transistor behavior can be viewed as two diodes, one between the base and emitter, which tends to direct another diode between base and collector reverse polarized. This means that entity will have a base and emitter voltages of the direct current of a diode, ie 0.6 to 0.8 V for a silicon transistor and a 0.4 for germanium.
the grace of the device is that the collector will have a current proportional to base current: I C = β I B, ie, current gain when β> 1. Normal transistor signal, β varies between 100 and 300.
Then, there are three amplifier configurations: common emitter
The signal is applied to the base of the transistor and extracted by the collector . The transmitter is connected to the masses of both the signal input and the output. This configuration has both voltage gain power and high input impedance. In case of emitter resistor, R E > 50 Ω, and for low frequencies, the gain in voltage approximates quite well by the following expression: 
As the base is connected to emitter diode live among them we can assume a constant voltage, V g . We also assume that β is constant. Then we have the emitter voltage is: V E = V B - V g
And the emitter current: 
The emitter current is equal to the collector plus the base: 
The output voltage, which is the collector is calculated as: 
As β>> 1, can be approximated: 
can we write as 
Finally, the gain is: 
input current, 
B V>> V g , we can write: 
and input impedance: 
To account for the influence of frequency should be used more sophisticated transistor models. It is very common to use the model in pi. Common Base
The signal is applied to the emitter of transistor and extracted by the collector. the base is connected to the masses of both the signal input and the output. In this configuration has only voltage gain. The input impedance is low and the current gain slightly less than one, because part of the emitter current exiting the base. By adding an emitter resistor, which can be your own output impedance of the signal source, a similar analysis made in the common-emitter case, gives the following approximate gain: 
The common ground is often used to bring sources of low-impedance signal output, for example, dynamic microphones.
common collector
The signal is applied to the base of the transistor and extracted by the issuer. The collector is connected to the masses of both the signal input and the output. In this configuration has current gain but no voltage is slightly lower than unit. This configuration multiplies the output impedance by 1 / β.
The transistor against the thermionic valve
Before the advent of the transistor engineers used active elements called thermionic valves. The valves have electrical characteristics similar to that of field effect transistors (FET): the current that passes depends on the voltage at the terminal command, called grid. The reasons why the transistor replaced the thermionic valve are several:
- The tubes need very voltages high, the order of hundreds of volts, which are lethal to humans.
- Valves energy intensive, which makes them particularly unhelpful for use with batteries.
- Probably one of the most important has been the weight. The chassis needed to house the valves and transformers required for operation amounted to a significant weight, ranging from some tens of kilos kilos.
- The mean time between failures of the thermionic valve is very short compared to transistors, especially because of the heat generated.
- valves have a certain delay in starting work, and they need to be hot to set the driving.
- The transistor is inherently insensitive to microphonic effect, very common in the valves.
- transistors are smaller than the valves, including the nuvistor . Although there is unanimity on this point, it should make an exception: in the case of power devices, they must keep a sink, so the size to be considered is the device (valve or transistor) plus the sink. As the valves can operate at higher temperatures, heat sink efficiency is higher in them than in the transistors, so just a much cooler small.
- working transistors with low impedance, or low voltages and high currents, while the valves have higher impedances and therefore work with low voltage high current.
- Finally, the cost of transistors was not only much lower, but had the promise that he would continue to fall (as actually happened) with sufficient research and development.
As an example of all these drawbacks can be cited to the first digital computer, called ENIAC. It was a team that weighed over thirty tons and consumed 200 kilowatts, enough to power a small city. Was about than 18,000 valves, of which some were burned every day, requiring a major logistics and organization.
When the bipolar transistor was invented in 1947, was considered a revolution. Small, fast, reliable, inexpensive, sober in its energy needs, gradually replaced the thermionic valve during the 1950's, but not quite. In fact, during the years 1960, some manufacturers continued to use equipment thermionic valves in high-end radio, as Collins and Drake, then moved to the valve transistor transmitter but not all of the RF amplifiers. Other manufacturers, audio equipment this time, as Fender continued to use valve amplifiers for guitars. The reasons for the survival of thermionic valves are several:
- The transistor does not have the characteristics of high power linearity of the thermionic valve, so it could not replace the transmission amplifiers professional and amateur radio.
- harmonics introduced by the nonlinearity of the valves are pleasing to the human ear, so they are preferred by audiophiles
- The transistor is very sensitive to electromagnetic effects of nuclear explosions, so they were still used thermionic valves in some control-command systems of Soviet-made fighters.
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