I'm trying to understand transistor amplification here.
Uhhh... I don't see how a transistor is "amplifying" anything. If an NPN transistor gets ~0.7V+ across its base the collector and emitter begin to allow current to flow through. The collector and emitter are hooked up to a completely separate power source. For example, it's not taking the 0.7V produced from my 5V USB power supply and amplifying that directly into a 12V power source. How is this amplification?
It's about as much "amplification" as my little Arduino is doing by sending a 5V signal to a relay that "amplifies" a 120V AC circuit connected on the other side, but I don't ever hear people say that a relay is amplifying voltage or current.
It seems to me that a transistor is merely controlling a greater voltage or current with its own smaller voltage and current. Am I missing something? I can't expect to take a single little 5V 1A power supply and through a transistor turn it into 12V or 5A, right?
In the common emitter configuration, a small base current controls a larger collector current.
Base current * the transistors gain is how much collector current 'could' flow.
The collector resistor determines the maximum current that can flow through the collector.
If the base current was 1mA and the gain was 100 a collector current of 100mA 'could' flow.
If your collector relay coil was 100 ohms and the supply voltage was 5 volts, (5 - .2Vsat)/100R = 48mA.
i.e. the transistor would work like a switch and 48mA load current would be allowed.
So you have 1mA of input current controlling 48 mA of output current, this is a current gain.
To get a base current of 1mA from a 5V Arduino: (5V - .7Vbe) / 1mA = 4.3K for the base resistor.
In the common emitter configuration, a small base current controls a larger collector current.
Base current * the transistors gain is how much collector current 'could' flow.
The collector resistor determines the maximum current that can flow through the collector.
If the base current was 1mA and the gain was 100 a collector current of 100mA 'could' flow.
If your collector resistor was 100 ohms and the supply voltage was 5 volts, (5 - .2Vsat)/100R = 48mA.
i.e. the transistor would work like a switch and 48mA load current would be allowed.
So you have 1mA of input current controlling 48 mA of output current, this is a current gain.
To get a base current of 1mA from a 5V Arduino: (5V - .7Vbe) / 1mA = 4.3K for the base resistor.
So using this definition of amplification, a relay is also amplifying current, just in a binary fashion compared to the Active operational mode of a transistor where the current from collector to emitter is proportional to the current flowing into the base. A 40mA current on a relay pin coming from a 5V source gets amplified to a 5A 120VAC current when the relay switches on a microwave. A 5A current could flow if the relay had a 40mA current.
"Amplification" may generally be defined as the ratio of "effect divided by control". In the case of the transistor that will be the ratio of collector current divided by base current.
It matters not whether the systems use the same power source or not.
Of course, the conservation of energy always applies. Any increase in power/energy has to come from "somewhere else".
Yes, a relay is something like an amplifier. So is the gas pedal in your car... When you drive your car, the strength of your foot isn't moving the car. Power steering amplifies the force of your arms. Power brakes amplify the force of your foot.
On the other hand, a transformer is like a lever. A step-up transformer converts to higher voltage at lower current and a step-down transformer reduces the voltage and increases the current. The amount of energy-out is slightly less than the energy-in because transformers are less than 100% efficient. (Note that I'm talking about AC transformers, not DC power supplies which can sometimes be quite a bit less efficient.)
fuzzybabybunny:
I'm trying to understand transistor amplification here.
Uhhh... I don't see how a transistor is "amplifying" anything. If an NPN transistor gets ~0.7V+ across its base the collector and emitter begin to allow current to flow through. The collector and emitter are hooked up to a completely separate power source. For example, it's not taking the 0.7V produced from my 5V USB power supply and amplifying that directly into a 12V power source. How is this amplification?
Its amplification because a small signal controls a large signal with more power.
It's about as much "amplification" as my little Arduino is doing by sending a 5V signal to a relay that "amplifies" a 120V AC circuit connected on the other side, but I don't ever hear people say that a relay is amplifying voltage or current.
But they do, but not as often because amplifying a digital signal is normally called "fanout" or "buffering" or "boosting".
It seems to me that a transistor is merely controlling a greater voltage or current with its own smaller voltage and current. Am I missing something? I can't expect to take a single little 5V 1A power supply and through a transistor turn it into 12V or 5A, right?
There's nothing mere about controlling, that's how nearly all amplifiers work (lasers and masers are the
exception). What's amplified is the power in the signal, we don't care whether that signal shares a
power supply with the input signal, we only care about the actual signal.
It seems to me that a transistor is merely controlling a greater voltage or current with its own smaller voltage and current. Am I missing something? I can't expect to take a single little 5V 1A power supply and through a transistor turn it into 12V or 5A, right?
You are correct.
It sounds to me you are stuck on the definition of amplification as "literally" increasing the power of the signal. It does not. For all the context's I've seen/heard of amplification there are usually two "givens or assumptions"
There is some power source supplying the energy to create the amplified signal
There is a continuous capability. By that I mean your relay example is more properly termed a "switching" action as opposed to an amplification action. Transistors and hydraulic spool valves and audio amplifiers and..... all can provide various levels of amplification. A relay cannot.
Maybe you are misunderstanding the term analogue in analogue amplifier. In recent times the word has become to mean the opposite of digital that is a continuous function instead of a discreet function. But that is not what it really means.
The current flowing from collector to emitter in a transistor is an analogue of the current flowing in the base to emitter. That is it is "like" the current but subject to some function, the simplest of which is gain, the multiplication by a constant.
Rather than a relay being an amplifier, it is in fact the amplifier that is acting like a switch when you are dealing with digital signals.
""A relay can be compared to a vacuum tube and solid state transistor amplifier because both can use a small voltage and current in one circuit to control a large voltage and current in another circuit. However, unlike tubes and transistors, a relay cannot produce a variable output; It can only switch on and off similar to a logic circuit in a computer. But the relay has the advantage of being able to completely isolate its input circuit from its output.""
fuzzybabybunny:
I'm trying to understand transistor amplification here.
Uhhh... I don't see how a transistor is "amplifying" anything.
A transistor, by itself can't do anything at all, except being a paper weight for a very small piece of paper.
It needs to be connected up to something, or some things in order to do something.
What we can do with the transistor depends on what is connected to it. For some circuits, involving transistors connected to resistors, and wires, and DC power source, and some capacitors, and a sinusoidal source supplying a relatively small sinusoidal signal ------- people can create a voltage amplifier .... a small sinusoidal signal is applied to the input to the transistor amplifier circuit, and the output of the circuit could be a version of the input signal, except maybe larger by some factor --- such as a factor of -3 or even -4 or even -10.
How does it manage to achieve this? The answer comes out by learning or finding out about general behaviours of the transistor, and basic circuit theory (involving transistors and resistors and voltages and currents). Circuit theory can allow some nice equations to be derived that allows you to predict in advance the amplification factor in terms of some component values (or values related to the components).
Other circuits allow the transistor's current amplification feature to be studied. For some conditions, if a DC voltage is applied to collector terminal while the emitter terminal is grounded, and a relatively small DC current is deliberately made to flow into the base (after some particular experimental setup is carried out of course)........ then there will be conditions where the collector current will be linearly proportional to the base current. The multiplication factor (or constant of proportionality) is given some label, beta .... or something like that. And it turns out that the collector current is generally very similar to the emitter current under these conditions. Whatever the effects we want to consider ....current amplification or voltage amplication (which will be linked) ----- these relatively linear amplification properties are very useful in electronics.
The section of the webpage you referred to that you might want to look more closely at is the one that discusses this:
Saturation – The transistor acts like a short circuit. Current freely flows from collector to emitter.
Cut-off – The transistor acts like an open circuit. No current flows from collector to emitter.
Active – The current from collector to emitter is proportional to the current flowing into the base.
Reverse-Active – Like active mode, the current is proportional to the base current, but it flows in reverse. Current flows from emitter to collector (not, exactly, the purpose transistors were designed for).
Nearly all the examples we deal with here in Arduino-land reside in the digital realm where the transistors "active amplification mode" is mostly irrelevant because we are using it in "saturation" mode. This is why we see so few good "analog amplification" examples in our little Arduino world. We are using them as switches where they behave in essentially a full-on or full-off switch mode to do things like drive a relay.
You also seldom see just a "single" transistor analog amplifier because with just one transistor you don't get a lot of "result" if you are aiming for some serious punch. The issue is that you have to fight with the parameters to keep the transistor operating in the Active region by employing careful use of signal biasing (often just a set of resistors) and signal coupling (usually done with capacitors) in a bipolar signal scenario... like your home stereo. High power BJT's usually have low gain and high base current requirements. This is often a killer to a small signal needing to be amplified so, as a result, multiple, carefully designed amplification stages of transistors are used to ultimately achieve the desired signal. (Driving a speaker, for example)
By utilizing the transistors ability to take a small signal at the base pin and create a proportionally larger "result" from it, you get the amplification result that was not obvious to you. This is something you DON'T get from a transistor when it is used as a switch. With amplification, you have to keep it OUT of saturation mode.
For example, it's not taking the 0.7V produced from my 5V USB power supply and amplifying that directly into a 12V power source. How is this amplification?
Well because if you want amplification according to your definition then it is impossible to amplify anything at all.
MarkT:
Not even laser action? Travelling wave tube? Seems pretty close to the definition as in both cases the
signal is being added to as it goes through...
Well a laser is not increasing the energy each photon caries, that is fixed by it's frequency. What the laser does is add more photons of the same energy to the stream.
So, philosophically does a transistor amplify a current the same way? Yes. A few (well, a lot, but on the grander scale ...) electrons cause the flow of more electrons due to the way they interact with quantum transitions in the atoms of the 'amplifying medium'. Lasers do that with photons, transistors can do that with electrons.
I used italics above because as pwillard says above, "Nearly all the examples we deal with here in Arduino-land reside in the digital realm where the transistors "active amplification mode" is mostly irrelevant because we are using it in "saturation" mode". That's true for me. I use transistors as kind of a relay. It's either on or off.
For the OP........ many things are based on definition. So as long as whoever talks about 'amplification' is clear about what they mean (ie. mathematically, or whatever), then that's fine. If the transistor circuit has a defined input.... voltage ..... or maybe current, and if the output is defined (ie. a voltage or a current)..... and if circuit theory allows us to describe the behaviour between output and input in some kind of mathematical way, or some kind of graphical way, then that's a nice start. Then, on top of that, if changes in output happens to be relatively linear with changes in input, and if the output happens to be significantly larger than the input, then that could be what you call linear amplification. Otherwise, if relatively non-linear relation is seen, then it could still be considered as amplification if the output is still larger than the output.....except it could be called non-linear amplification, or some kind of amplification with a limiting effect.
To maybe just clarify the switching/saturation concept a bit. When you reach this point in transistor operation... there would no longer be a relationship between base current and and collector current. Once a transistor is fully conducting, adding MORE base current has no resulting effect on the output. You have left the operating region where an analog (or proportional) relationship exists between the base pin and the other pins. While you can overdrive this state to gain some wiggle room to "stay" in saturation state... adding a much larger than necessary base current can also have the effect of harming the transistor.
This is why many of us cringe when we see a diagram containing BJT that neglects to use base resistor to limit current to a safe value.
