Google serves cookies to analyze traffic to this site and for serving personalized ads. Learn more. Skip to content. Last updated: 7th Nov ' Well, let's add a couple of resistors.
Resistors are cheap, but if you want to make money you can try to sell them expensive by calling them "voltage-to-current converters" 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. Ib will depend on Vin and Re. Re we have found from DC analysis. By looking at this, you can make out that the output signal will be an amplified one, right? Note: This was just to give you an intuitive idea that amplification does take place.
But whether you will get amplification or not depends on whether the transistor is in linear amplifier , saturation or cut off switch. Again, what will be amplified current or voltage depends on type of configuration. So that all comprises of chapters of any standard book on analog theory.
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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.
That is enough voltage on the base, along with ground on the emitter and the large positive voltage on the collector, to properly bias the transistor. This action results in a positive output voltage, which has the same characteristics as the input except that it has been amplified and the polarity is reversed.
The input signals in the preceding circuits were amplified because the small change in base current caused a large change in collector current. And, by placing resistor RL in series with the collector, voltage amplification can be gained. In this condition if we place a transistor in the circuit, which has a gain of , the signal will be boosted. This current will now be sufficient to light our LED in the circuit. Pages Blog. Child pages. Applied Physics for Information Technology.
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