One way to explain the working principle of the transistor is to observe it as a means of controlling the diode conductivity. If we imagine conductivity as a value of 1 and non-conductivity as a value of 0, we have arrived at a binary, digital representation of a state, in one bit of information. It is fascinating that the most important component of digital electronics is the one that can control the state of 1 bit, and in a broader sense, the information represented by any number of bits. This role was played brilliantly by the vacuum tube until the appearance of the transistor, which provided adequate replacement with its reliability and efficiency. In digital electronics, the voltage values that define the values 0 and 1 are discrete, finite, and clearly separated, as opposed to analog in which the signal value varies continuously. Therefore, in building digital electronic devices, the transistor represents the most important building block.
The transistor we will first present is called an NPN bipolar transistor. If we understand the concept behind it, it will not be difficult for us to understand its other realizations. Like a diode, a transistor is built by connecting N-type and P-type semiconductors. Its conductivity is caused by electrons as the majority carriers and cavities as the minority carriers and is hence called bipolar.
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It is clear from the figure that emitter E and collector C are composed of N-type semiconductors, while base B is composed of P-type. If we connect a voltage source of any polarity to the emitter and collector of the transistor, the current cannot flow due to the isolating NP and PN layers, so it can be considered that the circuit behaves as two diodes connected in opposite directions. The following figure shows the aforementioned phenomena – when we connect collector C to the positive part of the source, and emitter E to the negative part of the source, the current cannot flow.
But if we bring enough positive bias voltage to the base, sufficient for the occurrence of permeable polarization between the P-type from the base and the N-type of the emitter, the current between the base and the emitter will flow because the emitter electrons will rush to combine with the base cavities. Finally, although the connection between the base and the collector is still impermeable polarized, part of the electrons from the emitter will still be attracted to the more positively charged collector and current will flow between the collector and the emitter. It is important to notice that the current between collector and emitter is much greater than the current between base and emitter. This happens because when electrons from the emitter arrive at the base, they do not have time to recombine with the cavities from the base but rush to recombine with the cavities from the collector which has much greater potential. Et voilà! By bringing the pulse in a form of bias voltage from the outside, we were able to control the state, as shown in the following figure.
Please note that the figure shows the direction of the electrons, but the conventional way of showing the direction of the current is quite the opposite. This can be noted from the transistor image and the direction of the arrow. The reason for this is of historical nature and dates back to Benjamin Franklin’s experiments which assumed that direction for the flow.
The following figure shows how the state of the transistor can be controlled.
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The picture requires a little more attention so we will try to explain it. RB is a base resistor that controls the amount of current IB on the base, and RL resistor controls the current IC on the collector when the transistor conducts. The total current of the emitter IE is equal to the sum of IB and IC. It follows that the NPN transistor is a current-controlled device because the current must flow between the base and the emitter in order for the current to flow between the collector and the emitter. In other words, the base current dictates the collector current.
Increasing the current IB (by decreasing the value of the RB resistor) will increase the bias voltage, and thus the current Ic will rise. This implies that the transistor is a natural amplifier because small fluctuations in the base current cause larger fluctuations in the collector current. The amount of amplification is called the gain, and for current, it is expressed by the ratio of collector and base current:
The phenomena of amplification are characteristic for every transistor which will show this behavior until it enters saturation mode in which collector current is at its maximum. On the other hand, decreasing the current IB below the value needed for biasing the transistor, the transistor cuts off. If we take into consideration only cut-off and saturation mode, it is very easy to understand that the transistor can act also as a switch. In a cut-off mode, the transistor acts as an open switch, and in saturation mode, the transistor acts as a closed switch. This is extremely important for us because it allows us to control the state of a bit, as previously mentioned. Having this control, we can build logic gates out of transistors, which then become building blocks of logical circuits. Later experiments will demonstrate how a transistor can be used as an amplifier, and as a switch.
To understand the operation of the PNP transistor, we can think of it as a mirror image of the NPN transistor, with polarity reversed. If we bring a negative bias voltage to the base, sufficient for the occurrence of permeable polarization between the P-type of the emitter and the N-type of the base, the current between the emitter and the base will flow, causing current to flow between the emitter and collector.
(Source: referenced)
Prior to getting our hands dirty with transistor experiments, we must try to understand the working principle of the potentiometer and the switches.
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