When calculating amplification stages on semiconductor elements, one needs to know a lot of theory. But if you want to make the simplest ULF, then it is enough to choose transistors for current and gain. This is the main thing, you still need to decide in which mode the amplifier should work. It depends on where you plan to use it. After all, you can amplify not only sound, but also current - an impulse for controlling any device.
Types of Amplifiers
When the designs of amplifier stages on transistors are realized, several important issues need to be addressed. Immediately determine which mode the device will work in:
- A is a linear amplifier, there is current at the output at any time.
- In - current passes only during the first half-cycle.
- C - at high efficiency, nonlinear distortions become stronger.
- D and F - modes of operation of amplifiers in the "key" (switch) mode.
Common circuits of transistor amplifier stages:
- With a fixed current in the base circuit.
- With fixing the voltage in the base.
- Stabilization of the collector circuit.
- Stabilization of the emitter circuit.
- ULF differential type.
- Push-pull bass amplifiers.
To understand the principle of operation of all these schemes, it is necessary to at least briefly consider their features.
Fixing current in the base circuit
This is the simplest amplifier circuit that can be used in practice. Due to this, it is widely used by beginner amateur radio enthusiasts - it will not be difficult to repeat the design. The base and collector circuits of the transistor are powered from a single source, which is an advantage of the design.
But it also has drawbacks - this is a strong dependence of the nonlinear and linear parameters of the ULF from:
- Supply voltage.
- The degree of variation in the parameters of the semiconductor element.
- Temperatures - when calculating the amplification stage, this parameter must be taken into account.
There are a lot of shortcomings, they do not allow the use of such devices in modern technology.
Base voltage stabilization
In mode A, amplifying cascades on bipolar transistors can work. But if you fix the voltage at the base, then you can even use field workers. Only it will be the voltage fixing not of the base, but of the gate (the names of the terminals for such transistors are different). Instead of a bipolar element, a field is installed in the circuit, you will not have to redo anything. You just need to pick up the resistance of the resistors.
Such cascades do not differ in stability, its main parameters are violated during operation, and very much. Due to extremely poor parameters, such a scheme is not used; instead, it is better to use in practice structures with stabilization of the collector or emitter circuits.
Collector circuit stabilization
When using circuits of amplifying cascades on bipolar transistors with stabilization of the collector circuit, it turns out to save about half of the supply voltage at its output. Moreover, this happens in a relatively large range of supply voltages. This is done due to the fact that there is negative feedback.
Such cascades are widespread in high-frequency amplifiers - URCh, UPCH, buffer devices, synthesizers. Such schemes are used in heterodyne radio receivers, transmitters (including mobile phones). The scope of such schemes is very large. Of course, in mobiles, the circuit is implemented not on a transistor, but on a composite element - one small silicon crystal replaces a huge circuit.
Emitter stabilization
These schemes can often be found, since they have obvious advantages - high stability of characteristics (when compared with all those that were described above). The reason is the very large depth of current feedback (constant).
Amplification stages on bipolar transistors, made with the stabilization of the emitter circuit, are used in radios, transmitters, microcircuits to increase the parameters of devices.
Differential Amplifier Devices
The differential amplifier stage is used quite often, such devices have a very high degree of resistance to interference. To power such devices, you can use low-voltage sources - this allows you to reduce the size. A diffuser is obtained by connecting the emitters of two semiconductor elements on the same resistance. The “classic” differential amplifier circuit is shown in the figure below.
Such cascades are very often used in integrated circuits, operational amplifiers, amplifiers, FM receivers, radio paths of mobile phones, frequency mixers.
Push-pull amplifiers
Push-pull amplifiers can work in almost any mode, but V. is most often used. The reason is that these cascades are installed exclusively at the outputs of devices, and there you need to increase efficiency to ensure a high level of efficiency. A push-pull amplifier circuit can be implemented both on semiconductor transistors with the same type of conductivity, and with different types. The “classic” circuit of a push-pull transistor amplifier is shown in the figure below.
Regardless of the operating mode the amplifier stage is in, it is possible to significantly reduce the number of even harmonics in the input signal. This is the main reason for the widespread occurrence of such a scheme. Push-pull amplifiers are often used in CMOS elements and other digital elements.
The scheme with a common base
Such a transistor switching circuit is relatively common, it is a four-terminal device - two inputs and the same number of outputs. Moreover, one input is simultaneously an output, connected to the output of the "base" of the transistor. A single output from the signal source and a load (for example, a speaker) are connected to it.
To power a cascade with a common base, you can apply:
- Base current fixing circuit.
- Base voltage stabilization.
- Collector stabilization.
- Emitter stabilization.
A feature of circuits with a common base is a very low input resistance value. It is equal to the resistance of the emitter junction of the semiconductor element.
Common collector circuit
Designs of this type are also used quite often, it is a four-terminal device, which has two inputs and the same number of conclusions. There are a lot of similarities with the common amplifier circuit of the cascade. Only in this case, the collector is a common point of connection of the signal source and load. Among the advantages of such a circuit, one can single out its high input resistance. Due to this, it is often used in low-frequency amplifiers.
In order to power the transistor, it is necessary to use current stabilization. For this, emitter and collector stabilization are ideal. It must be taken into account that such a circuit cannot invert the incoming signal, does not amplify the voltage, and for this reason it is called the "emitter follower". Such schemes have a very large stability of parameters, the depth of the OS in direct current (feedback) is almost 100%.
Common emitter
Amplifier stages with a common emitter have a very large gain. It is with the use of such circuit solutions that high-frequency amplifiers are used that are used in modern technology - GSM, GPS systems, in wireless Wi-Fi networks. The four-terminal (cascade) has two inputs and the same number of outputs. Moreover, the emitter is connected simultaneously with one output of the load and the signal source. To supply cascades with a common emitter, it is desirable to use bipolar sources. But if it is impossible to do this, the use of unipolar sources is allowed, only it is unlikely to achieve high power.