Transistors are normally used as amplifiers. Some transistor circuits are current amplifiers, with a small load resistance, other circuits are designed for voltage amplification and have a high load resistance and others amplify power.
Considering the above diagram:. The small current travels from the voltage source into the base of the transistor. A current at the base turns on the transistor. 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. This is a less-used parameter than beta. All these are low frequency analysis parameters of BJT. The Value for these varies from one transistor to another and is provided by the manufacturer for characteristic analysis of a Transistor.
Hfe is the value from the data sheet which is typical maximum current gain from that transistor in the common emitter configuration. Beta is amongst other things, dependent on the configuration of the transistor. Beta is also usually specified in the common emitter configuration, so Hfe is usually the same as DC beta.
It is the factor by which current is amplified in the circuit. So on a multimeter, it indicates a mode where the meter can measure probably crudely , the HFE of a transistor. Alpha of a transistor is defined as the current gain in the common base configuration which is in turn defined as the ratio of change in the collector current to change in the emitter current.
It can possess a maximum value of one. Also, beta is the current gain in Common Emitter configuration. It is easy to understand what DC current gain means here.
That's why often additional measures have to be taken to get a constant amplification e. A transistor is a current controlled device. This means that you have the freedom to alter the current in any transistor circuit. I assume that you already know that the transistor can be configured into three different modes namely the common emitter, common collector and common base. For my explanation, I am going to consider the common emitter configuration because the other two modes are not essentially amplifiers.
In a common emitter configuration, the input in the base, collector is the output and the emitter is grounded common to both input and output circuit. Now we have decided the mode of operation. Next we need to determine the region of operation by providing a DC bias to it. Since the transistor has three terminals, 4 different combination of biasing are possible. However, we prefer to bias it such that the emitter is forward biased and collector is in reverse bias. This ensures that the transistor is driven into the active region of operation.
Now after all these designing are complete, when we apply a small alternating signal to the base of the transistor input in this case , the signal gets multiplied with the amplification factor that is decided by the transistor's current gain and the way in which it is biased.
Sign up to join this community. The best answers are voted up and rise to the top. This forward bias is maintained regardless of the polarity of the signal.
The below figure shows how a transistor looks like when connected as an amplifier. The low resistance in input circuit, lets any small change in input signal to result in an appreciable change in the output. 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.
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