Practical Activity for A minute current in the base-emitter circuit is used to control a much larger current in the collector-emitter circuit. It will help if the transistor is mounted on a base with three 4 mm terminals. Otherwise, make connections to it using crocodile clips. Resistors are really great things, because next to "voltage-to-current converters" you an also use them as "current-to-voltage converters"!
Due to Ohm's Law:. The term between the brackets is a constant which we're not interested in at the moment. Then again ignoring the constant factor. So the output voltage is 10 times the input voltage plus a constant bias.
Looks like we can use the transistor for voltage amplification as well. The signal is being amplified. Depending on the design of the transistor amplifier the actual base current may or may not be part of the output current. Don't get hung up on a definition of amplification that requires every input electron to get larger and then pass to the output Amplifiy sound, and you're amplifying the energy-flow: the input watts of sound become larger output watts.
Note that an electrical transformer doesn't amplify. It can step up voltage, but it cant increase the watts. Transistors and any sort of valve or switch can amplify.
They do it by using a tiny wattage to control a power supply which can output a huge wattage. The large output comes from the power supply, while the input signal is valving the transistpr on and off. If you have a giant hydraulic press, you can crush cars by touching a valve switch with your little finger.
The valve amplified your finger motion to mash Chevys. But actually it was the hundreds-HP haudraulic supply which provided the increased wattage. With NPNs, same idea. Transistors are valves for flowing charge instead of flowing haudraulic fluid. What is my understanding is that for a transistor to amplify you need to bias it properly. Forward biasing of BE junction makes it a conducting diode so input resistance is less.
Reverse biasing CE junction makes it non conducting diode so output resistance is high. And if Ic is almost equal to Ie then the current causes a low voltage drop at input and large one at output. This is why its called an Amplifier. With a transistor, you can achieve this: Give a small signal ac at input, and get a larger valued higher amplitude signal at output. But this is not all. You have to give DC supply at collector and base; emitter if required.
This is called biasing the dc point. The rms power you get at the output will be less than the dc power you have supplied. DC analysis: don't consider any ac signal. Find out the values of all diode currents based on dc voltage at various nodes Collector, base , emitter. This is done by using KVL along various loops. Going further, the diode has forward resistance. So the actual model will be like this:. From DC analysis, you must have found the value of Ie. Vout will depend on Ic.
Ic will depend on Ib. The emitter current caused by the input signal contributes the collector current, which when flows through the load resistor R L , results in a large voltage drop across it. Thus a small input voltage results in a large output voltage, which shows that the transistor works as an amplifier.
Let there be a change of 0. This emitter current will obviously produce a change in collector current, which would also be 1mA. Hence it is observed that a change of 0. As the common emitter mode of connection is mostly adopted, let us first understand a few important terms with reference to this mode of connection.
As the input circuit is forward biased, the input resistance will be low. The input resistance is the opposition offered by the base-emitter junction to the signal flow. The current is then amplified and travels from the emitter of the transistor to the collector.
The amplified current is large enough to turn on and light the LED. A separate power source must be connected to the collector of the transistor because a transistor is an active device, which means it needs power in order to operate.
The amplifier is complex circuit which is exploiting properties of a transistor. It has input node and output node. The output voltage from the amplifier, taken at the collector of Q1 with respect to the emitter, is a negative alternation of voltage that is larger than the input, but has the same sine wave characteristics. During the negative alternation of the input signal, the transistor current increases because the input voltage aids the forward bias.
The output for the negative alternation of the input is a positive alternation of voltage that is larger than the input but has the same sine wave characteristics. The direct current flowing through the circuit develops more than just base bias; it also develops the collector voltage VC as it flows through Q1 and RL.
Thus, the output is a negative alternation of voltage that varies at the same rate as the sine wave input, but it is opposite in polarity and has a much larger amplitude. This permits the collector voltage VC to change with an input signal, which in turn allows the transistor to amplify voltage.
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