Noob Question about Transistors

I apologize for the total noob question. I’ve been reading articles all day long, and I still don’t have a good understanding.

With a typical NPN transistor (e.g. TIP120), does the voltage /current applied to the base have to “match” the voltage / current that’s going from the collector → emitter? Or is it a fixed amount of voltage / current that will “close” the circuit and allow current to flow from C->E?

Thank you!

The voltage certainly doesn't have to match: that's what makes them useful as switches. You can switch your 12V or whatever motor with the 5V of the controller.

And the currents don't match either: the gain is the ratio of the two and that's what makes a transistor an amplifier when it's not saturated as they call it.

Depends on the transistor configuration. . You sound like you want to sink current in which case on an npn yes the collector needs to match what's on your base voltage..

Sinking can be done with 0.7v or higher 300ohms should put that transistor into high conduction from a 5v source eg arduino.

Two things are involved; the base-emitter "step" voltage, and "current gain".

In normal operation, the base-emitter junction is a diode with a threshold voltage of about 0.65V. You need to provide this much voltage for it to conduct, but once you do, you can cause quite significant current (which you must limit to a reasonable specified amount) to flow from base to emitter without the voltage across the junction increasing a great deal. Just as with a common silicon diode, and this is largely independent of whether you have anything connected to the collector (but not totally independent due to the resistance of the emitter itself).

If you then have a circuit connected to the collector which allows current to flow to the emitter, then the transistor will pass such current proportional to the amount of current being fed into the base. The ratio of collector current to base current is the "current gain" or "?" (beta) but this ratio is not entirely linear, so is specified for one or more particular collector current.

At any point when the circuit connected to the collector cannot supply the (sufficient) amount of current that would be proportional to the particular base current, then the voltage on the collector will at that point be pulled down very close to the emitter voltage - of the order of 0.25V, never quite zero, and the transistor is "saturated". This limitation is obviously caused not by the transistor itself, but by the circuit connected to the collector - clearly no more current can be drawn than that circuit can provide. And incidentally, the transistor may take a small but electronically significant time to leave the "saturated" state when the base current is reduced due to storage of charge in the base junction.

So the collector current will be suitably proportional to the base current except that when the collector circuit cannot supply enough current (and it clearly must be deliberately limited in some way), the transistor will saturate.

And of course the transistor has limits on the amount of current that it can safely conduct in saturation as well as limits on the amount of power that it can dissipate when the collector voltage is above saturation where the power dissipated is the product of that voltage and the amount of current the transistor is conducting at the time.

Normally you think of the bipolar junction transistor as a current-controlled device,
you never attempt to control the base-emitter voltage directly, but convert a control
voltage to a current using a base resistor.
That's a good description, here's my take, probably saying the same things slightly
differently:

There are two ways to use such a transistor - as an amplifier or as a switch.

As an amplifier the collector current is controlled by the base current and is typically
100 to 500 times larger (or 10000 times if a darlington pair). Usually feedback is
used to tame this raw gain and produce more linear response. The simplest form
of feedback used is shunt feedback by adding an emitter resistor.

In amplifier mode the Vce is usually > 1V for best linearity.

When used as a switch you put about 5% of the load current into the base to turn
the device on hard, bringing the the Vce voltage down to around 0.05 to 0.2V - this
is "saturation", and the device is highly non-linear in this region, but generally you
just think of the transistor as being "on" or "off".

By contrast a MOSFET is a voltage controlled device (in fact no current flows into
the gate except to charge it up and discharge during switching). Modern power
MOSFETs are designed for switching only, not linear amplification, except for RF
power MOSFETs.

Vbe for a transistor is usually fairly constant (like voltage drop across a diode).

So long as your base voltage is higher than Vbe then you're good as far as volts go.

You do have to worry about how much current is going to pass through the base - a resistor is usually needed for this.

I don't think they do . You just give some current (has a minimum) to the base and collector => emitter is open and you give no voltage (or very low) and the collector => emitter is closed .

Arman5592:
I don't think they do . You just give some current (has a minimum) to the base and collector => emitter is open and you give no voltage (or very low) and the collector => emitter is closed .

It's very non-linear, like a diode.

Below a certain voltage, no current will flow.

When you reach a certain level, massive amounts of current will flow and the voltage doesn't really go up any more.

If he is asking what I think he is asking, then yes the voltages must "match".

If you have 50 V on the collector and 20 V on the emitter, you must have about 20.7 V on the base to turn it on. You cannot apply just 0.7 V to the base.

Relays are different, since they are electro-magnetic-mechanical devices with no electrical connection between the coil and the contacts, there is no real limit to the voltages. The coil can be at 0-5 V, and the contacts at 100-200 V.

Safety, don't try this at home yadda, yadda yadda.

KeithRB:
If he is asking what I think he is asking, then yes the voltages must "match".

If you have 50 V on the collector and 20 V on the emitter, you must have about 20.7 V on the base to turn it on. You cannot apply just 0.7 V to the base.

Yes, of course. All voltages at the base should relative to the emitter, not relative to GND.

I think he's asking if the voltage on BASE and COLLECTOR has to be the same , which is not necessary , and that's exactly why we use transistors - here's an example :
You're going to drive 9v with a transistor . 9v goes to the collector . You want to control it with arduino . Your desired arduino pin goes to the base . You need the 9v , right ? So you collect the 9v (and all excess from the 5v given - depends on switching voltage and voltage drop) from the emitter . If we just needed 5v then why not use the arduino pin itself ? We used the transistor since were working with 9v , not 5v .

Arman5592:
I think he's asking if the voltage on BASE and COLLECTOR has to be the same , which is not necessary , and that's exactly why we use transistors

Yeah that's what I was getting at here:

JimboZA:
The voltage certainly doesn't have to match: that's what makes them useful as switches. You can switch your 12V or whatever motor with the 5V of the controller.

Yes I noticed :grin:

Of course the other reason to use a transistor, even if you are using the same voltage as your Arduino is for current capability. If you were driving the coil of a 5V relay and it needed more than the 20mA the Arduino can supply, you use a buffer in the form of a transistor or MOSFET.

Once you learn the advantages of MOSFETs they are really superior in almost any application. The only time a bipolar might win is if your drive voltage is very low. Many "logic level" MOSFETs guarantee operation with the gate at a mere 2.4V, which means even the 3.3V DUE can be used with them.

rmetzner49:
The only time a bipolar might win is if your drive voltage is very low.

Or when your collector/emitter voltage or current requirements are low. The Vce drop on BJTs usually isn't as good as a MOSFET, but for many applications it is within an acceptable range and a BJT can be cheaper than a logic-level MOSFET.

As for the OP, the base voltage can be lower on an NPN BJT. But watch out when using PNP BJTs -- switching a 12V load with a 5V Arduino pin via a PNP BJT is not going to work well!

Mosfets are usually more expensive. .. they tend to require high 5 -10v to switch them on.

100 bjts for 2 bucks... fets are not needed with leds 10 watts or under.

.. they tend to require high 5 -10v to switch them on.

Oh but these wonderful things called "logic level" FETs have been around for a few years now. Most switch on at a mere 2-3V.

I just said... cost