When calculating amplifying stages on semiconductor elements, you need to know a lot of theory. But if you want to make the simplest ULF, then it is enough to select 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 the sound, but also the current - an impulse to control any device.
Types of amplifiers
When the designs of amplifying stages on transistors are implemented, several important issues need to be resolved. Immediately decide in which of the modes the device will work:
- A is a linear amplifier, there is current at the output at any time during operation.
- V - current flows only during the first half cycle.
- C - with high efficiency, non-linear distortions become stronger.
- D and F - operating modes of amplifiers in the "key" mode(switch).
Common transistor amplifier circuits:
- With a fixed current in the base circuit.
- With fixing the voltage in the base.
- Stabilization of the collector circuit.
- Emitter circuit stabilization.
- ULF differential type.
- Push-pull bass amplifiers.
To understand the principle of operation of all these schemes, you need to at least briefly consider their features.
Fixing the current in the base circuit
This is the simplest amplifying stage circuit that can be used in practice. Due to this, it is widely used by novice radio amateurs - it will not be difficult to repeat the design. The base and collector circuits of the transistor are powered from the same source, which is an advantage of the design.
But it also has disadvantages - this is a strong dependence of the nonlinear and linear parameters of the ULF on:
- Power supply.
- Degrees of dispersion of semiconductor element parameters.
- Temperatures - when calculating the amplifying stage, this parameter must be taken into account.
There are quite a few shortcomings, they do not allow the use of such devices in modern technology.
Base voltage stabilization
In mode A, amplifying stages on bipolar transistors can work. But if you fix the voltage on the base, then you can even use field workers. Only this will fix the voltage not of the base, but of the gate (the names of the pins for such transistors are different). in the diagram instead ofthe bipolar element is installed field, nothing will have to be redone. You just need to choose the resistance of the resistors.
Such cascades do not differ in stability, its main parameters are violated during operation, and very strongly. Due to the extremely poor parameters, such a circuit is not used; instead, it is better to use designs with stabilization of the collector or emitter circuits in practice.
Stabilization of the collector circuit
When using circuits of amplifying stages on bipolar transistors with stabilization of the collector circuit, it turns out to keep 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 widely used in high-frequency amplifiers - UFC, IF, buffer devices, synthesizers. Such circuits are used in heterodyne radio receivers, transmitters (including mobile phones). The scope of such schemes is very large. Of course, in mobile devices, the circuit is implemented not on a transistor, but on a composite element - one small silicon crystal replaces a huge circuit.
Emitter stabilization
These circuits can often be found, as they have clear advantages - high stability of characteristics (compared to all those described above). The reason is the very large depth of current feedback (DC).
Amplifyingcascades on bipolar transistors, made with stabilization of the emitter circuit, are used in radio receivers, transmitters, microcircuits to increase the parameters of devices.
Differential amplifying devices
The differential amplifying stage is used quite often, such devices have a very high degree of immunity to interference. To power such devices, you can use low-voltage sources - this allows you to reduce the size. A dif-amplifier is obtained by connecting the emitters of two semiconductor elements to 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, mobile phone radio paths, frequency mixers.
Push-pull amplifiers
Push-pull amplifiers can operate in almost any mode, but B is most often used. The reason is that these stages are installed exclusively at the outputs of devices, and there it is necessary to increase efficiency in order to ensure a high level of efficiency. It is possible to implement a push-pull amplifier circuit both on semiconductor transistors with the same type of conductivity, and with different ones. The “classic” circuit of a push-pull transistor amplifier is shown in the figure below.
Regardless of the operating mode of the amplifying stage, it turns out to significantly reducethe number of even harmonics in the input signal. This is the main reason for the widespread use of such a scheme. Push-pull amplifiers are often used in CMOS and other digital components.
Scheme with a common base
This transistor switching circuit is relatively common, it is a four-terminal circuit - two inputs and the same number of outputs. Moreover, one input is also an output, it is connected to the “base” terminal of the transistor. One 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 use:
- Scheme for fixing the base current.
- Basic voltage stabilization.
- Collector stabilization.
- Emitter stabilization.
A feature of circuits with a common base is a very low value of the input resistance. It is equal to the resistance of the emitter junction of the semiconductor element.
Common collector circuit
Constructions of this type are also used quite often, this is a four-terminal network, which has two inputs and the same number of outputs. There are a lot of similarities with the common base amplifier circuit. Only in this case, the collector is a common connection point for the signal source and the load. Among the advantages of such a circuit, one can single out its high input resistance. Because of this, it is often used in bass amplifiers.
In order to power the transistor, it is necessaryuse current stabilization. Emitter and collector stabilization is ideal for this. It should be noted that such a circuit cannot invert the incoming signal, does not amplify the voltage, for this reason it is called the "emitter follower". Such circuits have a very high stability of parameters, the depth of the DC feedback (feedback) is almost 100%.
Common emitter
Amp stages with a common emitter have a very high gain. It is with the use of such circuit solutions that high-frequency amplifiers are built, used in modern technology - GSM, GPS systems, in wireless Wi-Fi networks. A quadripole (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 power cascades with a common emitter, it is desirable to use bipolar sources. But if this is not possible, the use of unipolar sources is allowed, only it is unlikely to achieve high power.
