Linear solid state amplifier BJT? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: June 30, 2022, 09:58:14 pm
content: a dumb question but, is there any silicon device that acts like a triode tube? really linear?for some reason I was under the impression that JFET's were linear, but they only are when the voltage across them is really low.and I don't want to have like 500mV headroom before saturation and cutoff.so is there some BJT that is linear throughout it's operating voltages?and yes I know that I can add DC feedback from the drain to gate, but where's the fun in that

author: TimFox
author_rank: Super Contributor
timestamp: June 30, 2022, 10:12:01 pm
content: The vacuum triode is the only common device that, under certain reasonable conditions, is theoretically linear.To achieve a linear voltage gain, anidealtriode needs to be operated into a high load resistance, ideally a constant-current load.The vacuum pentode into a "normal" load resistance is similar to a triode into alowload resistance, very roughly 3/2-power, due to the constant voltage on the screen grid.A BJT as a voltage amplifier, with a "normal" load resistance, is inherently exponential.A FET (J or MOS) as a voltage amplifier, with a "normal" load resistance, is approximately quadratic.Degeneration (cathode, emitter, or source resistor, respectively) will help linearize a single device by providing local feedback.A BJTmayhave a reasonably constant current gain over a wide range of collector current: to make a voltage gain, you can add a resistor in series with the base and a load resistor in the collector.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2022, 10:14:28 pm
content: Yes, but not very common, mostly a relic. The Static Induction Transistor is the nearest analog to a triode.Tim

author: Benta
author_rank: Super Contributor
timestamp: June 30, 2022, 10:22:07 pm
content: Quote from: T3sl4co1l on June 30, 2022, 10:14:28 pmWow! Never heard of that one before. Thanks!

author: mag_therm
author_rank: Frequent Contributor
timestamp: June 30, 2022, 11:05:13 pm
content: Is a Triode a standard for linearity?For triode common cathode with no feedback:Ia=A+BVg+cVg^2. To get ABC these days, one way is with curve fitting along load line drawn on published Ia vs Vg curvesDuncan Amps has a large data base .These parameters can be used for both single tone linearity Vg =ASin(W1*t) and two tone intermodulation, Vg =ASin(W1*t)+BSin(W2*t)A cathode follower has excellent linearity but no gain.A grounded grid is a high frequency connection, but can have improved linearity too:iRL/e_in = (1+u)RL/(Rl+ra+(1+u)Rin) note the u on numerator and denominator.I converted an old mixer with ECC83 to grounded grid to match 4000 ~12000 Ohm from acoustic, bass and StratocastersSound quality is second to LOUD and your ears will be ringing when the teens are in the basement here.Of course, all the above can apply to BJT and FET.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2022, 11:54:30 pm
content: Because they suck? Triodes suck too... why don't they use 'em anymore, right?I don't know any SIT characteristics offhand, but evidently they simply aren't competitive to MOSFETs and BJTs. So, yeh.Your concern is misplaced -- linearity is a non sequitur for amplifying devices.As long as the operating range is continuous and one-to-one, it doesn't really matter. What does matter is how much gain is available in general, what saturation voltage is, and device capacitance or inductance. We can fix everything else by simply using a couple more devices and applying negative feedback to clean it up, and maybe a few more to re-distort it back to whatever other characteristic we want.Whereas no amount of feedback will gain you back the pitiful efficiency that triodes have, or the high impedance and relatively low bandwidth of vacuum tubes in general. (You can use a triode in positive-grid operation to reclaim some efficiency -- which costs some distortion due to grid rectification loading the driver -- but this takes more effort, and again probably needs feedback to manage the distortion.)RF amps are the most stringent, as there's no time (that is to say, phase margin) for anything more than local feedback -- usually shunt feedback as a output-to-input resistor in a common-source configuration (often called "neutralization", but it's not just compensating for reactance when it's used in this way). In this case, it does help that conventional devices are much better behaved than they possibly could be -- an exponential transfer function is a far sight better than some lumpy sort of thing you might imagine. So we get the basic calculus advantage of, small signals span less curvature so have less distortion; and a few tricks like push-pull or other phase shift hybrids (for narrowband RF mostly) to cancel out some harmonics/IMD. And if you don't mind spending a lot of time figuring things out, you can predistort the signal to perfectly cancel it out with the final's distortion, which is how particularly high linearity power amps are made, for cell towers etc.Tim

author: mclute0
author_rank: Regular Contributor
timestamp: July 01, 2022, 12:16:08 am
content: like SIT_2SK77B

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 01, 2022, 12:22:21 am
content: As I recall, there were presentations about SIT in 1984 at the world power electronics conference in France.It sounded too good to be true, I think they were used in prototypes ~ 2000~ 3000kW locomotive drives in Japan in the low kHz.But nothing eventuated as far as devices on the market.Then ...later....IGBT came along.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2022, 12:22:41 am
content: Meh -- at least tubes look nice. If you put a WE300B in a TO-247 package, no one's going to buy it. Even if the electrical characteristics were perfectly identical.Tim

author: WatchfulEye
author_rank: Regular Contributor
timestamp: July 01, 2022, 12:27:13 am
content: Lateral FETs might be worth a mention. Not as linear as SITs, but not quite as esoteric and expensive.Still actively produced and in stock in a convenient TO247 package for a fiver.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2022, 06:21:17 am
content: BJTs are almost all ideal; hFE can be "linear" in the sense that it remains high even at low Ic, but that really doesn't matter much. Ic is still exp(Vbe), give or take R_E and R_B.You can always make a "worse" BJT by degenerating the emitter (adding R_E), or other things.Tim

author: magic
author_rank: Super Contributor
timestamp: July 01, 2022, 09:38:00 am
content: Ic(Vbe) characteristics of power BJTs look quite linear above a few hundred mA due to internal emitter resistance, whether it's parasitic or introduced deliberately to improve current sharing across the die.Of course an external degeneration resistor achieves the same, and possibly with better thermal stability of gain.