as a 2nd order UG Sallen Key low pass filter and appropriate
lossy resistors in series with caps.
However I assume this topology would violate your "no FB
anywhere except local degen" rule.
cheers
Terry
Charles Hansen said:
Charles,
Sounds like quite a journey.
One way to run open loop triple BJT sans OP network is to configure
it as a 2nd order UG Sallen Key low pass filter and appropriate
lossy resistors in series with caps.
However I assume this topology would violate your "no FB
anywhere except local degen" rule.
cheers
Terry
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