The best sound amplifier circuit. How does an audio amplifier work?
Amateur radio amplifier unit
Basic specifications power amplifier:
Rated output power, W, ........................2x25 (2x60)
Power band, kHz.................................................... 0.02 ...150(100)
Nominal input voltage, V.................................... 1(1)
Harmonic coefficient, %, at frequency, kHz:
1 .............................................................................. 0,1(0,1)
2 ............................................................................ 0,14(0,55)
10 ............................................................................ 0,2(0,9)
20 ............................................................................. 0,35(1,58)
Intermodulation distortion coefficient, %,......... 0.3(0.47)
Input impedance, kOhm................................................... .150
Quiescent current of the output stage, mA............................................. 50(50 )
The signal voltage amplification stage is made on op-amp A1. As can be seen from the diagram, part of the output signal is supplied to its power supply circuit through the circuit R6C3C4R4R5 (together with the zener diodes V6, V7, the elements of this circuit, except for the resistor R6, provide stabilization and filtering of the supply voltages). As a result, the voltages at the power terminals of the op-amp at the maximum signal are shifted (relative to the common wire) in the corresponding direction and the range of the output signal of the op-amp increases significantly. The large common-mode signals that arise at the inputs of the op-amp do not pose a danger, since the op-amp suppresses them well (typical value of the attenuation coefficient is 70...90 dB). When applying a signal to the inverting input, the stabilized supply voltages should not exceed +-28 V, on inverting inputs - a value equal to (11in + 28 V), where 11in is the amplitude of the input signal. In any case, the unused input must be connected to the common wire. The K140UD8A op-amp in power amplifiers can be replaced with K140UD8B, K140UD6, K140UD10, K140UD11, K544UD1. The worst results are obtained by using the K140UD7 OU. It is not at all recommended to use OU K140UD1B, K140UD2A, K140UD2B, K153UD1. Instead of KS518A zener diodes, you can use D814A, D814B zener diodes connected in series with a total stabilization voltage of about 18V.
High quality ULF
The amplifier described below is suitable for amplifying powerful audio signals as part of high-end audio reproduction installations, as well as for use as a powerful wideband operational amplifier.
Main technical characteristics of the amplifier:
Rated output power, W, with load resistance,
Ohm: 8................................................... ...........................................48
4..........................................................................................60
Reproducible frequency range with frequency response unevenness of no more than 0.5 dB and output power of 2 W, Hz...................10...200000
THD at rated power
in the range 20...20000 Hz, %.................................... ............0.05
Rated input voltage, V................................... 0.8
Input impedance, kOhm................................................... .......47
Output impedance, Ohm................................................... ....0.02
The input stage of the amplifier consists of two differential amplifiers (connected in parallel), made on transistors VT1, VT3 and VT2, VT4 of opposite structure. Current generators on transistors VT5, VT6 ensure stability of the values (about 1 mA) of the total emitter currents of differential pairs, as well as decoupling of power circuits. The signal to the output amplifier is supplied from controlled current generators (VT7, VT7), which operate in antiphase. This inclusion doubled the drive current, reduced nonlinear distortion and improved the frequency properties of the amplifier as a whole. Each of the arms of the symmetrical output amplifier is made according to the Darlington circuit, and is a three-stage amplifier (in two stages the transistors are connected according to a circuit with a common emitter and in one - with a common collector). The amplifier is covered by a frequency-dependent feedback loop, which determines its voltage transfer coefficient, which in the audio range is close to three. Since the signal feedback, taken from resistor R39 (R40), is proportional to changes in the current of the output transistor, then in addition a rather strict stabilization of the operating point of this transistor is carried out. The bias voltage of the output stage is determined by the resistance of the collector-emitter junction of transistor VT9 and is regulated by resistor R24. The bias voltage is thermally stabilized by a VD4 diode, which is mounted on the heat sink of one of the powerful transistors.
Correction elements R16, C4, C6 - C11 ensure the stability of the amplifier and level its frequency response. Passive filter low frequencies R2C1 prevents RF signals from entering the input. Chain C12R45L1R47 compensates for the reactive component of the load resistance. A unit for protecting output transistors from overcurrent and voltage overloads is assembled on transistors VT12 and VT13. Resistor R1 allows, if necessary, to limit the output power in accordance with the signal level from the preamplifier and the capabilities of the speaker used.
The amplifier can also use other low-power high-frequency silicon transistors, for example KT342A, KT342B and KT313B, KT315 and KT361 (with indices from B to E). Transistors VT14 and VT15 (possible replacement - KT816V, KT816G and KT817V, KT817G or KT626V and KT904A) are equipped with finned heat sinks measuring 23x25x12 mm. As output transistors, you can use KT818GM and KT819GM transistors, which allow you to receive powers of over 70 W when the supply voltage is increased. Zener diode VD1 can also be D816G or 2S536A, VD2 and VD3 - KS147A (with appropriate correction of the resistances of resistors R11 and R14).
AF power amplifier
Rated (maximum) power, W.................... 60(80)
Nominal frequency range, Hz................................ 20...20000
Harmonic coefficient in the nominal frequency range, % 0.03
Nominal input voltage, V...................................0.775
Output resistance, Ohm, no more.................................... 0.08
Output voltage slew rate, V/µs....... 40
The main voltage gain is provided by a cascade based on high-speed op-amp DA1. The pre-terminal stage of the amplifier is assembled using transistors VT1 - VT4. Unlike the prototype, the described amplifier has an output emitter follower made on transistors VT5, VT6 operating in mode “B”. Temperature stability is achieved by including relatively higher resistance resistors R19, R20 in the collector circuits of transistors VT3, VT4. Each arm of the pre-terminal stage is covered by a local feedback loop with a depth of at least 20 dB. The OOS voltage is removed from the collector loads of transistors VT3, VT4 and through dividers R11R14 and R12R15 is supplied to the emitter circuits of transistors VT1, VT2. Frequency correction and stability in the OOS circuit is provided by capacitors SYU, C11. Resistors R13, R16 and R19, R20 limit the maximum currents of the pre-final and final stages of the amplifier during a short circuit of the load. For any overload, the maximum current of transistors VT5, VT6 does not exceed 3.5...4 A, and in this case they do not overheat, since fuses FU1 and FU2 have time to burn and turn off the power to the amplifier.
The reduction in harmonic distortion was achieved by introducing a deep (at least 70 dB) general feedback loop, the voltage of which is removed from the output of the amplifier and fed through the divider C3C5R3R4 to the inverting input of the op-amp DA1. Capacitor C5 adjusts the frequency response of the amplifier through the OOS circuit. The R1C1 circuit connected at the amplifier input limits its bandwidth to 160 kHz. The maximum possible linearization of the frequency response frequency in the range of 10...200 Hz was achieved by appropriate selection of the capacitance of capacitors C1, SZ, C4.
Instead of those indicated in the diagram, you can use op-amps K574UD1A, K574UD1V and transistors of the same types as in the diagram, but with the indices G, D (VT1, VT2) and B (VT3 - VT6).
UMZCH with output stage on field-effect transistors
Main technical characteristics:
Nominal (maximum) output power, W. 45(65)
Harmonic coefficient, %, no more,.................................... 0.01
Nominal input voltage, mV................................. 775
Nominal frequency range, Hz, ......................... 20...100000
Output voltage slew rate, V/µs, ................60
Signal-to-noise ratio, dB.................................................... ......... 100
The input stage of the amplifier is made using op-amp DA1. To increase the amplitude of the output voltage, the UMZCH output transistors are controlled via the op-amp power circuits. The output signal is taken from the positive power supply terminal DA1 and, through the transistor VT1 connected according to the OB circuit, is supplied to one of the inputs of the differential cascade on transistors VT2, VT4. Its second input receives a stabilized voltage from a divider formed by diodes VD2 - VD5 and resistor R13.
The described amplifier does not require any special measures to protect the output transistors from short circuits in the load, since the maximum voltage between source and gate is only twice the same voltage in quiescent mode and corresponds to a current through the output transistor of approximately 9 A. This current the transistors used reliably withstand the time required to blow the fuses and disconnect the UMZCH from the power source.
Coil L1 is wound in one layer on a toroidal frame with an outer diameter of 20, an inner diameter of 10 and a height of 10 mm and contains 28 turns of PEV-2 1.0 wire.
In UMZCH it is advisable to use the KR544UD2A op amp, as the most broadband domestic op amp with internal frequency correction. Transistors KT3108A are interchangeable with KT313A, KT313B, and KP912B with KP912A and KP913, KP920A. 
High quality power amplifier
When designing the amplifier described below, the Kvod-405 amplifier was taken as a basis, successfully combining high technical characteristics and circuit simplicity. Structural scheme The amplifier mainly remained unchanged, only devices for protecting the transistors of the output stage from overload were excluded. Practice has shown that devices of this kind do not completely eliminate transistor failures, but often introduce nonlinear distortions at maximum output power. The transistor current can be limited in other ways, for example, by using overcurrent protection in voltage stabilizers. At the same time, it seems advisable to protect loudspeakers in the event of failure of the amplifier or power supplies. To improve the symmetry of the amplifier, the output stage is made on a complementary pair of transistors, and to reduce nonlinear step-type distortions, diodes VD5, VD6 are included between the bases of transistors VT9, VT10. This ensures fairly reliable closing of the output stage transistors in the absence of a signal. Slightly modified input circuit. The non-inverting input of op-amp DA1 was used as a signal signal, which made it possible to increase the input resistance of the amplifier (it is determined by the resistance of resistor R1 and is equal to 100 kOhm.) It should be noted, however, that even in the non-inverting version, the stability of the amplifier remains high. To prevent clicks in the speakers caused by transient processes when turning on the power, as well as to protect the speakers from DC voltage when the amplifier or power supplies fail, a simple, well-proven device (VT6 - VT8) is used, used in the industrial amplifier "Brig - 001" ". When this device is triggered, one of the lamps HL1, HL2 lights up, signaling the presence of a DC voltage of one polarity or another at the output of the amplifier. Basically, the circuit of the described amplifier does not differ from the circuit of the “Kvod -405” amplifier. The coils are wound with PEV-2 1.0 wire on frames with a diameter of 10 mm and contain: L1 and L3 - 50 turns each (inductance - 5...7 μH), L2 - 30 turns (3 μH).
Instead of those indicated in the diagram, the amplifier can use op-amps K574UD1B, K574UD1V, K544UD2, and also (with some deterioration in the parameters) K544UD1 and K140UD8A - K140UD8V; transistors KT312V, KT373A(VT2), KT3107B, KT3107I, KT313B, KT361V, KT361K (VT1, VT3, VT4), KT315V (VT6, VT8), KT801A, KT801B (VT7). Each of the KT825G transistors can be replaced with composite transistors KT814V, KT814G+KT818V, KT818G, and KT827A with composite transistors KT815V, KT815G+KT819V, KT819G. Diodes VD3 - VD6, VD11, VD12 - any silicon with a maximum forward current of at least 100 mA, VD7 - VD10 - the same, but with maximum current not less than 50 mA. In the absence of KS515A zener diodes, it is permissible to use D814A, D814B or KS175A zener diodes connected in series.
Maximum output power, W, into a 4 Ohm load..... 2x70
Nominal input voltage, V.................................... 0.2
Upper limit of the frequency range, kHz.................................... 50
Output voltage slew rate, V/µs......5.5
Signal-to-noise ratio (unweighted), dB.................................... 80
Harmonic coefficient, %, no more, ..............................................0, 05
Amplifier with multi-loop feedback
Main technical characteristics:
Nominal frequency range, Hz, .................................... 20...20000
Nominal load resistance, Ohm.................................... 4
Nominal (maximum) output. power, W, with load resistance, Ohm:
4 .................................................................................. 70(100)
8 ........................................................................................40(60)
Frequency range, Hz, ................................................... ........ 5...100000
Output voltage rise rate, V/µs, no less... 15 Harmonic coefficient, %, no more, at frequency, Hz:
20...5000 .................................................................................. 0,001
10000 ................................................................................ 0,003
20000 ................................................................................. 0,01
Harmonic coefficient, %, no more, ....................................... 0.01
Rated input voltage, V.................................................... 1
Input impedance, kOhm, min..................................................... 47
The first stage is assembled on an operational amplifier (op-amp) DA1, the rest - on transistors (the second and third - on VT1, VT3, respectively, the fourth - on VT8, VT11 and VT10, VT12, the fifth - on VT13, VT14). In the fourth (pre-final) stage, transistors of different structures are used, connected according to a composite emitter follower circuit, which made it possible to introduce local feedback into it and thus increase linearity and reduce output resistance. To reduce transient distortion at high frequencies, the output stage operates in AB mode, and the resistance of the bias circuit resistors (R30, R33) is limited to 15 Ohms. All transistor stages of the amplifier are covered by a local feedback loop with a depth of at least 50 dB. The OOS voltage is removed from the output of the amplifier and fed through the divider R10R12 into the emitter circuit of transistor VT1. Frequency correction and stability in the OOS circuit are provided by capacitor C4. The introduction of local OOS made it possible, even with the most unfavorable combinations of amplification properties of transistors, to limit the harmonic coefficient of this part of the amplifier to 0.2%. The protection device consists of a trigger on transistors VT6, VT7 and a threshold element on transistor VT9. As soon as the current through any of the output transistors exceeds 8...9 A, transistor VT9 opens, and its collector current opens trigger transistors VT6, VT7.
AF power amplifier
The AF amplifier offered to the attention of radio amateurs has very low coefficients of harmonic and intermodulation distortion, it is relatively simple, can withstand short-term short circuits in the load, and does not require remote elements for thermal stabilization of the current of the output stage transistors.
Main technical characteristics:
Maximum power at 4 Ohm load, W.................................... 80
Nominal frequency range, Hz...................20....20000
Harmonic distortion at maximum output power 80 W, %, at frequency:
1 kHz................................................... ................................ 0.002
20..................................................................................... 0,004
Intermodulation distortion coefficient, %...................0.0015
Output voltage slew rate, V/µs...................................40
To increase the input resistance, transistors VT1, VT2 were introduced into the AF amplifier. This facilitated the operation of op-amp DA1 and made it possible to ensure a stable base-emitter voltage of transistors VT3, VT4 when the temperature changes.
Resistor R14 sets the symmetry of the arms of the amplifier's output stage. 
Simple power amplifier
Main technical characteristics:
Input voltage, V................................................... ................1.8
Input impedance, kOhm................................................... .......10
Rated output power, W, .................................... 90
Nominal frequency range, Hz................................ 10...20000
Harmonic coefficient, %, at frequency, Hz:
200 .................................................................................... 0,01
2000 ............................................................................ 0,018
20000 ............................................................................... 0,18
Relative noise level, dB, no more........................... -90
Output voltage slew rate, V/µs.................................. 17
The power amplifier consists of a voltage amplification stage on a high-speed op-amp DA1 and an output cascade on transistors VT1 - VT4. The transistors of the complementary pair of the pre-terminal stage (VT1 - VT2) are connected according to a circuit with a common base, and the final stage (VT3 - VT4) - with a common emitter. This inclusion of powerful composite transistors of the final stage provides signal amplification not only in current, but also in voltage. The symmetry of the arms of the output stage helps to reduce the nonlinear distortions introduced by the amplifier. For the same purpose, it is covered by a common OOS circuit, the voltage of which is removed from the output of the amplifier and, through resistor R3, supplied to the non-inverting input of the op-amp. Capacitors C4, C5, shunt resistors R6, R7 reduce step-type distortion. The R12C6 circuit prevents self-excitation of the amplifier in the region of higher audio frequencies and increases the stability of its operation under the reactive nature of the load. The gain depends on the ratio of the resistances of resistors R2, R3. With the denominations indicated in the diagram, it is equal to 10.
Any unstabilized bipolar source with a voltage of 25...45 V is suitable to power the amplifier. Instead of KT503D transistors, you can use KT503E, instead of KT502D - KT502E. Transistors KT827B and KT825D can be replaced by composite transistors KT817G + KT819GM and KT816G + KT818GM, respectively.
200 W power amplifier with power supply
Main technical characteristics:
Nominal frequency range, Hz.................................... 20...20000
Maximum output power, W, into a 4 Ohm load........ 200
Harmonic coefficient, %, with output power 0.5..150 W at frequency, kHz
1 ..........................................................................................0,1
10 .................................................................................... 0,15
20 .................................................................................... 0,2
Efficiency, %................................................... ............................................... 68
Rated input voltage, V.................................................... 1
Input impedance, kOhm................................................... .. 10
Output voltage slew rate, V/µs.................................... 10
The pre-amplification stage is made on a high-speed op-amp DA1 (K544UD2B), which, along with the necessary voltage gain, ensures stable operation of the amplifier with deep feedback. Feedback resistor R5 and resistor R1 determine the gain of the amplifier. The output stage is made using transistors VT1 - VT8. Zener diodes VD1, VD2 stabilize the op-amp supply voltage, which is simultaneously used to create the required bias voltage of the output stage. Capacitors C4, C5 are corrective. As the capacitance of capacitor C5 increases, the stability of the amplifier increases, but at the same time nonlinear distortion increases, especially at higher audio frequencies. The amplifier remains operational when the supply voltage is reduced to 25 V.
A conventional bipolar power supply can be used as a power source, circuit diagram which Powerful composite transistors VT7 and VT8, connected according to the emitter follower circuit, provide fairly good filtering of supply voltage ripples with the mains frequency and stabilization of the output voltage thanks to the zener diodes VD5 - VD10 installed in the transistor base circuit. Elements L1, L2, R16, R17, C11, C12 eliminate the possibility of high-frequency generation. Resistors R7, R12 of the power supply represent a segment copper wire PEL, PEV-1 or PELSHO with a diameter of 0.33 and a length of 150 mm, wound on the body of the MLT-1 resistor. The power transformer is made on a toroidal magnetic core made of electrical steel E320, 0.35 mm thick, strip width 40 mm, internal diameter of the magnetic core 80, outer diameter 130 mm. The network winding contains 700 turns of PELSHO 0.47 wire, the secondary winding contains 2x130 turns of PELSHO 1.2 mm wire.
Instead of the K544UD2B op-amp, you can use K544UD2A, K140UD11 or K574UD1. Each of the KT825G transistors can be replaced with composite transistors KT814G, KT818G, and KT827A with composite transistors KT815G, KT819G. Diodes VD3 - VD6 UMZCH can be replaced with any high-frequency silicon diodes, VD7, VD8 - with any silicon diodes with a maximum forward current of at least 100 mA. Instead of KS515A zener diodes, you can use D814A (B, C, D, D) and KS512A zener diodes connected in series.
BP
Today it is no longer considered fashionable to solder various shiny parts on a homemade circuit board, as it was twenty years ago. However, in our cities there are still amateur radio circles, and specialized magazines are published in offline and online modes.
Why has interest in radio electronics dropped sharply? The fact is that in modern stores everything that is required is sold, and there is no longer any need to study something or look for ways to acquire it.
But not everything is as simple as we would like. There are excellent speakers with active amplifiers and subwoofers on sale, wonderful imported stereo systems and multi-channel mixers with a wide range of capabilities, but low-power amplifiers are completely absent. As a rule, they are used to connect instruments at home, so as not to destroy the psyche of neighbors. It’s quite expensive to buy a device as part of a powerful device; the rational solution would be the following: put in a little effort and create homemade amplifier without outside help. Fortunately, today this is possible, and the Internet will be happy to help with this.
Amplifier "assembled on the knee" 
Attitude towards yourself assembled devices today is somewhat negative, and the expression “collect on your knees” is overly negative. But let’s not listen to the envious people, but let’s immediately turn to the first stage. 
First you need to select a scheme. Homemade type ULF can be made using transistors or a microcircuit. The first option is highly not recommended for beginning radio amateurs, as it will clutter the board and make repairing the device more difficult. It is best to replace a dozen transistors with one monolithic chip. This homemade amplifier will please the eye, it will be compact, and it will take a little time to assemble.
Today, the most popular and reliable chip is the TDA2005 type. It is already a two-channel ULF in itself; you just need to organize the power supply and supply the input and output signals. Such a simple homemade amplifier will cost no more than a hundred rubles, along with other parts and wires.
The output power of the TDA2005 ranges from 2 to 6 watts. This is enough to listen to music at home. The list of parts used, their parameters and, in fact, the diagram itself is shown below.
When the device is assembled, it is recommended to screw a small aluminum screen to the chip. This way, when heated, the heat will be better dissipated.
This homemade amplifier is powered by 12 volts. To implement it, purchase a small power supply or electrical adapter with the ability to switch output voltage values. The device current is no more than 2 amperes.
You can connect speakers with a power of up to 100 watts to such a ULF amplifier. The amplifier input can be supplied with a signal from mobile phone, DVD player or computer. At the output, the signal is received through a standard headphone jack.
Thus, we figured out how to assemble an amplifier in a short time for little money. A rational decision by practical people!
Good afternoon dear habrauser, I want to tell you about the basics of building amplifiers audio frequency. I think this article will be interesting to you if you have never worked in radio electronics, and of course it will be funny to those who never part with a soldering iron. And so I will try to talk about this topic as simply as possible and, unfortunately, omitting some of the nuances.An audio amplifier or low frequency amplifier, in order to understand how it works and why there are so many transistors, resistors and capacitors, you need to understand how each element works and try to find out how these elements are arranged. In order to assemble a primitive amplifier, we will need three types of electronic elements: resistors, capacitors and, of course, transistors.
Resistor
So, our resistors are characterized by resistance to electric current and this resistance is measured in Ohms. Each electrically conductive metal or metal alloy has its own resistivity. If we take a wire of a certain length with a high resistivity, then we will get a real wirewound resistor. To make the resistor compact, the wire can be wound around the frame. This way we get a wirewound resistor, but it has a number of disadvantages, so resistors are usually made of metal-ceramic material. This is how resistors are designated electrical diagrams:The upper version of the designation is adopted in the USA, the lower one in Russia and Europe.
Capacitor
The capacitor consists of two metal plates separated by a dielectric. If we apply to these plates constant pressure, then an electric field will appear, which, after turning off the power, will maintain positive and negative charges on the plates, respectively.
The basis of the capacitor design is two conductive plates, between which there is a dielectric
In this way, the capacitor is able to accumulate electrical charge. This ability to accumulate electrical charge is called electrical capacitance, which is the main parameter of a capacitor. Electrical capacitance is measured in Farads. What is also characteristic is that when we charge or discharge a capacitor, it goes through electricity. But as soon as the capacitor is charged, it stops passing electric current, and this is because the capacitor has accepted the charge of the power source, that is, the potential of the capacitor and the power source are the same, and if there is no potential difference (voltage), there is no electric current. Thus, a charged capacitor does not allow direct electric current to pass through, but does alternating current, since when you connect it to alternating electric current, it will be constantly charged and discharged. On electrical diagrams it is designated as follows:
Transistor
In our amplifier we will use the simplest bipolar transistors. A transistor is made from a semiconductor material. The material property we need is the presence of free carriers of both positive and negative charges. Depending on which charges are greater, semiconductors are divided into two types according to conductivity: n-type and p-type (n-negative, p-positive). Negative charges are electrons released from outer shells atoms of the crystal lattice, and the positive ones are the so-called holes. Holes are vacancies, remaining in electron shells after electrons leave them. Conventionally, we denote atoms with an electron in the outer orbit by a blue circle with a minus sign, and atoms with a vacant place by an empty circle:
Each bipolar transistor consists of three zones of such semiconductors, these zones are called base, emitter and collector.
Let's look at an example of how a transistor works. To do this, connect two 1.5 and 5 volt batteries to the transistor, with the plus to the emitter and the minus to the base and collector, respectively (see figure):
An electromagnetic field will appear at the contact between the base and emitter, which literally pulls out electrons from the outer orbit of the base atoms and transfers them to the emitter. Free electrons leave behind holes and occupy vacant places already in the emitter. This same electromagnetic field has the same effect on the atoms of the collector, and since the base in the transistor is quite thin relative to the emitter and collector, the collector electrons quite easily pass through it to the emitter, and in much greater quantities than from the base.
If we turn off the voltage from the base, then there will be no electromagnetic field, and the base will act as a dielectric, and the transistor will be closed. Thus, by applying a sufficiently low voltage to the base, we can control the higher voltage applied to the emitter and collector.
The transistor we considered pnp-type, since he has two p-zones and one n-zone. There are also npn-transistors, the principle of operation in them is the same, but the electric current flows in them in the opposite direction than in the transistor we considered. Like this bipolar transistors are indicated on electrical diagrams, the arrow indicates the direction of the current: 
ULF
Well, let's try to design a low-frequency amplifier from all this. First, we need a signal that we will amplify; it can be a computer sound card or any other audio device with a linear output. Let's say our signal with a maximum amplitude of approximately 0.5 volts at a current of 0.2 A, something like this:
And in order for the simplest 4-ohm 10-watt speaker to work, we need to increase the signal amplitude to 6 volts, at current strength I = U / R= 6 / 4 = 1.5 A.
So, let's try to connect our signal to a transistor. Remember our circuit with a transistor and two batteries, now instead of a 1.5 volt battery we have a line output signal. Resistor R1 acts as a load so that there is no short circuit and our transistor did not burn out.
But here two problems arise at once, firstly our transistor npn-type, and opens only when the half-wave value is positive, and closes when the half-wave is negative.
Secondly, a transistor, like any semiconductor device, has nonlinear characteristics in terms of voltage and current, and the lower the current and voltage values, the stronger these distortions:
Not only is there only a half-wave left of our signal, but it will also be distorted:
This is the so-called step-type distortion.
To get rid of these problems, we need to shift our signal to work area transistor, where the entire sinusoid of the signal will fit and nonlinear distortions will be negligible. To do this, a bias voltage, say 1 volt, is applied to the base using a voltage divider made up of two resistors R2 and R3.
And our signal entering the transistor will look like this:
Now we need to remove our useful signal from the collector of the transistor. To do this, install capacitor C1:
As we remember, a capacitor allows alternating current to pass through and does not allow direct current to pass through, so it will serve as a filter that passes only our useful signal - our sine wave. And the constant component that has not passed through the capacitor will be dissipated by resistor R1. Alternating current, our useful signal, will tend to pass through the capacitor, so the resistance of the capacitor for it is negligible compared to resistor R1.
This is the first transistor stage of our amplifier. But there are two more small nuances:
We don’t know 100% what signal is entering the amplifier, what if the signal source is faulty, anything can happen, again static electricity or a constant voltage passes along with the useful signal. This may cause the transistor to not work properly or even cause it to break. To do this, we will install capacitor C2; like capacitor C1, it will block direct electric current, and the limited capacitance of the capacitor will not allow large amplitude peaks to pass through, which can damage the transistor. These power surges usually occur when the device is turned on or off.
And the second nuance is that any signal source requires a certain specific load (resistance). Therefore, the input impedance of the cascade is important for us. To adjust the input resistance, add resistor R4 to the emitter circuit:
Now we know the purpose of each resistor and capacitor in the transistor stage. Let's now try to calculate what element values need to be used for it.
Initial data:
- U= 12 V - supply voltage;
- U bae~ 1 V - Emitter-base voltage of the transistor operating point;
- Pmax= 200 mW - maximum power dissipation;
- Imax= 100 mA - maximum D.C. collector;
- Umax= 18 V - maximum permissible voltage collector-base / collector-emitter (We have a supply voltage of 12 V, so there is enough to spare);
- U eb= 5 V - maximum permissible emitter-base voltage (our voltage is 1 volt ± 0.5 volt);
- h21= 75-225 - base current amplification factor, the minimum value is accepted - 75;
- We calculate the maximum static power of the transistor, it is taken 20% less than the maximum power dissipation, so that our transistor does not work at the limit of its capabilities:
P st.max = 0,8*Pmax= 0.8 * 200mW = 160 mW;
- Let's determine the collector current in static mode (without a signal), despite the fact that no voltage is supplied to the base through the transistor, electric current still flows to a small extent.
I k0 =P st.max / U ke, Where U ke- collector-emitter junction voltage. Half of the supply voltage is dissipated on the transistor, the second half will be dissipated on resistors:
U ke = U / 2;
I k0 = P st.max / (U/ 2) = 160 mW / (12V / 2) = 26.7 mA;
- Now let's calculate the load resistance, initially we had one resistor R1, which performed this role, but since we added resistor R4 to increase the input resistance of the cascade, now the load resistance will be the sum of R1 and R4:
R n = R1 + R4, Where R n- total load resistance;
The ratio between R1 and R4 is usually taken to be 1 to 10:
R1 =R4*10;
Let's calculate the load resistance:
R1 + R4 = (U / 2) / I k0= (12V / 2) / 26.7 mA = (12V / 2) / 0.0267 A = 224.7 Ohm;
The closest resistor values are 200 and 27 Ohms. R1= 200 Ohm, a R4= 27 Ohm.
- Now let’s find the voltage at the collector of the transistor without a signal:
U k0 = (U ke0 + I k0 * R4) = (U - I k0 * R1) = (12V -0.0267 A * 200 Ohm) = 6.7 V;
- Transistor control base current:
I b = I to / h21, Where I to- collector current;
I to = (U / R n);
I b = (U / R n) / h21= (12V / (200 Ohm + 27 Ohm)) / 75 = 0.0007 A = 0.07 mA;
- The total base current is determined by the base bias voltage, which is set by the divider R2 And R3. The current set by the divider should be 5-10 times greater than the base control current ( I b), so that the base control current itself does not affect the bias voltage. Thus, for the current divider value ( I cases) we accept 0.7 mA and calculate R2 And R3:
R2 + R3 = U / I cases= 12V / 0.007 = 1714.3 Ohm
- Now let's calculate the voltage at the emitter at rest of the transistor ( U e):
U e = I k0 * R4= 0.0267 A * 27 Ohm = 0.72 V
Yes, I k0 collector quiescent current, but the same current also passes through the emitter, so I k0 is considered the quiescent current of the entire transistor.
- We calculate the total voltage at the base ( U b) taking into account the bias voltage ( U cm= 1V):
U b = U e + U cm= 0.72 + 1 = 1.72 V
Now, using the voltage divider formula, we find the resistor values R2 And R3:
R3 = (R2 + R3) * U b / U= 1714.3 Ohm * 1.72 V / 12 V = 245.7 Ohm;
The closest resistor value is 250 ohms;
R2 = (R2 + R3) - R3= 1714.3 Ohm - 250 Ohm = 1464.3 Ohm;
We select the resistor value in the direction of decrease, the closest R2= 1.3 kOhm.
- Capacitors C1 And C2 Usually set to at least 5 µF. The capacitance is chosen such that the capacitor does not have time to recharge.
Conclusion
At the output of the cascade, we receive a proportionally amplified signal in both current and voltage, that is, in power. But one stage is not enough for us to achieve the required gain, so we will have to add the next and the next... And so on.The considered calculation is quite superficial and such an amplification circuit, of course, is not used in the construction of amplifiers; we must not forget about the range of transmitted frequencies, distortion and much more.