Low-frequency amplifier (hereinafter referred to as ULF) is an electronic device designed to amplify low-frequency oscillations to the one that is necessary for the consumer. They can be performed on various electronic elements such as transistors of various types, lamps or operational amplifiers. All ULFs have a number of parameters that characterize the effectiveness of their work.
This article will talk about the use of such a device, its parameters, methods of construction using various electronic components. Also, the circuitry of low-frequency amplifiers will be considered.
ULF application
Most often, ULF is used in equipment for reproducing sound, because in the art it is often necessary to amplify the signal frequency to that which the human body can perceive (from 20 Hz to 20 kHz).
Other applications for ULF:
- measuring technique;
- flaw detection
- analog computing technology.
In general, low-frequency amplifiers are found as components of various electronic circuits, for example, radio receivers, acoustic devices, televisions or radio transmitters.
Options
The most important parameter for an amplifier is the gain. It is calculated as the ratio of the output signal to the input. Depending on the considered value, distinguish:
- current gain = output current / input current;
- voltage gain = output voltage / input voltage;
- power gain = output power / input power.
For some devices such as operational amplifiers, the value of this coefficient is very large, but working with too large (as well as too small) numbers in the calculations is inconvenient, therefore, often the amplification factors are expressed in logarithmic units. For this, the following formulas apply:
- power gain in logarithmic units = 10 * decimal logarithm of the desired power gain;
- current gain in logarithmic units = 20 * decimal logarithm of the desired current gain;
- voltage gain in logarithmic units = 20 * decimal logarithm of the desired voltage gain.
Coefficients calculated in this way are measured in decibels. The abbreviated name is dB.
The next important parameter of the amplifier is the signal distortion coefficient. It is important to understand that signal amplification occurs as a result of its transformations and changes. Not the fact that always these transformations will occur correctly. For this reason, the output signal may differ from the input, for example, in shape.
Ideal amplifiers do not exist, so distortion always takes place. True, in some cases they do not go beyond permissible boundaries, and in others they go beyond. If the harmonics of the signals at the output of the amplifier coincide with the harmonics of the input signals, then the distortions are linear and are reduced only to a change in amplitude and phase. If new harmonics appear at the output, then the distortions are nonlinear, because they lead to a change in the waveform.
Simply put, if the distortion is linear and the signal “a” was at the input of the amplifier, then the signal “A” will be output, and if it is non-linear, then the signal “B” will be output.
The final important parameter characterizing the operation of the amplifier is the output power. Varieties of power:
- Rated.
- Passport noise.
- Maximum short-term.
- Maximum long-term.
All four types are standardized by various GOSTs and standards.
Lamp amplifiers
Historically, the first amplifiers were created on electron tubes, which belong to the class of electric vacuum devices.
Depending on the electrodes located inside the sealed bulb, the following can be distinguished:
- diodes;
- triodes;
- tetrodes;
- pentodes.
The maximum number of electrodes is eight. There are also electrovacuum devices such as klystrons.
Triode Amplifier
To begin with, it is worthwhile to figure out the inclusion circuit. The circuit description of the low frequency amplifier on the triode is given below.
Voltage is applied to the filament, which heats the cathode. Also, voltage is applied to the anode. Electrons are knocked out of the cathode under the influence of temperature, which rush to the anode, which has a positive potential (the potential is negative for electrons).
Part of the electrons is intercepted by the third electrode - the grid, to which voltage is also applied, only alternating. Using the grid, the anode current is regulated (current in the circuit as a whole). If a large negative potential is applied to the grid, all the electrons from the cathode will settle on it, and no current will flow through the lamp, because the current is the directed motion of the electrons, and the grid blocks this movement.
The lamp gain controls the resistor that is connected between the power source and the anode. It sets the desired position of the operating point on the current-voltage characteristic, on which the gain parameters depend.
Why is the position of the operating point so important? Because it depends on how much the current and voltage (therefore, power) in the low-frequency amplifier circuit are amplified.
The output signal on the triode amplifier is removed from the area between the anode and the resistor connected in front of it.
Klystron amplifier
The principle of operation of the low frequency amplifier on the klystron is based on the modulation of the signal first in speed and then in density.
The klystron is arranged as follows: in the flask there is a cathode heated by a filament, and a collector (anode analog). Between them are the input and output resonators. The electrons emitted from the cathode are accelerated by the voltage supplied to the cathode, and rush to the collector.
Some electrons will move faster, others slower - this is how modulation in speed looks. Due to the difference in the speed of motion, the electrons are grouped into beams - this is how density modulation is manifested. The density-modulated signal enters the output resonator, where it produces a signal of the same frequency but with a higher power than the input resonator.
It turns out that the kinetic energy of the electrons is converted into the energy of microwave oscillations of the electromagnetic field of the output resonator. So there is a signal amplification in the klystron.
Features of Vacuum Amplifiers
If we compare the quality of the same signal amplified by a tube device and VLF on transistors, the difference will be visible to the naked eye not in favor of the latter.
Any professional musician will say that tube amplifiers are much better than their advanced counterparts.
Vacuum devices have long gone out of mass consumption, they have been replaced by transistors and microcircuits, but this is irrelevant for the field of sound reproduction. Due to temperature stability and vacuum inside, lamp devices better amplify the signal.
The only drawback of the ULF tube is the high price, which is logical: it is expensive to produce items that are not in high demand.
Bipolar transistor amplifier
Often amplifier stages are assembled using transistors. A simple low-frequency amplifier can be assembled from only three main elements: a capacitor, a resistor and an npn transistor.
To assemble such an amplifier, you need to ground the emitter of the transistor, connect a capacitor to it base in series, and in parallel - a resistor. The load should be placed in front of the collector. It is advisable to connect a limiting resistor to the collector in this circuit.
The permissible supply voltage of such a low-frequency amplifier circuit varies from 3 to 12 volts. The value of the resistor should be chosen experimentally, taking into account the fact that its value should be at least 100 times greater than the load resistance. The capacitor rating can vary from 1 to 100 μF. Its capacity affects the magnitude of the frequency with which the amplifier can work. The larger the capacitance, the lower the frequency rating that the transistor can amplify.
The input signal of the low-frequency amplifier on a bipolar transistor is supplied to the capacitor. The positive power pole must be connected to the connection point of the load and the resistor connected in parallel with the base and the capacitor.
To improve the quality of such a signal, it is possible to connect a parallel-connected capacitor and resistor playing the role of negative feedback to the emitter.
Amplifier on two bipolar transistors
To increase the gain, you can combine two single VLF transistors into one. Then the gains of these devices can be multiplied.
Although if we continue to increase the number of amplifier stages, the chance of self-excitation of amplifiers will increase.
Field effect transistor amplifier
Low-frequency amplifiers are also assembled on field-effect transistors (hereinafter referred to as PT). The circuits of such devices are not much different from those that are assembled on bipolar transistors.
An example will be considered an amplifier on an field-effect transistor with an insulated gate with an n-channel (MIS type).
A capacitor is connected in series to the substrate of this transistor, and a voltage divider is connected in parallel. A resistor is connected to the source of the PT (you can also use a parallel connection of the capacitor and resistor, as described above). A limiting resistor and power are connected to the drain, and a load output is created between the resistor and the drain.
The input signal to the low-frequency amplifiers on field-effect transistors is fed to the gate. This is also done through a capacitor.
As can be seen from the explanation, the circuit of the simplest amplifier on a field-effect transistor is no different from the circuit of a low-frequency amplifier on a bipolar transistor.
However, when working with PT it is worth considering the following features of these elements:
- The PT has a high R input = I / U gate-source . Field effect transistors are controlled by an electric field, which is formed due to voltage. Consequently, PTs are controlled by voltage, not current.
- DTs almost do not consume current, which entails a slight distortion of the original signal.
- Field-effect transistors do not have charge injection, so the noise level of these elements is very low.
- They are resistant to temperature changes.
The main disadvantage of field effect transistors is their high sensitivity to static electricity.
Many people are familiar with the situation when, seemingly, non-conductive things shock a person with an electric current. This is a manifestation of static electricity. If such an impulse is applied to one of the contacts of the field-effect transistor, you can disable the element.
Thus, when working with PTs, it is better not to touch the contacts with your hands, so as not to accidentally damage the element.
Operational amplifier device
An operational amplifier (hereinafter referred to as op amp) is a device with differentiated inputs, which has a very high gain.
Signal amplification is not the only function of this element. It can also work as a signal generator. Nevertheless, for working with low frequencies, its amplifying properties are of interest.
In order to make a signal amplifier from an op-amp, it is necessary to correctly connect a feedback circuit to it, which is an ordinary resistor. How to understand where to connect this circuit? To do this, refer to the transfer characteristic of the op-amp. It has two horizontal and one linear sections. If the operating point of the device is located on one of the horizontal sections, then the op-amp operates in generator mode (pulse mode), if it is on a linear section, then the op-amp amplifies the signal.
To put the op-amp in linear mode, you need to connect a feedback resistor with one contact to the output of the device, and the other to the inverting input. This inclusion is called negative feedback (OOS).
If it is required that the low-frequency signal is amplified and does not change in phase, then the inverting input from the OOS should be grounded, and an amplified signal should be fed to the non-inverting input. If it is necessary to amplify the signal and change its phase by 180 degrees, then the non-inverting input must be grounded, and the input signal must be connected to the inverting one.
At the same time, we must not forget that it is necessary to supply power to the operational amplifier of opposite polarities. To do this, he has special contact pins.
It is important to note that working with such devices is sometimes difficult to select elements for the low frequency amplifier circuit. Their careful coordination is required not only in terms of nominal values, but also in the materials from which they are made in order to achieve the necessary gain parameters.
Microchip amplifier
ULF can be assembled on electro-vacuum elements, and on transistors, and on operational amplifiers, only electronic lamps are the last century, and the rest of the circuits are not without drawbacks, the correction of which inevitably entails the complication of the design of the amplifier. It is not comfortable.
Engineers have long found a more convenient option for creating ULF: the industry produces off-the-shelf microcircuits that act as amplifiers.
Each of these circuits is a set of op-amps, transistors and other elements connected in a certain way.
Examples of some VLF series in the form of integrated circuits:
- TDA7057Q.
- K174UN7.
- TDA1518BQ.
- TDA2050.
All the above series are used in audio equipment. Each of the models has different characteristics: supply voltage, output power, amplification factors.
They are made in the form of small elements with many leads that are convenient to place on the board and mount.
To work with a low-frequency amplifier on a chip, it is useful to know the basics of the algebra of logic, as well as the principles of operation of logical elements AND-NOT, OR-NOT.
Almost any electronic devices can be assembled on logical elements, but in this case many circuits will turn out to be bulky and inconvenient for installation.
Therefore, the use of ready-made integrated circuits that perform the function of ULF seems to be the most convenient practical option.
Circuit improvement
An example was given above of how to improve the amplified signal when working with bipolar and field-effect transistors (by connecting a parallel connection of a capacitor and a resistor).
Similar structural upgrades can be made with almost any circuit. Of course, the introduction of new elements increases the voltage drop (loss), but due to this, the properties of various circuits can be improved. For example, capacitors are excellent frequency filters.
On resistive, capacitive or inductive elements, it is recommended to assemble simple filters that filter out frequencies that should not fall into the circuit. By combining resistive and capacitive elements with operational amplifiers, it is possible to assemble more efficient filters (integrators, differentiators according to the Sullen-Key scheme, notch and bandpass filters).
Finally
The most important parameters of frequency amplifiers are:
- gain;
- signal distortion factor;
- output power.
Low-frequency amplifiers are most often used in audio equipment. You can collect device data practically on the following elements:
- on vacuum tubes;
- on transistors;
- on operational amplifiers;
- on finished chips.
The characteristics of low frequency amplifiers can be improved by introducing resistive, capacitive or inductive elements.
Each of the circuits listed above has its own advantages and disadvantages: some amplifiers are expensive to assemble, some can go into saturation, for some it is difficult to coordinate the elements used. There are always features that a person who designs amplifiers has to reckon with.
Using all the recommendations that are given in this article, you can assemble your own amplifier for home use instead of buying this device, which can cost a lot of money when it comes to high-quality devices.