Bridge amplifier. Bridged Audio Power Amplifier Bridged Amplifier Output
There is a class of amplifiers called bridge amplifiers, in which an ungrounded load is connected to the outputs of the amplifier with anti-phase output signals. The advantages of such circuits include quadrupling the maximum output power at the same supply voltage, compared to power amplifiers with a single output and a grounded load. In addition, such circuits create symmetrical current ripples along the power circuits with a double signal frequency, which simplifies the construction of power supplies (of the corresponding power), eliminating possible conditions for the appearance of distortions in the output bipolar voltages. This is true for UPT-type amplifiers and not only. In addition, bridge amplifiers do not cause high-current signal currents to appear on the "common" wire, which greatly improves the compatibility of nodes in multi-channel (for example, stereo) equipment.
Bridge amplifier circuits are also found in some recommendations for the use of power amplifier circuits. If we disassemble, for example, the datasheet scheme on the TDA2030 by "bones", we get two power amplifier connected in series. The first amplifier is non-inverting, the second is inverting. A load is connected between their outputs. It is clear that the output of the second amplifier will have an increased level of harmonics, since input signal will pass through a chain of two reinforcing links. In addition, the second amplifier will add a time delay for the time the signal passes through it. The resulting disadvantages are obvious.
Symmetrical bridge amplifier circuits with crossovers are known. For example, the circuit from P. Shkritek's book "Reference Guide to Audio Circuitry" (Chapter 13. Power Amplifiers) is good in many respects, except for one - the operating point of power amplifiers is not set by anything. Mentally set a voltage at the outputs of such an amplifier, for example, close to the supply voltage (simultaneously) - and the balance of the circuit will not be disturbed, since it suppresses common-mode noise both at the input of the amplifier and at the output :-), due to its symmetrical topology. A special servo circuit is needed to maintain the operating point of the output stages. AT otherwise on the output arms of the amplifier there will be an imbalance in terms of power dissipation and, in the end, such a device may fail.
In the INUN circuit I proposed, this drawback was eliminated by adding two resistors (R3, R4) between the differential inputs of the amplifiers. Now the common-mode departure of the output voltages from zero will cause a voltage imbalance between the differential. amplifier inputs and return them to the initial state. Otherwise, in terms of topology, the circuits are identical. The advantages of symmetrical bridge circuits include the fact that without alteration they can be used in circuits with both balanced and unbalanced inputs. In addition, symmetrical bridge circuits are characterized by reduced levels of even harmonics. The disadvantages include the need for accurate selection of circuit ratings. The voltage gain of this circuit will be equal to Ku=-R5/(R1+R3/2), input impedance Rin=2*R1+R3/2.
ITUN (Voltage Controlled Current Source) is built on a similar principle. Current sensors (R7, R8) are introduced into the output circuits, and the signals feedback taken from voltage dividers. Thus, when the load is connected, with the appearance of the input signal, the imbalance of the bridge, formed by the described elements, occurs, which is eliminated due to negative feedback. In this case, regardless of the magnitude of the load (theoretically), the current through it will not change, since the balance of the circuit is maintained only when the output current specified by the input signal flows through the resistors - current sensors. The main parameter of the ITUN is the conversion steepness, it can be calculated for this circuit using the formula Si = -R1 / (R7 * R5). For the indicated ratings Si=-4.68 A/V. Rin=R1+R2, neglecting the values of R3 and R4 due to their relative smallness.
INUN and ITUN schemes in MC7 format are available.
Using the same current sensors and replacing the OOS with POS, with the adjusted values of the resistive dividers, you can get an amplifier with a negative output impedance. Those who wish can analyze his work on their own :-)
A power amplifier with a negative output impedance is used in sound engineering in cases where it is necessary to increase the amount of electrical damping, that is, to get rid of the increased quality factor in the speaker, for example, at the speaker resonance frequency. According to the definition of negative output resistance, when the load resistance increases, the voltage across it drops (gain decreases), and when it decreases, it increases (gain increases). This is realized by positive current feedback in the amplifier. In this regard, there is a danger of self-excitation of such an amplifier if the load resistance modulo becomes less than the value of the negative output resistance, since the gain in this case becomes infinite :-).
Without going into details of the derivation of formulas based on Kirchhoff's laws, the symmetry of the circuit (R1=R2, R5=R6, R7=R8, R9=R10), taking into account the fact that R3, R4 - have little effect on the result, the circuit parameters can be calculated by the following formulas:
Rin=R1+R2
Ku=-R5*R9/(R1*(R5-R9)) at no load.
Rout=2*R7*R9/(R9-R5)
For the denominations indicated in the diagram, respectively, we obtain:
Rin = 40 kOhm;
Ku= -39.16 or 31.85 dB
Rout = -4.7 Ohm.
P.S. I must say that the type of microcircuit shown in the example (TDA2050) does not play any decisive role, you can use any microcircuit (or discrete) UN that is suitable in terms of parameters, made according to the circuitry of a powerful op-amp. It is advisable to comply general recommendations datasheet to include one or another type of chip.
For example, on the basis of TDA7293, a bridge ITUN for a subwoofer with EMOS was assembled according to the following scheme:
View of the board from the parts (in PCAD2006) in the following figure:
The printed circuit board can be downloaded in pdf format or in SprintLayout5.0 program format
When assembled it looks like this:
For systems with EMOS, an ITUN with a frequency-dependent frequency response, or rather, with a frequency-dependent impedance, is desirable. As the frequency increases, the output impedance of the PA should fall. An implementation example is a bridge UMZCH with cross connections that implements this principle: due to the introduction of capacitor C8, the circuit acquires the necessary properties. When using TDA2050, the optimal load resistance is 8 ohms.
Below is its printed circuit board (protective diodes are added to the TDA2050 outputs):
And photo assembled amplifier. Note that the layout of the elements is somewhat different from the above printed circuit board. Just in the process of fine-tuning the circuit, one of the elements (it is no longer on the circuit diagram) had to be completely removed.
Transistors for a wide range of applications high power, for example, types KT903 and KT812 with different letter indices can provide the output power of a transformerless cascade up to 100-120 W. A further increase in output power requires the parallel connection of two or three transistors of the same type or the use of forced air cooling of heat sinks. All this complicates the design and operation of amplifiers.
A method of increasing the output power of amplifiers has long been known, which consists in using two identical power amplifiers connected in such a way that the input signal is fed to their inputs in the form of two oscillations equal in amplitude but opposite in sign, and the load is switched on directly between the outputs of the amplifiers. Such amplifiers are called balanced bridge amplifiers. AT different years on the pages of amateur radio magazines of the USSR, the GDR, Poland and other countries, descriptions of such amplifiers appeared, however, for a power of no more than 10 watts.
The figure shows a schematic diagram of a balanced bridge amplifier low frequency power at 250 W with a harmonic distortion coefficient of about 2% in the frequency band from 30 Hz to 16 kHz. The design is based on two identical low-frequency amplifiers (A and B), assembled on bipolar silicon transistors. There is no terminal transistor protection device and no differential input stage. The change in the phase of the input signal to the Gn1 socket is carried out using a phase inverter on a T8 transistor, assembled according to a shared load circuit. The transfer coefficient of such a stage for the collector load is -1, for the emitter load +1. This means that the signal voltages supplied from the outputs of the cascade on the transistor T8 are equal in amplitude, but opposite in sign, which is required for the normal operation of the amplifier in a bridge circuit. The power supply (common for amplifiers A and B) is made according to a full-wave circuit on a step-down transformer Tr1 and two diodes D1, D2. The filter circuit consists of three 2500 uFx100 V electrolytic capacitors connected in parallel. Load resistance 12-15 ohms. The load is connected directly between the outputs of both amplifiers. Repetition of the design is possible when using domestic silicon high voltage transistors type KT626V (T1), KT801A (T3), KT312A (T2), KT802A (T4), KT903A (T6, T7), KT626V (T5). Diodes D1 and D2 must be rated for current up to 10 A, for example, type D242B. All electrolytic capacitors, except for C1, can be for an operating voltage of 60 V. Transformer Tr1 has a Sh50x70 core. The primary winding contains 218 turns of PEV-2 wire with a diameter of 1.1 mm, the secondary - 120 turns with a tap from the middle, PEV-2 wire with a diameter of 1.9 mm.
To ensure the normal operation of the amplifier, transistors T3-T7 must have efficient heat sinks. You can use the simplest plate heat sinks made of blackened sheet duralumin. The dimensions of the heat sinks, as indicated in the primary sources, should be as follows: for T6 and T7 transistors - 3x160x160 mm; for transistors T4 and T5 - 2x60x60 mm; for transistor T2 - 2x15x15 mm. If the KT602A transistor is used as T2, then an additional heat sink is not required.
Setting up the assembled amplifier begins with checking the installation and connections. Then turn on the power and set the operating modes of each of the amplifier channels separately with the signal and load turned off. First with a variable resistor R 9 set the current consumed by channel B to 60 mA. Next, a variable resistor R 5 ensure that the constant voltage at the output of channel B is 30 V. Then similar operations are performed with channel A.
Then turn on the load and measure the DC voltage on it. This voltage is allowed to be no more than ±0.3 V. Otherwise, the variable resistor R5 of channels A and B is again corrected so that the constant voltage at the load returns to normal. And only after that it is possible to test the amplifier with the signal source.
Of course, in most cases of amateur practice output power 250W is not required. But the principle of building low-frequency balanced bridge power amplifiers described above can be useful when creating lower power amplifiers (40-50 W) based on two low-power amplifiers. It is only necessary that both original amplifiers be of the same type, have the same characteristics, and the power supply allows you to get the required power. On average, we can assume that the power of the rectifier and transformer should be at least twice the maximum output power of the amplifier as a whole.
In conclusion, it must be pointed out that the quality of any low-frequency power amplifier largely depends on the source of the amplified signal, the previous control and correction stages, on the electro-acoustic installation itself, in which this amplifier is used, as well as on the power, input impedance and quality of the loudspeaker ( or loudspeakers, if there are several).
One of the main problems that a designer of tube amplifiers faces is the manufacture of output transformers. While the power transformer only has to provide the necessary voltages and currents and can be wound, in a pinch even by hand, the output transformer has a huge impact on the performance of the amplifier. The method of winding the windings, the dimensions of the core, even the thickness of the core plates and the thickness of the spacers between the windings - everything affects such important parameters amplifier like output power, frequency bandwidth and harmonic distortion.
The desire to make the output transformer less critical to the quality of its manufacture or to completely abandon its use has led to the emergence of bridge amplifier circuits in which the DC output lamps are connected in series, and in parallel in the alternating current. Since the output lamps in such a circuit operate in the cathode follower mode, and the constant component on the load is excluded, it becomes possible to match the load resistance using a simple autotransformer with only one winding.
A diagram of such a bridge power amplifier is shown in Fig. 1.
Fig.1. Bridge Power Amplifier Circuit
The input stage on the lamp L1.1 type 6N8S is built according to the scheme with a common cathode and has no features. Its purpose is to provide the necessary level of sensitivity. If the signal source has output voltage not less than 4 V, then the input stage can be excluded and the input signal can be applied directly to the input of the phase inverter.
The phase inverter (lamp L2 tin 6N9S) is built on the basis of a balanced one. Such a phase inverter is characterized by high gain and symmetry of the divided signal. If you want to have a balanced XLR input in the amplifier, which has greater noise immunity compared to the single-ended RCA input, you can remove the capacitor that grounds the second phase inverter input and apply a signal to it.
The output stage is made on two beam tetrodes L3 and L4. 6P6S or 6P3S lamps can be used as output lamps. With the first, the output power will be about 12-13 W, with the second - up to 25 W per channel. You can further increase the output power by using 6P27S lamps, which have a maximum anode voltage of up to 800 V and a much higher anode current. But for this it is necessary to increase the power of the power transformer and change the design of the amplifier.
Due to the parallel connection of lamps on alternating current the optimal load resistance decreases by a factor of 4 and is about 900 ohms for this circuit.
The output autotransformer is wound on a core from a standard TP-208-6 transformer with a cross section of 7.0 cm2. The primary winding has 650 turns of wire with a diameter of 0.33 mm, the secondary - 84, the third - 35 turns of wire with a diameter of 1.0 mm, the fourth - 531 turns of wire with a diameter of 0.33 mm. All windings must be wound in one direction. Their location on the coil is shown in Fig.2.
Fig.2. The location of the windings of the output transformer
The arms of the output stage are powered by separate rectifiers. In the manufacture two-channel amplifier four anode power windings are required, which must be taken into account when selecting a transformer.
The power supply circuit of a two-channel amplifier is shown in Fig. 3.
Fig.3. Power Supply Diagram
The power transformer is wound on a core with a cross section of at least 16 cm2 and has eight windings. The primary winding has 650 turns of wire with a diameter of 0.5 mm; the second, third, fourth and fifth windings each have 700 turns of wire with a diameter of 0.2 mm; filament windings - the sixth and seventh - each have 19 turns of wire with a diameter of 1.0 mm; the eighth winding has 36 turns of wire with a diameter of 0.2 mm and is used to power the anode power on delay device.
The power-on delay device is made according to the scheme in Fig. 4. For a two-channel amplifier, this device must have two relays of the RES22 type. Depending on the operating voltage, the relay windings are connected in parallel or in series.
Fig.4. Anode voltage supply delay circuit
The rectifiers and the power-on delay device are assembled on a common board, the figure of which is shown in Fig. 5.
Fig.5. Power supply circuit board
As you know, the main disadvantage of lamps compared to transistors is the rather low stability of the parameters. So, the resource of most lamps is 500-1000 hours of continuous operation. During this period, the main parameters of the lamp change significantly - the steepness of the characteristic decreases, the output power drops, and the internal resistance changes. This effect is especially unpleasant in push-pull output stages, since changing the parameters of the lamps leads to an imbalance in the shoulders of the push-pull stage, the appearance of direct current through the output transformer and an increase in non-linear distortion. Stabilization of the anode power in this case does not help, since the DC lamp is a resistance and a change internal resistance lamp causes instability of the quiescent current. Most amplifiers are either adjusted only once during manufacture, or have trimmers to set the quiescent current during the life of the amplifier, which requires periodic maintenance with the use of special equipment and some qualification from the user of the tube equipment.
For the described amplifier, I developed a simple device that automatically maintains a given quiescent current of the output lamps. The diagram of this device is shown in Fig.6.
Fig.6. Output lamp quiescent current stabilizer circuit
The device is a current stabilizer and consists of several functional units. Resistor Rdt is a current sensor, which creates a drop voltage proportional to the quiescent current of the lamp. On transistors VT1 and VT2, a low-power reference voltage source is assembled, with the help of which the quiescent current of the lamp is set. This scheme of the reference voltage source is characterized by low current consumption (0.5-0.7 mA), which is important, since the current of the reference voltage source does not pass through the current sensor and, therefore, leads to a small error in setting the quiescent current. If desired, the reference voltage source can be replaced by an LED that will indicate the normal mode of the lamp. In this case, you need to use an LED with a working current of not more than 1 mA. A device for comparing and controlling the current is assembled on a composite transistor VT3VT4. With a decrease in the quiescent current of the lamp, the voltage drop across the resistor of the current sensor Rdt decreases. Since the voltage at the base of the transistor VT3 is stabilized by a reference voltage source, a decrease in the voltage at the emitter VT3 causes the transistors VT3 and VT4 to open, which shunt the resistor Rk and reduce the total resistance in the lamp cathode circuit, thereby increasing its anode current. With an increase in the anode current, transistors VT3 and VT4 close and increase the resistance in the cathode circuit. To eliminate the influence of the variable component of the cathode current on D.C. rest resistor R5 is shunted by a large capacitor C1.
This device is connected to the cathode circuit of the lamp instead of the auto-bias resistor and is powered by the bias voltage. When tested with several lamps of the 6P6S and 6P3S types, such a current stabilizer ensured a constant quiescent current with an accuracy of 2%. For alternating current, this device is shunted by a large capacitor and does not have any effect on the amplification of audio frequencies. For each output lamp, such a current regulator is made on a small printed circuit board and installed in place of the cathode resistor. By setting the output stage quiescent current to 25-30 mA, you can use an amplifier in class A or AB by installing 6P6S or 6P3S lamps in the output stage, respectively. No adjustments are needed when changing bulbs.
All transformers and lamps are mounted directly on the amplifier housing. Transformers are covered with casings, which are also attached to the body. The installation dimensions of the power transformer depend on the design of the transformer itself and therefore are not indicated on the drawing of the amplifier case. All transformers must have holes drilled for wiring. Their size and position are also quite arbitrary. The power supply board is mounted in the basement of the case under power transformer on the screws securing the transformer casing. The amplifier stages are mounted in a hinged way on the terminals of the lamp panels. Additional contact plates made of textolite are fixed on the screws for fastening the lamp panels, on which the contact pads are cut with a cutter.
The procedure for mounting and adjusting the amplifier is the same as for.
Dmitry Klimov
Tube amplifiers. Method of calculation and design
Pretty simple, Even a person who is not very strong in electrical engineering can repeat it. ULF on this chip will be ideal for use as part of a speaker system for home computer, TV, cinema. Its advantage is that it does not require fine tuning and tuning, as is the case with transistor amplifiers. And what can we say about the difference from lamp structures - the dimensions are much smaller.
No high voltage is required to power the anode circuits. Of course, there is heating, as in lamp designs. Therefore, if you plan to use the amplifier for a long time, it is best to install, in addition to an aluminum radiator, at least a small fan for forced airflow. Without it, on the TDA7294 microassembly, the amplifier circuit will work, but there is a high probability of switching to temperature protection.
Why TDA7294?
This chip has been very popular for over 20 years. It has won the trust of radio amateurs, since it has very high characteristics, amplifiers based on it are simple, anyone, even a beginner radio amateur, can repeat the design. The amplifier on the TDA7294 chip (the diagram is given in the article) can be either monophonic or stereophonic. Internal organization IC consists of Amplifier audio frequency, built on this chip, belongs to the class AB.
Advantages of the microcircuit
Benefits of using a microchip for:
1. Very high output power. About 70 W if the load has a resistance of 4 ohms. In this case, the usual scheme for switching on the microcircuit is used.
2. Approximately 120W into 8 ohms (bridged).
3. Very low level extraneous noise, distortion is insignificant, reproducible frequencies lie in the range completely perceived by the human ear - from 20 Hz to 20 kHz.
4. The microcircuit can be powered from a source constant voltage 10-40 V. But there is a small drawback - you must use a bipolar power supply.
It is worth paying attention to one feature - the distortion factor does not exceed 1%. On the TDA7294 microassembly, the power amplifier circuit is so simple that it is even surprising how it allows you to get such high-quality sound.
The purpose of the pins of the microcircuit
And now in more detail about what conclusions the TDA7294 has. The first leg is the “signal ground”, it is connected to the common wire of the entire structure. Conclusions "2" and "3" - inverting and non-inverting inputs, respectively. The "4" pin is also a "signal ground" connected to ground. The fifth leg is not used in audio frequency amplifiers. The "6" leg is a volt additive, an electrolytic capacitor is connected to it. "7" and "8" conclusions - plus and minus the power supply of the input stages, respectively. Leg "9" - standby mode, used in the control unit.
Similarly: "10" leg - mute mode, also used when designing an amplifier. "11" and "12" conclusions are not used in the design of audio frequency amplifiers. From the "14" output, the output signal is taken and fed to acoustic system. "13" and "15" pins of the microcircuit are "+" and "-" for connecting the power supply of the output stage. On the TDA7294 chip, the circuit is no different from those proposed in the article, it is only supplemented by which it is connected to the input.
Features of microassembly
When designing an audio frequency amplifier, you need to pay attention to one feature - the power minus, and these are the legs "15" and "8", electrically connected to the microcircuit case. Therefore, it is necessary to isolate it from the heat sink, which in any case will be used in the amplifier. For this purpose it is necessary to use a special thermal pad. If you use a bridge amplifier circuit on the TDA7294, pay attention to the version of the case. It can be vertical or horizontal type. The most common is the version designated as TDA7294V.
Protective functions of the TDA7294 chip
The microcircuit provides several types of protection, in particular, against a drop in the supply voltage. If the supply voltage suddenly changes, the microcircuit will go into protection mode, therefore, there will be no electrical damage. The output stage is also protected against overloads and short circuits. If the body of the device heats up to a temperature of 145 degrees, the sound is turned off. When it reaches 150 degrees, it goes into standby mode. All pins of the TDA7294 chip are protected from electrostatics.
Amplifier
Simple, accessible to everyone, and most importantly - cheap. In just a few hours, you can collect very good amplifier audio frequency. And most of the time you will spend on etching the board. The structure of the entire amplifier consists of power and control units, as well as 2 ULF channels. Try to use as few wires as possible in the design of the amplifier. Follow these simple guidelines:
1. Required condition- this is the connection of a power source by wires to each UZCH board.
2. Bundle the power wires. With this, it will be possible to slightly compensate for the magnetic field that is created electric shock. To do this, you need to take all three supply wires - “common”, “minus” and “plus”, with a slight tension weave them into one pigtail.
3. In no case do not use the so-called "earth loops" in the construction. This is the case when a common wire connecting all blocks of the structure closes in a loop. The ground wire must be connected in series, starting from the input further to the UZCH board, and must end at the output connectors. It is extremely important to connect the input circuits with shielded wires in isolation.
Standby and mute control unit
This chip also has muting. It is necessary to control the functions using the conclusions "9" and "10". The mode is turned on if there is no voltage on these legs of the microcircuit, or it is less than one and a half volts. To enable the mode, it is necessary to apply a voltage to the microcircuit legs, the value of which exceeds 3.5 V. In order for the amplifier boards to be controlled simultaneously, which is important for bridge-type circuits, one control unit is assembled for all cascades.
When the amplifier turns on, all the capacitors in the power supply are charged. In the control unit, one capacitor also accumulates a charge. When the maximum possible charge is accumulated, the standby mode is turned off. The second capacitor used in the control unit is responsible for the operation of the mute mode. It charges a little later, so the mute mode is disabled second.
Bridge ULF on TDA2030 - 150 watts.
Schematic diagram of a bridge amplifier on TDA2030A chips:
As you can see in the diagram, the amplifier consists of two identical cascades, in which each TDA2030 chip has a pair of transistors at the output, due to which the amplification occurs, and these cascades are connected into a bridge circuit, due to which the power is doubled. This amplifier showed not bad results when used as an amplifier for a subwoofer.
The printed circuit board of the bridge amplifier on TDA2030 microcircuits with power transistors is shown below:
Microcircuits and transistors are fastened to the heatsink through insulating gaskets using KPT-type paste. The fastening bolts of the elements also have insulating washers.
The appearance of the amplifier board assembly for the subwoofer is shown in the following pictures:
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