An audio amplifier is a critical component of many electronic devices. This could be the playback of music files, fire and security alarm systems, or sound sensors for various toys. Appliances equipped with built-in low-frequency channels, but for home design electronic homemade products You may need to make this device yourself.

DIY transistor audio amplifier circuit

The range of sound frequencies that are perceived by the human ear is in the range of 20 Hz-20 kHz, but a device made on a single semiconductor device, due to the simplicity of the circuit and the minimum number of parts, provides a narrower frequency band. In simple devices, it is enough to listen to music frequency range 100 Hz-6,000 Hz. This is enough to play music on a miniature speaker or earphone. The quality will be average, but for mobile device quite acceptable.

The circuit of a simple audio amplifier using transistors can be assembled using silicon or germanium products of direct or reverse conductivity (p-n-p, n-p-n). Silicon semiconductors are less critical to supply voltage and have less dependence of characteristics on junction temperature.

Audio amplifier circuit with 1 transistor

The simplest audio amplifier circuit using a single transistor includes the following elements:

• Transistor KT 315 B
• Resistor R1 – 16 kom
• Resistor R2 – 1.6 kom
• Resistor R3 – 150 ohm
• Resistor R4 – 15 ohm
• Capacitor C1 – 10.0 µF
• Capacitor C2 – 500.0 uF

This is a device with a fixed base bias voltage, which is set by the divider R1-R2. The collector circuit includes resistor R3, which is the load of the cascade. Between pin X2 and the plus of the power source, you can connect a miniature speaker or earphone, which should have a high resistance. A low-impedance load cannot be connected to the cascade output. A correctly assembled circuit starts working immediately and does not need to be configured.

A higher quality ULF can be assembled using two devices.

The amplifier circuit with two transistors includes more components, but can work with low level input signal, since the first element performs the function of a preliminary stage.

An alternating audio frequency signal is supplied to potentiometer R1, which plays the role of a volume control. Next, through an isolation capacitor, the signal is supplied to the base of the first stage element, where it is amplified to a value that ensures normal operation of the second stage. The collector circuit of the second semiconductor includes a sound source, which can be a small-sized earphone. The bias on the bases is set by resistors R2 and R4. In addition to KT 315, any low-power silicon semiconductors can be used in the audio amplifier circuit with two transistors, but depending on the type of products used, selection of bias resistors may be required.

If you use a push-pull output you can achieve a good volume level and good frequency response. This circuit is made on three common KT 315 silicon devices, but other semiconductors can be used in the device. The big advantage of the circuit is that it can operate on a low-impedance load. Miniature speakers with a resistance of 4 to 8 ohms can be used as a sound source.

The device can be used in conjunction with a player, tuner or other household appliance. The supply voltage of 9 V can be obtained from a Krona battery. If you use KT 815 in the output stage, then at a 4 ohm load you can get up to 1 watt of power. In this case, the supply voltage will need to be increased to 12 volts, and the output elements will need to be mounted on small aluminum heat sinks.

It is almost impossible to obtain good electrical characteristics in an amplifier assembled on a single semiconductor, therefore quality devices gather at several semiconductor devices. Such designs produce tens and hundreds of watts at low-impedance loads and are intended for operation in Hi-Fi complexes. When choosing a device, the question may arise as to which transistors can be used to make a sound amplifier. These can be any silicon or germanium semiconductors. ULFs assembled on field-effect semiconductors have become widespread. For low-power devices with low-voltage power supply, you can use silicon products KT 312, KT 315, KT 361, KT 342 or germanium products of the old MP 39-MP 42 series.

A do-it-yourself power amplifier using transistors can be made using the complementary pair KT 818B-KT 819B. This design will require a pre-block, an input stage and a pre-final block. The pre-assembly includes signal level control and tone control for high and low frequencies or a multi-band equalizer. The voltage at the output of the preliminary unit must be at least 0.5 volts. The input node of the power block can be assembled on a high-speed operational amplifier. In order to swing the final part, you will need a pre-terminal cascade, which is assembled on a complementary pair of medium-power devices KT 816-KT 817. The designs of powerful low-frequency amplifiers are distinguished by complex circuitry and big amount components. To properly adjust and configure such a unit, you will need not only a tester, but an oscilloscope and an audio frequency generator.

The circuits of low-frequency amplifiers differ little from each other, except in the capacity of the capacitors used. Despite the fact that usually a low-frequency amplifier has at least a couple of stages, to gain experience you can try to assemble a simple amplifier with just one transistor (and, accordingly, with one cascade).

The single-stage amplifier circuit proposed below is extremely simple, and can be equally well executed using either wall-mounted (based on conventional wires and leads) or printed circuit (based on printed wires, electrically conductive strips) installation.

Fig. 1: Single-stage transistor circuit

Circuits for assembling transistors have a number of symbols:

• R1 (2, 3, 4...) – resistors;
• C1 (2, 3, 4...) – capacitors;
• B1 (2, 3, 4...) – speaker, telephone, etc.;
• T1 (2, 3, 4…) – .

The feasibility of assembling a single-stage amplifier is justified solely by the need to obtain experimental experience, and its practical application will demonstrate sufficient low quality sound similar to that observed in modern Chinese technology.

To assemble a simple amplifier you will need a number of parts:

• Transistor KT 817 (or similar);
• 5 kOhm resistor, 0.25 Watt;
• Film capacitor 0.22 - 1 microfarad;
• A speaker delivering a load of 4-8 Ohms (1 - 3 Watts);
• 9 Volt power supply;
• Signal source (1 channel and ground).

The value of the bias resistor R1 reaches tens of kOhms and is determined experimentally. The fact is that this indicator is calculated taking into account the supply voltage of the device, the resistance of the telephone capsule, and the transmission coefficient characteristic of the selected type of transistor. The starting point can be a load resistance increased by at least a hundred times.

The capacitor (in the diagram is designated as C1) and the level of its capacitance varies in the range from 1 to 100 microfarads, with increasing capacitance the device gains the ability. The purpose of a capacitor (also called a decoupling capacitor) is to pass alternating current and filter out direct current, preventing the circuit from shorting out.

For this circuit, it is appropriate to use a bipolar transistor with an n-p-n structure and a power of average and high level. It is advisable to use a film capacitor. The received signal can be received through the output of the MP3 player. The device assembled according to this scheme can be equipped with a potentiometer (50,000 Ohms), which allows you to adjust the volume.

If not in the power supply electrolytic capacitor with a large capacity, you will need to install an electrolyte of 1000 - 2200 microfarads, which has an operating voltage greater than in the circuit.

Anyone who has no experience working with electronics should know that when soldering, components can very easily overheat. To prevent this from happening, it is best to use 25 Watt soldering irons, and you need to stop soldering after every 10 seconds of continuous exposure.

Compared to the given circuit of a single-stage low-frequency amplifier, a two-stage one has much best characteristics, but its assembly is not much more difficult. To construct it you only need to connect two simple cascades in series. However, they can be used different kinds connections, which, of course, affect the quality and characteristics of signal transmission. But in the simplest version, you can simply connect the output of the first stage to the input of the second stage directly or through a resistor. A connection of this type is respectively called direct or resistor. The degree of signal amplification in this case is equal to the multiplied gain factors of each of the stages. Unfortunately, a subsequent increase in the number of stages in the amplifier does not give a similar effect. The problem is that the gain value is determined in a complex manner and depends quite strongly on the time delay, that is, the phase change.

Modern modifications of low-frequency amplifiers, usually listed in magazines for radio amateurs, are designed to reduce the level of nonlinear distortion and increase output power, as well as modify other parameters in order to increase the efficiency of the device.

But at the same time, if the task is to establish the operation of certain devices, as well as solve some controversial issues experimentally, it may be necessary simplest option amplifier, assembled in literally a quarter of an hour. The main requirement for such a device will be a minimum number of scarce components, as well as the ability to operate with a wide range of voltage and resistance levels.

When operating a low-frequency amplifier, do not forget that its performance is highly dependent on temperature conditions, especially for home-made devices.

Write comments, additions to the article, maybe I missed something. Take a look at, I will be glad if you find something else useful on mine.

Low frequency amplifier (LF) is integral part most radio devices such as TVs, players, radios and various household appliances. Let's consider two simple circuits two-stage ULF on.

The first version of ULF on transistors

In the first version, the amplifier is built on silicon npn transistors conductivity. The input signal comes through variable resistor R1, which in turn is load resistance for the signal source circuit. connected to the collector circuit of the amplifier transistor VT2.

Setting up the amplifier of the first option comes down to selecting resistances R2 and R4. The resistance value must be selected such that the milliammeter connected to the collector circuit of each transistor shows a current in the region of 0.5...0.8 mA. According to the second scheme, it is also necessary to set the collector current of the second transistor by selecting the resistance of resistor R3.

In the first option, it is possible to use transistors of the KT312 brand, or their foreign analogues, however, it will be necessary to set the correct voltage bias of the transistors by selecting resistances R2, R4. In the second option, in turn, it is possible to use silicon transistors of the KT209, KT361 brands, or foreign analogues. In this case, you can set the operating modes of the transistors by changing the resistance R3.

Instead of headphones, it is possible to connect a high-impedance speaker to the collector circuit of transistor VT2 (both amplifiers). If you need to get more powerful sound amplification, you can assemble an amplifier that provides amplification of up to 15 W.

Scheme No. 1

Selecting an amplifier class . Let us immediately warn the radio amateur - we will not make a class A amplifier using transistors. The reason is simple - as stated in the introduction, the transistor amplifies not only the useful signal, but also the bias applied to it. Simply put, it amplifies direct current. This current, together with the useful signal, will flow through the acoustic system (AS), and speakers, unfortunately, are able to reproduce this direct current. They do this in the most obvious way - by pushing or pulling the diffuser from its normal position to an unnatural one.

Try to press the speaker cone with your finger - and you will see what a nightmare the sound produced will turn into. Direct current in its action successfully replaces your fingers, so it is absolutely contraindicated for a dynamic head. You can separate direct current from an alternating signal by only two means - a transformer or a capacitor - and both options, as they say, are worse than the other.

Schematic diagram

The circuit of the first amplifier that we will assemble is shown in Fig. 11.18.

This is an amplifier with feedback, the output stage of which operates in mode B. The only advantage of this circuit is its simplicity, as well as the uniformity of the output transistors (no special complementary pairs are required). However, it is quite widely used in low-power amplifiers. Another advantage of the scheme is that it does not require any configuration, and if the parts are in good working order, it will work immediately, and this is very important for us now.

Let's consider the operation of this circuit. The amplified signal is supplied to the base of transistor VT1. The signal amplified by this transistor from resistor R4 is supplied to the base of the composite transistor VT2, VT4, and from it to resistor R5.

Transistor VT3 is turned on in emitter follower mode. It amplifies the positive half-waves of the signal on resistor R5 and supplies them through capacitor C4 to the speaker.

The negative half-waves are enhanced by the composite transistor VT2, VT4. In this case, the voltage drop across diode VD1 closes transistor VT3. The signal from the amplifier output is fed to the feedback circuit divider R3, R6, and from it to the emitter of the input transistor VT1. Thus, transistor VT1 plays the role of a comparison device in the feedback circuit.

It amplifies direct current with a gain equal to unity (because the resistance of capacitor C to direct current is theoretically infinite), and the useful signal with a gain equal to the ratio R6/R3.

As you can see, the capacitance value of the capacitor is not taken into account in this formula. The frequency from which the capacitor can be neglected in calculations is called the cutoff frequency of the RC circuit. This frequency can be calculated using the formula

F = 1 / (R×C).

For our example, it will be about 18 Hz, i.e. more low frequencies the amplifier will amplify worse than it could.

Pay . The amplifier is assembled on a board made of single-sided fiberglass 1.5 mm thick with dimensions 45×32.5 mm. Wiring printed circuit board V mirror image and the parts layout diagram can be downloaded. You can download a video about the operation of the amplifier in MOV format for viewing. I want to immediately warn the radio amateur - the sound reproduced by the amplifier was recorded in the video using the microphone built into the camera, so, unfortunately, it will not be entirely appropriate to talk about the sound quality! Appearance amplifier is shown in Fig. 11.19.

Element base . When manufacturing an amplifier, transistors VT3, VT4 can be replaced with any transistors designed for a voltage not less than the supply voltage of the amplifier, and a permissible current of at least 2 A. The diode VD1 must also be designed for the same current.

The remaining transistors are any with permissible voltage no less than the supply voltage, and a permissible current of no less than 100 mA. Resistors - any with a permissible power dissipation of at least 0.125 W, capacitors - electrolytic, with a capacitance not less than indicated in the diagram, and an operating voltage less than the supply voltage of the amplifier.

Radiators for amplifier . Before we try to make our second design, let us, dear radio amateur, focus on radiators for the amplifier and present here a very simplified method for calculating them.

First, we calculate the maximum power of the amplifier using the formula:

P = (U × U) / (8 × R), W,

Where U- amplifier supply voltage, V; R- speaker resistance (usually it is 4 or 8 ohms, although there are exceptions).

Secondly, we calculate the power dissipated on the collectors of the transistors using the formula:

P race = 0.25 × P, W.

Thirdly, we calculate the radiator area required to remove the corresponding amount of heat:

S = 20 × P race, cm 2

Fourthly, we select or manufacture a radiator whose surface area will be no less than the calculated one.

This calculation is very approximate, but for amateur radio practice it is usually sufficient. For our amplifier, with a supply voltage of 12 V and an AC resistance of 8 Ohms, the “correct” radiator would be an aluminum plate measuring 2x3 cm and at least 5 mm thick for each transistor. Keep in mind that a thinner plate does not transfer heat well from the transistor to the edges of the plate. I would like to warn you right away - the radiators in all other amplifiers must also be of “normal” sizes. Which ones exactly - count for yourself!

Sound quality . After assembling the circuit, you will find that the sound of the amplifier is not entirely clear.

The reason for this is the “pure” class B mode in the output stage, the characteristic distortions of which even feedback is not able to completely compensate. For the sake of experiment, try replacing transistor VT1 in the circuit with KT3102EM, and transistor VT2 with KT3107L. These transistors have a significantly higher gain than KT315B and KT361B. And you will find that the amp's sound has improved significantly, although some distortion will still be noticeable.

The reason for this is also obvious - a higher gain of the amplifier as a whole provides greater accuracy of the feedback and a greater compensating effect.

Continue reading

A simple transistor amplifier can be a good tool for studying the properties of devices. The circuits and designs are quite simple; you can make the device yourself and check its operation, take measurements of all parameters. Thanks to modern field-effect transistors, it is possible to make a miniature one from literally three elements. microphone amplifier. And connect it to a personal computer to improve sound recording parameters. And the interlocutors during conversations will hear your speech much better and more clearly.

Frequency characteristics

Low (audio) frequency amplifiers are found in almost all household appliances - stereo systems, televisions, radios, tape recorders, and even personal computers. But there are also RF amplifiers based on transistors, lamps and microcircuits. The difference between them is that the ULF allows you to amplify the signal only at the audio frequency that is perceived by the human ear. Transistor audio amplifiers allow you to reproduce signals with frequencies in the range from 20 Hz to 20,000 Hz.

Consequently, even the simplest device can amplify the signal in this range. And it does this as evenly as possible. The gain depends directly on the frequency of the input signal. The graph of these quantities is almost a straight line. If a signal with a frequency outside the range is applied to the amplifier input, the quality of operation and efficiency of the device will quickly decrease. ULF cascades are assembled, as a rule, using transistors operating in the low and mid-frequency ranges.

Classes of operation of audio amplifiers

All amplifying devices are divided into several classes, depending on the degree of current flow through the cascade during the period of operation:

1. Class “A” - current flows non-stop during the entire period of operation of the amplifier stage.
2. In work class "B" current flows for half a period.
3. Class “AB” indicates that current flows through the amplifier stage for a time equal to 50-100% of the period.
4. In "C" mode electricity less than half of the operating time has elapsed.
5. ULF mode “D” has been used in amateur radio practice quite recently - a little over 50 years. In most cases, these devices are implemented based on digital elements and have a very high efficiency - over 90%.

The presence of distortion in various classes of low-frequency amplifiers

The working area of ​​a class “A” transistor amplifier is characterized by fairly small nonlinear distortions. If the incoming signal spits out higher voltage pulses, this causes the transistors to become saturated. In the output signal, higher ones begin to appear near each harmonic (up to 10 or 11). Because of this, a metallic sound appears, characteristic only of transistor amplifiers.

If the power supply is unstable, the output signal will be modeled in amplitude near the network frequency. The sound will become harsher on the left side of the frequency response. But what better stabilization power supply to the amplifier, the more complex the design of the entire device becomes. ULFs operating in class “A” have a relatively low efficiency - less than 20%. The reason is that the transistor is constantly open and current flows through it constantly.

To increase (albeit slightly) efficiency, you can use push-pull circuits. One drawback is that the half-waves of the output signal become asymmetrical. If you transfer from class “A” to “AB”, nonlinear distortions will increase by 3-4 times. But the efficiency of the entire device circuit will still increase. ULF classes “AB” and “B” characterize the increase in distortion as the signal level at the input decreases. But even if you turn up the volume, this will not help completely get rid of the shortcomings.

Work in intermediate classes

Each class has several varieties. For example, there is a class of amplifiers “A+”. In it, the input transistors (low voltage) operate in mode “A”. But high-voltage ones installed in the output stages operate either in “B” or “AB”. Such amplifiers are much more economical than those operating in class “A”. There is a noticeably lower number of nonlinear distortions - no higher than 0.003%. Better results can be achieved using bipolar transistors. The operating principle of amplifiers based on these elements will be discussed below.

But there is still a large number of higher harmonics in the output signal, causing the sound to become characteristically metallic. There are also amplifier circuits operating in class “AA”. In them, nonlinear distortions are even less - up to 0.0005%. But the main drawback of transistor amplifiers still exists - the characteristic metallic sound.

"Alternative" designs

This is not to say that they are alternative, but some specialists involved in the design and assembly of amplifiers for high-quality sound reproduction are increasingly giving preference to tube designs. Tube amplifiers have the following advantages:

1. Very low level of nonlinear distortion in the output signal.
2. There are fewer higher harmonics than in transistor designs.

But there is one huge disadvantage that outweighs all the advantages - you definitely need to install a device for coordination. The fact is that the tube stage has a very high resistance - several thousand Ohms. But the speaker winding resistance is 8 or 4 Ohms. To coordinate them, you need to install a transformer.

Of course, this is not a very big drawback - there are also transistor devices that use transformers to match the output stage and speaker system. Some experts argue that the most effective scheme is a hybrid one - in which single ended amplifiers, not covered by negative feedback. Moreover, all these cascades operate in ULF class “A” mode. In other words, a power amplifier on a transistor is used as a repeater.

Moreover, the efficiency of such devices is quite high - about 50%. But you shouldn’t focus only on efficiency and power indicators - they don’t talk about high quality sound reproduction by amplifier. The linearity of the characteristics and their quality are much more important. Therefore, you need to pay attention primarily to them, and not to power.

Single-ended ULF circuit on a transistor

The simplest amplifier, built according to a common emitter circuit, operates in class “A”. The circuit uses a semiconductor element with an n-p-n structure. A resistance R3 is installed in the collector circuit, limiting the flow of current. The collector circuit is connected to the positive power wire, and the emitter circuit is connected to the negative wire. In the case of using semiconductor transistors with a structure pnp circuit will be exactly the same, you just need to change the polarity.

Using a decoupling capacitor C1, it is possible to separate the alternating input signal from the direct current source. In this case, the capacitor is not an obstacle to the flow of alternating current along the base-emitter path. The internal resistance of the emitter-base junction together with resistors R1 and R2 represent the simplest supply voltage divider. Typically, resistor R2 has a resistance of 1-1.5 kOhm - the most typical values ​​for such circuits. In this case, the supply voltage is divided exactly in half. And if you power the circuit with a voltage of 20 Volts, you can see that the value of the current gain h21 will be 150. It should be noted that HF ​​amplifiers on transistors are made according to similar circuits, only they work a little differently.

In this case, the emitter voltage is 9 V and the drop in the “E-B” section of the circuit is 0.7 V (which is typical for transistors on silicon crystals). If we consider an amplifier based on germanium transistors, then in this case the voltage drop in the “E-B” section will be equal to 0.3 V. The current in the collector circuit will be equal to that flowing in the emitter. You can calculate it by dividing the emitter voltage by the resistance R2 - 9V/1 kOhm = 9 mA. To calculate the value of the base current, you need to divide 9 mA by the gain h21 - 9 mA/150 = 60 μA. ULF designs usually use bipolar transistors. Its operating principle is different from field ones.

On resistor R1, you can now calculate the drop value - this is the difference between the base and supply voltages. In this case, the base voltage can be found using the formula - the sum of the characteristics of the emitter and the “E-B” transition. When powered from a 20 Volt source: 20 - 9.7 = 10.3. From here you can calculate the resistance value R1 = 10.3 V/60 μA = 172 kOhm. The circuit contains capacitance C2, which is necessary to implement a circuit through which the alternating component of the emitter current can pass.

If you do not install capacitor C2, the variable component will be very limited. Because of this, such a transistor-based audio amplifier will have a very low current gain h21. It is necessary to pay attention to the fact that in the above calculations the base and collector currents were assumed to be equal. Moreover, the base current was taken to be the one that flows into the circuit from the emitter. It occurs only if a bias voltage is applied to the base output of the transistor.

But it must be taken into account that collector leakage current absolutely always flows through the base circuit, regardless of the presence of bias. In common emitter circuits, the leakage current is amplified by at least 150 times. But usually this value is taken into account only when calculating amplifiers based on germanium transistors. In the case of using silicon, in which the current of the “K-B” circuit is very small, this value is simply neglected.

Amplifiers based on MOS transistors

The field-effect transistor amplifier shown in the diagram has many analogues. Including using bipolar transistors. Therefore, we can consider, as a similar example, the design of an audio amplifier assembled according to a circuit with a common emitter. The photo shows a circuit made according to a common source circuit. R-C connections are assembled on the input and output circuits so that the device operates in class “A” amplifier mode.

The alternating current from the signal source is separated from the direct supply voltage by capacitor C1. It is imperative that the field-effect transistor amplifier must have a gate potential that will be lower than the same source characteristic. In the diagram shown, the gate is connected to the common wire via resistor R1. Its resistance is very high - resistors of 100-1000 kOhm are usually used in designs. Such a large resistance is chosen so that the input signal is not shunted.

This resistance almost does not allow electric current to pass through, as a result of which the gate potential (in the absence of a signal at the input) is the same as that of the ground. At the source, the potential turns out to be higher than that of the ground, only due to the voltage drop across resistance R2. From this it is clear that the gate has a lower potential than the source. And this is exactly what is required for the normal functioning of the transistor. It is necessary to pay attention to the fact that C2 and R3 in this amplifier circuit have the same purpose as in the design discussed above. And the input signal is shifted relative to the output signal by 180 degrees.

ULF with transformer at the output

You can make such an amplifier with your own hands for home use. It is carried out according to the scheme that works in class “A”. The design is the same as those discussed above - with a common emitter. One feature is that you need to use a transformer for matching. This is a disadvantage of such a transistor-based audio amplifier.

The collector circuit of the transistor is loaded primary winding, which develops an output signal transmitted through the secondary to the speakers. A voltage divider is assembled on resistors R1 and R3, which allows you to select the operating point of the transistor. This circuit supplies bias voltage to the base. All other components have the same purpose as the circuits discussed above.

Push-pull audio amplifier

It cannot be said that this is a simple transistor amplifier, since its operation is a little more complicated than those discussed earlier. In push-pull ULFs, the input signal is split into two half-waves, different in phase. And each of these half-waves is amplified by its own cascade, made on a transistor. After each half-wave has been amplified, both signals are combined and sent to the speakers. Such complex transformations can cause signal distortion, since the dynamic and frequency properties of two transistors, even of the same type, will be different.

As a result, the sound quality at the amplifier output is significantly reduced. When working push-pull amplifier in class “A” it is not possible to reproduce a complex signal with high quality. The reason is that increased current constantly flows through the amplifier's shoulders, the half-waves are asymmetrical, and phase distortions occur. The sound becomes less intelligible, and when heated, signal distortion increases even more, especially at low and ultra-low frequencies.

Transformerless ULF

A transistor-based bass amplifier made using a transformer, despite the fact that the design may have small dimensions, is still imperfect. Transformers are still heavy and bulky, so it's better to get rid of them. A circuit made on complementary semiconductor elements with different types conductivity. Most modern ULFs are made precisely according to such schemes and operate in class “B”.

The two powerful transistors used in the design operate according to an emitter follower circuit (common collector). In this case, the input voltage is transmitted to the output without loss or gain. If there is no signal at the input, then the transistors are on the verge of turning on, but are still turned off. When a harmonic signal is applied to the input, the first transistor opens with a positive half-wave, and the second one is in cutoff mode at this time.

Consequently, only positive half-waves can pass through the load. But the negative ones open the second transistor and completely turn off the first. In this case, only negative half-waves appear in the load. As a result, the power-amplified signal appears at the output of the device. Such an amplifier circuit using transistors is quite effective and can provide stable operation and high-quality sound reproduction.

ULF circuit on one transistor

Having studied all the features described above, you can assemble the amplifier with your own hands using a simple element base. The transistor can be used domestic KT315 or any of its foreign analogues - for example BC107. As a load, you need to use headphones with a resistance of 2000-3000 Ohms. A bias voltage must be applied to the base of the transistor through a 1 MΩ resistor and a 10 μF decoupling capacitor. The circuit can be powered from a source with a voltage of 4.5-9 Volts, a current of 0.3-0.5 A.

If resistance R1 is not connected, then there will be no current in the base and collector. But when connected, the voltage reaches a level of 0.7 V and allows a current of about 4 μA to flow. In this case, the current gain will be about 250. From here you can make a simple calculation of the amplifier using transistors and find out the collector current - it turns out to be equal to 1 mA. Having assembled this transistor amplifier circuit, you can test it. Connect a load to the output - headphones.

Touch the amplifier input with your finger - a characteristic noise should appear. If it is not there, then most likely the structure was assembled incorrectly. Double-check all connections and element ratings. To make the demonstration more clear, connect a sound source to the ULF input - the output from the player or phone. Listen to music and evaluate the sound quality.