Low-frequency amplifier (hereinafter referred to as ULF) is an electronic device designed to amplify low-frequency oscillations to the one that the consumer needs. They can be performed on various electronic elements such as different types of transistors, tubes 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. The circuitry of low frequency amplifiers will also be considered.
ULF application
ULF is most often used in sound reproduction equipment, because in this field of technology it is often necessary to amplify the signal frequency to that which the human body can perceive (from 20 Hz to 20 kHz).
Other ULF applications:
- measuring technology;
- defectoscopy;
- analog computing.
In general, bass amplifiers are found as components of various electronic circuits, such as radios, acoustic devices, televisions, or radio transmitters.
Parameters
The most important parameter for an amplifier is the gain. It is calculated as the ratio of the output to the input. Depending on the value under consideration, they distinguish:
- current gain=output current / input current;
- voltage gain=output voltage / input voltage;
- power gain=output power / input power.
For some devices, like op-amps, the value of this coefficient is very large, but it is inconvenient to work with too large (as well as with too small) numbers in calculations, so gains are often expressed in logarithmic units. The following formulas apply for this:
- power gain in logarithmic units=10logarithm of the desired power gain;
- current gain in logarithmic units=20decimal logarithm of desired current gain;
- voltage gain in logarithmic units=20logarithm of the desired voltage gain.
Coefficients calculated in this way are measured in decibels. Abbreviated name - dB.
The next important parameteramplifier - 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 signal, for example, in shape.
Ideal amplifiers don't exist, so distortion is always present. True, in some cases they do not go beyond the permissible limits, while in others they do. If the harmonics of the signals at the output of the amplifier coincide with the harmonics of the input signals, then the distortion is linear and is reduced only to a change in amplitude and phase. If new harmonics appear at the output, then the distortion is non-linear, because it leads to a change in the signal shape.
In other words, if the distortion is linear and there was an “a” signal at the input of the amplifier, then the output will be an “A” signal, and if it is non-linear, then the output will be a “B” signal.
The final important parameter that characterizes the operation of the amplifier is the output power. Power varieties:
- Rated.
- Passport noise.
- Maximum short-term.
- Maximum long-term.
All four types are standardized by various GOSTs and standards.
Vamplifiers
Historically, the first amplifiers were created on vacuum tubes, which belong to the class of vacuum devices.
Depending on the electrodes located inside the hermetic flask, the lamps are distinguished:
- diodes;
- triodes;
- tetrodes;
- pentodes.
Maximumthe number of electrodes is eight. There are also such electrovacuum devices as klystrons.
Triode amplifier
First of all, it is worth understanding the switching scheme. A description of the low-frequency triode amplifier circuit is given below.
The filament that heats the cathode is energized. Voltage is also applied to the anode. Under the action of temperature, electrons are knocked out from the cathode, which rush to the anode, to which a positive potential is applied (electrons have a negative potential).
Part of the electrons is intercepted by the third electrode - the grid, to which voltage is also applied, only alternating. With the help of the grid, the anode current (the current in the circuit as a whole) is regulated. 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 a directed movement of electrons, and the grid blocks this movement.
The lamp gain adjusts the resistor that is connected between the power supply 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 current and voltage (and hence power) will be amplified in the low-frequency amplifier circuit.
The triode amplifier output signal is taken from the area between the anode and the resistor connected in front of it.
Amplifier onklystron
The principle of operation of a low-frequency klystron amplifier is based on signal modulation first in speed and then in density.
The klystron is arranged as follows: the flask has a cathode heated by a filament, and a collector (analogous to the anode). Between them are the input and output resonators. Electrons emitted from the cathode are accelerated by a voltage applied to the cathode and rush to the collector.
Some electrons will move faster, others slower - this is how velocity modulation looks like. Due to the difference in the speed of movement, electrons are grouped into beams - this is how density modulation manifests itself. The density modulated signal enters the output resonator, where it creates a signal of the same frequency, but greater power than the input resonator.
It turns out that the kinetic energy of electrons is converted into the energy of microwave oscillations of the electromagnetic field of the output resonator. This is how the signal is amplified in the klystron.
Features of electrovacuum amplifiers
If we compare the quality of the same signal amplified by a tube device and ULF on transistors, the difference will be visible to the naked eye not in favor of the latter.
Any professional musician will tell you that tube amps are way better than their advanced counterparts.
Electrovacuum devices have long gone out of mass consumption, they were replaced by transistors and microcircuits, but this is irrelevant for the field of sound reproduction. Due to the temperature stability and vacuum inside, lamp devices amplify the signal better.
The only drawback of the tube ULF is the high price, which is logical: it is expensive to produce elements that are not in mass demand.
Bipolar transistor amplifier
Often amplifying stages are assembled using transistors. A simple low-frequency amplifier can be assembled from just three basic elements: a capacitor, a resistor, and an n-p-n transistor.
To assemble such an amplifier, you will need to ground the emitter of the transistor, connect a capacitor in series to its base, and a resistor in parallel. 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 allowable 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 must be at least 100 times the load resistance. The value of the capacitor can vary from 1 to 100 microfarads. Its capacitance affects the amount of frequency at which the amplifier can operate. The larger the capacitance, the lower the frequency rating that the transistor can amplify.
The input signal of the low frequency bipolar transistor amplifier is applied 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, you can connect a parallel-connected capacitor and resistor to the emitter, which play the role of negative feedback.
Amplifier with two bipolar transistors
To increase the gain, you can connect two single ULF transistors into one. Then the gains of these devices can be multiplied.
Although if you continue to increase the number of amplifying 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 n-channel FET (ITF) amplifier will be considered as an example.
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 FET (you can also use a parallel connection of a capacitor and a resistor, as described above). A limiting resistor and power are connected to the drain, and a load terminal is created between the resistor and the drain.
The input signal to low-frequency field-effect transistor amplifiers is applied to the gate. This is also done through a capacitor.
As you can see from the explanation, the simplest field-effect transistor amplifier circuit is no different from the low-frequency bipolar transistor amplifier circuit.
However, when working with PT, the following features of these elements should be taken into account:
- FET high Rinput=I / Ugate-source. Field-effect transistors are controlled by an electric field,which is generated by stress. Therefore, FETs are controlled by voltage, not current.
- FETs consume almost no current, which entails a slight distortion of the original signal.
- There is no charge injection in field-effect transistors, so the noise level of these elements is very low.
- They are temperature resistant.
The main disadvantage of FETs is their high sensitivity to static electricity.
Many people are familiar with the situation when seemingly non-conductive things shock a person. This is the manifestation of static electricity. If such an impulse is applied to one of the contacts of the field-effect transistor, the element can be disabled.
Thus, when working with the PT, it is better not to take the contacts with your hands so as not to accidentally damage the element.
OpAmp device
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, it is its amplifying properties that are of interest for working with low frequencies.
To make a signal amplifier out of an op amp, you need to correctly connect a feedback circuit to it, which is a regular resistor. How to understand where to connect this circuit? To do this, you need to refer to the transfer characteristic of the op-amp. It has two horizontal and one linear section. If the operating pointdevice is located on one of the horizontal sections, then the op-amp operates in generator mode (pulse mode), if it is located on a linear section, then the op-amp amplifies the signal.
To transfer the op-amp to linear mode, you need to connect the feedback resistor with one contact to the output of the device, and the other - to the inverting input. This inclusion is called negative feedback (NFB).
If it is required that the low frequency signal be amplified and not change in phase, then the inverting input with OOS should be grounded, and the amplified signal should be applied 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.
In this case, we must not forget that the operational amplifier must be supplied with power of opposite polarities. For this, he has special contact leads.
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 terms of the materials from which they are made, in order to achieve the desired gain parameters.
Amplifier on a chip
ULF can be assembled on electrovacuum elements, and on transistors, and on operational amplifiers, only vacuum tubes are the last century, and the rest of the circuits are not without flaws, the correction of which inevitably entails complicating the design of the amplifier. This is inconvenient.
Engineers have long found a more convenient option for creating ULF: the industry produces ready-made 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 ULF series in the form of integrated circuits:
- TDA7057Q.
- K174UN7.
- TDA1518BQ.
- TDA2050.
All of the above series are used in audio equipment. Each model has different characteristics: supply voltage, output power, gain.
They are made in the form of small elements with many pins, which are convenient to place on the board and mount.
To work with a low-frequency amplifier on a microcircuit, it is useful to know the basics of logic algebra, as well as the principles of operation of logical elements AND-NOT, OR-NOT.
Almost any electronic device 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 ULF function seems to be the most convenient practical option.
Scheme improvement
The above was an example of how you can improve the amplified signal when working with bipolar and field-effect transistors (by connecting a capacitor and resistor in parallel).
Such structural upgrades can be made with almost any scheme. Of course, the introduction of new elements increasesvoltage drop (losses), but thanks 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 collect the simplest filters that filter out frequencies that should not fall into the circuit. By combining resistive and capacitive elements with operational amplifiers, more efficient filters (integrators, Sallen-Key differentiators, notch and bandpass filters) can be assembled.
In conclusion
The most important parameters of frequency amplifiers are:
- gain;
- signal distortion factor;
- power output.
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 schemes 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 the amp designer has to deal with.
Using all the recommendations given in this article, you can build your own amplifier for home useinstead of buying this device, which can cost a lot of money when it comes to high quality devices.
