I know it's been asked 1,000,000 times how/what/why transistors work and I feel like I've read every answer but I still struggle with the basics. The struggle for me is the difference between transistors for switching and transistors for amplifying.
For switching, I gather that the base needs a certain voltage to allow the current to travel through. Say the voltage required is 1.5 volts and you give it 15 volts, does that affect the current between the collector and emitter?
For amplifying… Can only specific transistors amplify and specific ones switch? Also, one of my biggest challenges is grasping exactly how it's amplifying because it feels like something from nothing. I guess a good example logically is an electric guitar. You have a low voltage signal from the guitar and a high current feed to amplify it. Now are you using the power of your high current feed and in a sense mixing it in with you guitar signal to boost it? Also, if you had a pot to adjust the signal amplification, what exactly would you be adjusting, the current or voltage supplied.
Lastly I read that you can increase voltage and current with a transistor. Is this only in the case that the supply is of greater voltage and current that the signal being amplified? Is it possible to increase voltage and current above that of any of the supplies?
Best Answer
The most common model of transistor operation is the current-amplifier model, like all models it is something of a simplification and it represents just one way of viewing the way a transistor works. I'll assume your question is in the context of a typical silicon NPN bipolar junction transistor (BJT) used in a common-emitter configuration.
This is more or less correct, however the range of useful voltages is very small - you might as well think of it as fixed at 0.7 volts with tiny variations in normal use. This voltage is usually called Vbe (voltage across base and emitter). So long as the base voltage is enough to allow current into the base, the transistor can begin to operate. The more base current, the more current is able to cross from collector to emitter. like a valve, small amounts of base current allow large amounts of collector current. The ratio is known as the gain and might be 100x. Eventually the collector current reaches a maximum and the transistor is said to be in a saturated state. For switching applications you want to quickly drive the transistor into saturation (turning it "on") by quickly raising the base current to a suitable value.
Some transistors are better for some types of use. However many common small-signal transistors are used for both purposes.
No, that's not a useful way to think about what is happening. Think of a small flow of water turning a valve against a spring and controlling the a larger flow of water. It's about controlling not mixing.
Both usually. Voltage and current are dependent on one another. In the case of a resistive load they are related by the ohms-law equation V=IR. For a constant R, increasing V increases I (I=V/R).
As I said, the most common model for understanding transistor behaviour describes the transistor operation in terms of current amplification. However you can usually find a way of converting a current into a voltage.
The voltage at the collector of a transistor in common emitter configuration cannot be made greater than the supply voltage (Vcc) by the normal operation of the transistor itself.
Note: With an additional inductor you can create a circuit that creates higher voltages but this isn't an intrinsic capability of the transistor itself.