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.How does a transistor amplify current and voltage
No, it is not. The transistor is modulating and converting the energy supplied by the bias voltages in accordance with the signal voltage/current it receives as input. There is no free lunch involved.Isn't that like creating something from nothing?
If you put your thumb over the end of a water hose to spray your friend with the resulting high pressure water, you then can control the water with small movements of your thumb. You're amplifying small thumb movements and getting large differences in water output as a result, but have you created something from nothing?
If you put your thumb over the end of a water hose to spray your friend with the resulting high pressure water, you then can control the water with small movements of your thumb. You're amplifying small thumb movements and getting large differences in water output as a result, but have you created something from nothing?
BJT transistors are voltage responding devices, as are FETs and tubes. The only basic current responding device I know of is a magnetic amplifier. As any transistor book will show, collector current Ic = Is*e^(Vbe/Vt), where Vbe is the base-emitter voltage, Is is the saturation current, and Vt is the thermal voltage, of which both the last two are relatively constant. This shows that the Vbe controls the current in the transistor. The base current Ib is (Is/β)*e^(Vbe/Vt), where β is relatively constant for a particular transistor. It also shows that both Ic and Ib are exponentially responsive to Vbe. Dividing Ic by Ib cancels the exponential terms and gives the ratio β . So a BJT mimics a current amplifier, but its Ic is really and truly controlled by Vbe. The base current is the unavoidable "waste current" that is present during operation in the active region. If the β were super high or infinity, there would not be any base current at all to worry about. Therefore, the Ib is a linear indicator of what the collector current is, but it is not controlling the collector current. Now one can make voltage amplifiers out of basic current amplifiers and current amplifiers out of basic voltage amplifiers by the using external circuitry, but that does not define what the basic active element is.Transistor are current controlled devices.....
current = flow of charge...which control the flow of current thru a load, not voltage.
Is it kind of like the transistor allows you to draw more voltage or current from the battery than it's rating? Obviously it can't be just increasing the total energy in the circuit or there would be no energy crisis.
ericgibs,
BJT transistors are voltage responding devices, . I
Ratch
Nope, you are not getting it. A transistor acts like a fast changing variable resistor in a circuit. A transistor's name comes from the words "transfer resistance". It cannot deliver more voltage than the battery voltage, or more than the short-circuit current of the battery. It simply changes the energy flow from the battery according to a control signal. The control signal uses little energy and controls a relatively large amount of energy from the battery, but all energy sources have finite limits which are never exceeded. Stay dry and don't get confused by hydraulic analogies. Just remember that small amounts of control signal energy control large amounts of electrical energy to a load. That's amplification.Is it kind of like the transistor allows you to draw more voltage or current from the battery than it's rating?
jac4b,
Nope, you are not getting it. A transistor acts like a fast changing variable resistor in a circuit. A transistor's name comes from the words "transfer resistance". It cannot deliver more voltage than the battery voltage, or more than the short-circuit current of the battery. It simply changes the energy flow from the battery according to a control signal. The control signal uses little energy and controls a relatively large amount of energy from the battery, but all energy sources have finite limits which are never exceeded. Stay dry and don't get confused by hydraulic analogies. Just remember that small amounts of control signal energy control large amounts of electrical energy to a load. That's amplification.
Ratch
It is correct that a BJT can be viewed as a current controlled device for design and calculation purposes, but I am averring that it is physically a voltage controlled device. Nowhere does the article say that a BJT is a current controlled device. To successfully refute my assertion, you have to show where I am wrong in my statement which describes the equation showing Ic is dependent on Vbe, and that Ib is a waste product that has to be taken into account, but does not control the Ic. It only indicates what the Ic is. I know that a lot of literature says that a BJT is a current control device, but when the external circuitry is removed, the Vbe to Ic equation says otherwise.The collector–emitter current can be viewed as being controlled by the base–emitter current (current control), or by the base–emitter voltage (voltage control)
Just saying it is so does not make it so. I expounded on my assertion. Can you do the same for yours?I agree with Eric. Bipolar transistors are current controlled devices.
How so? What great math contortions? Which diode equation were you looking at? I am looking at the Shockley diode equation, which has only one main variable, voltage across the diode Vd. Diode - Wikipedia, the free encyclopedia The other "variables" are controlled in manufacture, and one is temperature dependent. They could be considered constants. In a transistor, Vbe controls Ic, not the other way around. Any good text like Sedra and Smith will confirm that.You go through great mathematical contortions to show that it is voltage controlled, because the diode equation of the base input has a voltage component, but that is a secondary result of the current operation of the transistor.
And what is the current gain of a BJT without any external circuitry? It is β of course. I said that already. How does knowing that a BJT is a voltage-controlled confuse the beginner? Anyone would be a fool not to use the useful relationship of Ic to Ib in calculations and design. That does not refute my assertion. The truth is not confusing.For normal amplifier design you are only concerned about the current gain of the bipolar transistor not its voltage gain (transimpedance, which is only sometimes used in RF design). If you want to still think of bipolar transistors as voltage controlled, go for it, but don't try confuse the beginners by promoting a viewpoint that is yours alone.
No, the winding is a relatively small impedance, so responds well to current input.By your logic a magnetic amplifier is also voltage controlled if you include the resistance of the control winding.
I am a electrical engineer, not a hydraulics engineer. I know how a BJT works. Do you find hydraulic engineers using electrical analogies?If you want to use the water analog for a transistor, think of a pipe with a valve. The pipe is the current through the collector-emitter. The valve is the base. Adjusting the value with a small amount of power can control the flow rate of a stream of water with a much larger amount of power. That's amplification.
It does not make me happy or sad. After all, it is just technical knowledge.I can see that this thread is going to spiral out of control, so if you want to believe a transistor is best considered as a voltage controlled device makes you happy, I don't have problem with that.
Bring them on. But I hope they can show me their reasoning, and not just throw out some links that only parrot the false viewpoint that has been published before.I think you will realise that there are a lot learned and experienced members who will try to convince you otherwise.
Good one. The man in the picture represents Vbe.I always liked this explanation of a transistor. Picture says it all.