The simplest low-frequency amplifiers using transistors. A simple transistor amplifier of class “A Unch” on 2 transistors of different conductivity circuit
Readers! Remember this author's nickname and never repeat his schemes.
Moderators! Before you ban me for insulting me, think that you “allowed an ordinary gopnik to the microphone, who should not even be allowed close to radio engineering and, especially, to teaching beginners.
Firstly, with such a connection scheme, a large direct current will flow through the transistor and speaker, even if the variable resistor is in the desired position, that is, music will be heard. And with a large current, the speaker is damaged, that is, sooner or later, it will burn out.
Secondly, in this circuit there must be a current limiter, that is, a constant resistor, at least 1 KOhm, connected in series with an alternating one. Any homemade product will turn the variable resistor knob all the way, it will have zero resistance and a large current will flow to the base of the transistor. As a result, the transistor or speaker will burn out.
A variable capacitor at the input is needed to protect the sound source (the author should explain this, because there was immediately a reader who removed it just like that, considering himself smarter than the author). Without it, only those players that already have such protection at the output will work normally. And if it is not there, then the player’s output may be damaged, especially, as I said above, if you turn the variable resistor “to zero”. In this case, the output of the expensive laptop will be supplied with voltage from the power source of this cheap trinket and it may burn out. Homemade people love to remove protective resistors and capacitors, because “it works!” As a result, the circuit may work with one sound source, but not with another, and even an expensive phone or laptop can be damaged.
The variable resistor in this circuit should only be tuning, that is, it should be adjusted once and closed in the housing, and not brought out with a convenient handle. This is not a volume control, but a distortion control, that is, it selects the operating mode of the transistor so that there is minimal distortion and so that no smoke comes out of the speaker. Therefore, it should under no circumstances be accessible from the outside. You CANNOT adjust the volume by changing the mode. This is something to kill for. If you really want to adjust the volume, it’s easier to connect another variable resistor in series with the capacitor and now it can be output to the amplifier body.
In general, for the simplest circuits - and to make it work right away and not to damage anything, you need to buy a TDA type microcircuit (for example TDA7052, TDA7056... there are many examples on the Internet), and the author took a random transistor that was lying around in his desk. As a result, gullible amateurs will look for just such a transistor, although its gain is only 15, and the permissible current is as much as 8 amperes (it will burn out any speaker without even noticing).
After mastering the basics of electronics, the novice radio amateur is ready to solder his first electronic designs. Audio power amplifiers are typically the most repeatable designs. There are quite a lot of schemes, each with its own parameters and design. This article will discuss several simple and fully working amplifier circuits that can be successfully repeated by any radio amateur. The article does not use complex terms and calculations; everything is simplified as much as possible so that no additional questions arise.
Let's start with a more powerful circuit.
So, the first circuit is made on the well-known TDA2003 microcircuit. This is a mono amplifier with an output power of up to 7 watts into a 4 ohm load. I want to say that the standard circuit for connecting this microcircuit contains a small number of components, but a couple of years ago I came up with a different circuit on this microcircuit. In this circuit, the number of components is reduced to a minimum, but the amplifier has not lost its sound parameters. After developing this circuit, I began making all my amplifiers for low-power speakers using this circuit.
The circuit of the presented amplifier has a wide range of reproducible frequencies, a supply voltage range from 4.5 to 18 volts (typical 12-14 volts). The microcircuit is installed on a small heat sink, since the maximum power reaches up to 10 Watts.
The microcircuit is capable of operating at a load of 2 ohms, which means that 2 heads with a resistance of 4 ohms can be connected to the amplifier output.
The input capacitor can be replaced with any other one, with a capacity from 0.01 to 4.7 μF (preferably from 0.1 to 0.47 μF), you can use both film and ceramic capacitors. It is advisable not to replace all other components.
Volume control from 10 to 47 kOhm.
The output power of the microcircuit allows it to be used in low-power speakers for PCs. It is very convenient to use the chip for stand-alone speakers for a mobile phone, etc.
The amplifier works immediately after switching on and does not require additional adjustment. It is recommended to additionally connect the power supply minus to the heat sink. It is advisable to use all electrolytic capacitors at 25 Volts.
The second circuit is assembled using low-power transistors and is more suitable as a headphone amplifier.
This is probably the highest quality circuit of its kind, the sound is clear, you can feel the entire frequency spectrum. With good headphones, it feels like you have a full-fledged subwoofer.
The amplifier is assembled with only 3 reverse conduction transistors; as the cheapest option, transistors of the KT315 series were used, but their choice is quite wide.
The amplifier can operate at a low-impedance load, up to 4 ohms, which makes it possible to use the circuit to amplify the signal of a player, radio, etc. A 9-volt Krona battery is used as a power source.
The final stage also uses KT315 transistors. To increase the output power, you can use KT815 transistors, but then you will have to increase the supply voltage to 12 volts. In this case, the amplifier power will reach up to 1 Watt. The output capacitor can have a capacity from 220 to 2200 µF.
The transistors in this circuit do not heat up, therefore, no cooling is needed. If you use larger output transistors, you may need small heat sinks for each transistor.
And finally - the third scheme. An equally simple, but proven version of the amplifier structure is presented. The amplifier is capable of operating from reduced voltage to 5 volts, in which case the PA output power will be no more than 0.5 W, and the maximum power with a 12 volt supply reaches up to 2 Watts.
The output stage of the amplifier is built on a domestic complementary pair. The amplifier is regulated by selecting resistor R2. To do this, it is advisable to use a 1 kOhm trimmer. Slowly rotate the regulator until the quiescent current of the output stage is 2-5 mA.
The amplifier does not have high input sensitivity, so it is advisable to use a pre-amplifier before the input.
The diode plays a significant role in the circuit; it is here to stabilize the mode of the output stage.
The output stage transistors can be replaced with any complementary pair of corresponding parameters, for example KT816/817. The amplifier can power low-power stand-alone speakers with a load resistance of 6-8 ohms.
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad | |
|---|---|---|---|---|---|---|---|
| Amplifier on TDA2003 chip | |||||||
| Audio amplifier | TDA2003 | 1 | To notepad | ||||
| C1 | 47 uF x 25V | 1 | To notepad | ||||
| C2 | Capacitor | 100 nF | 1 | Film | To notepad | ||
| C3 | Electrolytic capacitor | 1 uF x 25V | 1 | To notepad | |||
| C5 | Electrolytic capacitor | 470 uF x 16V | 1 | To notepad | |||
| R1 | Resistor | 100 Ohm | 1 | To notepad | |||
| R2 | Variable resistor | 50 kOhm | 1 | From 10 kOhm to 50 kOhm | To notepad | ||
| Ls1 | Dynamic head | 2-4 Ohm | 1 | To notepad | |||
| Transistor amplifier circuit No. 2 | |||||||
| VT1-VT3 | Bipolar transistor | KT315A | 3 | To notepad | |||
| C1 | Electrolytic capacitor | 1 uF x 16V | 1 | To notepad | |||
| C2, C3 | Electrolytic capacitor | 1000 uF x 16V | 2 | To notepad | |||
| R1, R2 | Resistor | 100 kOhm | 2 | To notepad | |||
| R3 | Resistor | 47 kOhm | 1 | To notepad | |||
| R4 | Resistor | 1 kOhm | 1 | To notepad | |||
| R5 | Variable resistor | 50 kOhm | 1 | To notepad | |||
| R6 | Resistor | 3 kOhm | 1 | To notepad | |||
| Dynamic head | 2-4 Ohm | 1 | To notepad | ||||
| Transistor amplifier circuit No. 3 | |||||||
| VT2 | Bipolar transistor | KT315A | 1 | To notepad | |||
| VT3 | Bipolar transistor | KT361A | 1 | To notepad | |||
| VT4 | Bipolar transistor | KT815A | 1 | To notepad | |||
| VT5 | Bipolar transistor | KT816A | 1 | To notepad | |||
| VD1 | Diode | D18 | 1 | Or any low power | To notepad | ||
| C1, C2, C5 | Electrolytic capacitor | 10 uF x 16V | 3 | ||||
The editors of the “Two Schemes” website present a simple but high-quality low-frequency amplifier based on MOSFET transistors. His circuit should be well known to radio amateurs and audiophiles, since it is already about 20 years old. The circuit was developed by the famous Anthony Holton, which is why it is sometimes called ULF Holton. The sound amplification system has low harmonic distortion, not exceeding 0.1%, with a load power of about 100 watts.
This amplifier is an alternative to the popular amplifiers of the TDA series and similar pop ones, because at a slightly higher cost you can get an amplifier with clearly better characteristics.
The big advantage of the system is its simple design and output stage, consisting of 2 inexpensive MOS transistors. The amplifier can work with speakers with impedance of both 4 and 8 ohms. The only adjustment that needs to be made during startup is to set the quiescent current value of the output transistors.
Schematic diagram of UMZCH Holton
Holton amplifier on MOSFET - circuit diagram The circuit is a classic two-stage amplifier; it consists of a differential input amplifier and a symmetrical power amplifier, in which one pair of power transistors operates. The system diagram is shown above.
PCB
ULF printed circuit board - finished view Here is an archive with PDF files of the printed circuit board - .
Amplifier operating principle
Transistors T4 (BC546) and T5 (BC546) operate in a differential amplifier configuration and are designed to be powered by a current source built on the basis of transistors T7 (BC546), T10 (BC546) and resistors R18 (22 kohm), R20 (680 Ohm) and R12 (22 rooms). The input signal is fed to two filters: a low-pass filter, built from elements R6 (470 Ohm) and C6 (1 nf) - it limits the high-frequency components of the signal and a bandpass filter, consisting of C5 (1 μF), R6 and R10 (47 kohm), limiting signal components at infra-low frequencies.
The load of the differential amplifier is resistors R2 (4.7 kΩ) and R3 (4.7 kΩ). Transistors T1 (MJE350) and T2 (MJE350) represent another amplification stage, and its load is transistors T8 (MJE340), T9 (MJE340) and T6 (BD139).
Capacitors C3 (33 pf) and C4 (33 pf) counteract the excitation of the amplifier. Capacitor C8 (10 nf) connected in parallel with R13 (10 kom/1 V) improves the transient response of the ULF, which is important for rapidly rising input signals.
Transistor T6, together with elements R9 (4.7 ohms), R15 (680 Ohms), R16 (82 Ohms) and PR1 (5 ohms), allows you to set the correct polarity of the amplifier output stages at rest. Using a potentiometer, it is necessary to set the quiescent current of the output transistors within 90-110 mA, which corresponds to a voltage drop across R8 (0.22 Ohm/5 W) and R17 (0.22 Ohm/5 W) within 20-25 mV. The total current consumption in idle mode of the amplifier should be around 130 mA.
The output elements of the amplifier are MOSFETs T3 (IRFP240) and T11 (IRFP9240). These transistors are installed as a voltage follower with a large maximum output current, so the first 2 stages must drive a sufficiently large amplitude for the output signal.
Resistors R8 and R17 were used mainly for quickly measuring the quiescent current of power amplifier transistors without interfering with the circuit. They may also be useful in case of expanding the system with another pair of power transistors, due to differences in the resistance of the open channels of the transistors.
Resistors R5 (470 Ohm) and R19 (470 Ohm) limit the charging rate of the pass transistor capacitance, and, therefore, limit the frequency range of the amplifier. Diodes D1-D2 (BZX85-C12V) protect powerful transistors. With them, the voltage at startup relative to the power supplies of the transistors should not be more than 12 V.
The amplifier board provides space for power filter capacitors C2 (4700 µF/50 V) and C13 (4700 µF/50 V).
Homemade transistor ULF on MOSFET The control is powered through an additional RC filter built on elements R1 (100 Ohm/1 V), C1 (220 μF/50 V) and R23 (100 Ohm/1 V) and C12 (220 μF/50 V).
Power supply for UMZCH
The amplifier circuit provides power that reaches a real 100 W (effective sine wave), with an input voltage of around 600 mV and a load resistance of 4 ohms.
Holton amplifier on a board with details The recommended transformer is a 200 W toroid with a voltage of 2x24 V. After rectification and smoothing, you should get bipolar power supply to the power amplifiers in the region of +/-33 Volts. The design presented here is a mono amplifier module with very good parameters, built on MOSFET transistors, which can be used as a separate unit or as part of.
Now on the Internet you can find a huge number of circuits of various amplifiers on microcircuits, mainly the TDA series. They have quite good characteristics, good efficiency and are not that expensive, which is why they are so popular. However, against their background, transistor amplifiers, which, although difficult to set up, are no less interesting, remain undeservedly forgotten.
Amplifier circuit
In this article we will look at the process of assembling a very unusual amplifier, operating in class “A” and containing only 4 transistors. This scheme was developed back in 1969 by the English engineer John Linsley Hood; despite its old age, it remains relevant to this day.Unlike amplifiers on microcircuits, transistor amplifiers require careful tuning and selection of transistors. This scheme is no exception, although it looks extremely simple. Transistor VT1 – input, PNP structure. You can experiment with various low-power PNP transistors, including germanium ones, for example, MP42. Transistors such as 2N3906, BC212, BC546, KT361 have proven themselves well in this circuit as VT1. Transistor VT2 - NPN structures, medium or low power, KT801, KT630, KT602, 2N697, BD139, 2SC5707, 2SD2165 are suitable here. Particular attention should be paid to the output transistors VT3 and VT4, or rather, their gain. KT805, 2SC5200, 2N3055, 2SC5198 are well suited here. You need to select two identical transistors with the gain as close as possible, and it should be more than 120. If the gain of the output transistors is less than 120, then you need to put a transistor with a high gain (300 or more) in the driver stage (VT2).
Selection of amplifier ratings
Some ratings in the diagram are selected based on the circuit supply voltage and load resistance; some possible options are shown in the table:
It is not recommended to increase the supply voltage above 40 volts; the output transistors may fail. A feature of class A amplifiers is a large quiescent current, and, consequently, strong heating of the transistors. With a supply voltage of, for example, 20 volts and a quiescent current of 1.5 amperes, the amplifier consumes 30 watts, regardless of whether a signal is supplied to its input or not. At the same time, 15 watts of heat will be dissipated on each of the output transistors, and this is the power of a small soldering iron! Therefore, transistors VT3 and VT4 need to be installed on a large radiator using thermal paste.
This amplifier is prone to self-excitation, so a Zobel circuit is installed at its output: a 10 Ohm resistor and a 100 nF capacitor connected in series between ground and the common point of the output transistors (this circuit is shown as a dotted line in the diagram).
When you first turn on the amplifier, you need to turn on an ammeter to monitor the quiescent current. Until the output transistors warm up to operating temperature, it may float a little, this is quite normal. Also, when you turn it on for the first time, you need to measure the voltage between the common point of the output transistors (collector VT4 and emitter VT3) and ground, there should be half the supply voltage there. If the voltage differs up or down, you need to twist the trimming resistor R2.
Amplifier board:
(downloads: 523)
The board is made using the LUT method.
Amplifier I built
A few words about capacitors, input and output. The capacitance of the input capacitor in the diagram is indicated as 0.1 µF, but such a capacitance is not enough. A film capacitor with a capacity of 0.68 - 1 μF should be used as the input, otherwise an unwanted cutoff of low frequencies is possible. The output capacitor C5 should be set to a voltage no less than the supply voltage; you should also not be greedy with the capacitance.
The advantage of the circuit of this amplifier is that it does not pose a danger to the speakers of the acoustic system, because the speaker is connected through a coupling capacitor (C5), this means that if a constant voltage appears at the output, for example, when the amplifier fails, the speaker will remain intact, After all, the capacitor will not allow DC voltage to pass through.