transistorsvacuum-tube
If you encounter an old text which uses tubes instead of transistors, and if you are not familiar with the tubes, which one of the transistor from the following transistor tree would you think of to understand the circuit?
I sometimes encounter an ancient text but when I see tubes in the diagram I used to skip reading. Should I instead think of it as an NPN transistor or a particular type of MOSFET? Which particular transistor in the above tree makes most sense for a tube circuit?
A faster BJT will probably help once you get the fundamentals sorted out.
There are two (probably) new miracle working friends that you should meet.
Anti saturation Schottky clamp
Speedup capacitor.
(1) Connect a small Schottky diode from base to collector
(Anode to base, Cathode to collector), so that the diode is reverse biased when the transistor is off.
When the transistor is turned on the collector cannot fall more than a Schottky "junction" drop below the base. The transistor this cannot go into saturation and charge accumulated is much smaller so is quicker to get rid of on turn off. Example of this from here
Look at the internal block diagrams for Schottky TTL. Note how this compares. This is primarily what allows Shottky TTL to be faster than standard TTL.
They note (more worthwhile material on page)
Reducing storage time. The biggest overall delay is storage time.
When a BJT is in saturation, the base region is flooded with charge carriers. When the input goes low, it takes a long time for these charge carriers to leave the region and allow the depletion layer to begin to form. The amount of time this takes is a function of three factors:
The physical characteristics of the device.
The initial value of Ic
The initial value of reverse bias voltage applied at the base.
Once again, we can't do much about the first factor, but we can do something about the other two. If we can keep just below saturation, then the number of charge carriers in the base region is reduced and so is . We can also reduce by applying a high initial reverse bias to the transistor.
Fall time. Like rise time, fall time () is a function of the physical characteristics of the transistor, and there is nothing we can do to reduce its value.
Putting all these statements together, we see that delay and storage time can be reduced by:
Applying a high initial value of (to decrease delay time) that settles down to some value lower than that required to saturate the transistor (to reduce storage time). Applying a high initial reverse bias (to reduce storage time) that settles down to the minimum value required to keep the transistor in cutoff (to reduce delay time). It is possible to meet all of these conditions simply by adding a single capacitor to a basic BJT switch. This capacitor, called a speed-up capacitor, is connected across the base resistor as shown in Figure 19-7. The waveforms in the figure are the result of adding the capacitor to the circuit.
When initially goes high, the capacitor acts like a short circuit around. As a result, the input signal is coupled directly to the base for a brief instant. This results in a high initial voltage spike being applied to the base, generating a high initial value of . As the capacitor charges, decreases to the point where is held just below the saturation point.
When the input first goes negative, the charge on the speed-up capacitor briefly drives the base to –5 V. This drives the transistor quickly into cutoff. As soon as the capacitor discharges, the base voltage returns to 0 V. This ensures that the base-emitter junction is not heavily reverse biased. In this way, all of the desired criteria for reducing switching time are met.
(3) See how that goes. If not good enough we can see if we can add some regenerative drive next.
LSTTL & even faster friends:
Warning !!!!!!!!!!!!
Looking in here whence the below diagram came from, is liable to result in you and your soldering iron and/or breadboard staying awake all night :-).
Many good ideas.
Can you do a Miller killer ? :-).
Note that low power Schottky uses Schottky diodes whereas the earlier Schottky TTL used Schottky transistors - an apparent step backwards.
Your question distills to "how should I use transistors?" and the answer fills whole books. Distributors carry tens of thousands of different transistors to meet the needs of designers. As a consequence, I don't believe you can create an infographic that is complete, correct, and useful at the same time.
What I can give you is a set of heuristics:
Choose...
...a BJT when:
you need a current amplifier¹
you have power to burn, so the required base current doesn't bother you
the "bottom leg" of the transistor can get pulled above the "top leg," and you need this path to not conduct when that happens, as it would in a MOSFET due to the body diode
you need to exploit the C-B or E-B diode²
...a JFET when:
you only need small-signal amplification³
the base current of a BJT would be a problem in your application⁴
you need a voltage-controlled resistance
you need higher transconductance or lower flicker noise than you'd get with a MOSFET
you need a small, discrete, low-component-count constant current source
...a MOSFET when:
you need a switch⁵
you need higher input impedance than a BJT, and can't find an IGBT that fits the bill
you can't find a JFET studly enough to control the power your circuit needs⁶
you were looking at JFETs, but then realized you needed enhancement mode operation⁷
you want to exploit the inherent body diode as a feature, rather than consider it a problem to cope with⁸
...an IGBT when:
I've largely ignored transistor combinations above, because once you start combining transistors, the possibilities literally become infinite. Today you have a range from Darlington pairs to billion-transistor ICs, with new circuit arrangements being designed all the time.
There are many, many more types of transistors, but for the most part, you want to figure out all the above before moving on to them. The others are usually derivatives of the above classes, so without the initial grounding, you will have no basis to select one of the exotics. Examples:
Need to exploit the fact that a BJT's base is light-sensitive when not encapsulated? Okay, that's a phototransistor.
Need multiple emitters or collectors on your BJT? Sure, no problem, that’s easy to create in silicon; it’s often useful and therefore often seen at the custom silicon level.
Need a BJT, but a whole lot faster? Okay, we just won't use doped silicon for everything, we'll combine materials into a heterojunction transistor.
Need to pack transistors in even tighter than normal, but running into trouble with the short channel effect? Okay, we'll give you a FinFET.
Need something that works kinda like a diode, and kinda like a BJT? Okay, we'll blur the lines for you and give you a UJT.
Need something like an IGBT but with lower loss and higher current gain? A SiC junction transistor might be what you're looking for.
Footnotes
You can also make voltage amplifiers with bipolars, of course.
Small-signal BJTs make great low-leakage diodes.
JFET-like power devices exist, such as Infineon's CoolSIC, but they're relatively new and exotic, thus expensive.
This is why there are so many JFET-input op-amps.
All the other transistor types can act as a switch, too, but MOSFETs excel at this.
A power MOSFET isn't quite the same thing as a power JFET,³ and it isn't the only alternative, but it is often the simplest, cheapest alternative.
JFETs are inherently depletion-mode devices, only.
One my answers here at EE.SE shows such a circuit.
Best Answer
None of the above. They are voltage-controlled, like FETs, but their characteristics are not so similar. Perhaps JFETs are closest, since a more negative voltage on the grid turns them 'off'.
You should just look at the I-V curves of vacuum tubes for a basic understanding.
Consider that if the anode current is fixed at 1.5mA and the grid voltage goes from -1V to -2V, the anode voltage goes from 160V to 260V - a gain of -100!