UMZCH on the TDA7294 chip. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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The TDA7294 integrated circuit from SGS-THOMSON MICROELECTRONICS is designed to work in sound reproducing systems as a power amplifier. The microcircuit is produced in plastic cases with a metal substrate and molded leads for vertical ("Multiwatt 15V") and horizontal ("Multiwatt 15H") mounting on printed circuit boards. Each package has 15 hard leads (Figure 1).

A simplified block diagram of the TDA7294 is shown in fig. 2.

One of the main features of this microcircuit is the use of field-effect transistors in the preliminary and output amplification stages. Its advantages include high output power (up to 100 W at a load of 4 ohms), the ability to work in a wide range of supply voltages (from 7,5 to 50 V), high technical characteristics (low distortion, low noise level, wide operating frequency range). etc.), minimum required external components and low cost. The microcircuit implements the functions of operational silent shutdown of the output signal ("Mute") and switching to standby mode ("Stand-woo"). There are also built-in protection units against short circuits in the load and overheating of the output transistors.

A typical switching circuit and pinout of the TDA7294 are shown in fig. 3. The purpose of the main pins of the microcircuit is clear from the figure. Pins 5, 11, 12 not shown on it are not used. The PIC circuit is connected to pin 6, the output stage blocking control circuit to pin 9, and the input stage blocking control circuit to pin 10, in the latter case, control is provided only for the non-inverting input. Pin 8 is connected to the metal substrate of the microcircuit housing.

The purpose of the main external elements of a typical TDA7294 switching circuit can be seen from fig. 3. Resistor R1 determines the input impedance of the UMZCH; resistors R2, R3 (or rather, their ratio R3 / R2) - its gain; elements R5C3 and R4C4 provide noiseless blocking of the input and output stages; from the ratings of capacitors C1. C2 depends on the frequency response of the amplifier in the region of low audio frequencies; C7, C9 - blocking capacitors of power circuits; C6, C8 - power supply filter capacitors; C5 performs the functions of a voltage boost in the PIC circuit.

All technical specifications UMZCH when using the switching scheme recommended by the company:

• Input current, μA, not more than......0,5
• Bias voltage, mV. no more ......±10
• Noise level reduced to the input, µV, not more than: weighted value according to curve "A"......1
• in the range of 20...20 Hz......000
• Output power, W, at harmonic coefficient 0,5%, supply voltage 2x35 and 2x27 V and load resistance respectively 8 and 4 Ohm.....60...70
• Nonlinear distortion factor, %: at output power of 5 W at a frequency of 1 kHz......0,005
• with an output power of 0.1...50 W in the frequency range of 20-20000 Hz,
• no more ...... 0,1
• Operating frequency range. Hz,
• with an output power of 1 W and uneven frequency response - 3 dB ...... 20 ... 20 000
• Output voltage slew rate, V/µs, not less......7... 10
• Voltage gain, dB: without OOS......80
• with environmental protection ...... 24 ... 40
• Power supply voltage, V......(2х7.5)...(2х40)
• Current. consumed by the amplifier in idle mode. mA......20.-.60
• Depth of suppression of supply voltage ripple with a frequency of 100 Hz, dB......60 ..75
• Tripping temperature of the thermal protection device. °С ... +145
• Threshold voltage of operation of blocking devices of input and output stages, V, in the modes: "Enabled"......+1,5
• "Off"......+3.5
• Reducing the output signal level when blocking devices are triggered, dB, cascades: input ...... 60 ... 80
• weekend ...... 70 ... 90
• Current consumed by the output stage blocking control circuit, mA. no more ...... 1-3
• Maximum permissible values ​​​​of the parameters of the TOA7294 microcircuit
• Maximum supply voltage, V......(2x50)
• Maximum output current. A......10
• Maximum power dissipation, W, at a temperature of the microcircuit case + 70 ° С ...... 50
• Maximum temperature of the crystal, °С......+150
• Thermal resistance crystal-case, ° С / W...... 1,5

On fig. 4 - 11 shows the dependence of the output power of the UMZCH on the supply voltage of each arm (Fig. 4, 5), the harmonic coefficient on the output power (Fig. 6, 7), the harmonic coefficient on the frequency (Fig. 8, 9), the power dissipation on the output power (Fig. 10, 11).

The TDA7294 switching circuit recommended by the manufacturer, in the opinion of the author, has a number of disadvantages. These include: the absence at the input of a low-pass filter microcircuit with a cutoff frequency of 50 ... 100 kHz, capable of protecting the input circuits of the amplifier from interference from powerful television and radio broadcasting stations and from signals induced from the amplifier output due to the POS; the use of oxide capacitors C2 - C4, which can reduce the reliability of the amplifier; the impossibility of a significant increase in the input impedance of the amplifier while maintaining the necessary margin of its resistance to the occurrence of generation at high frequencies; insufficient margin of stability when working on a load with a capacitive component.

We managed to get rid of these shortcomings by changing the recommended TDA7294 switching circuit (Fig. 12). At the input of the microcircuit, a low-pass filter R2C1 with a cutoff frequency of 50 ... 100 kHz is installed, oxide capacitors C2, C3 and C4 are replaced with polyethylene terephthalate ones, the input resistance is increased to 1 MΩ (R3 \u1d 9 M), the stability of the amplifier is increased due to the inclusion of a shunted capacitor at its output resistor R1 inductor L6, capacitors C12, C8 and resistor RXNUMX.

The amplifier uses resistors MLT-1.0 (R8) and MLT-0,25 (the rest), capacitors - K73-17 (C2-C5). CT-1 (C1, C6). KM-5 (C7, C8.C12), K50-16 (C9-C11).

The gain of the modified version of the amplifier is determined by the ratio of the resistances of the resistors R6 / R1 and can vary over a wide range by selecting the resistor R1. In this case, to ensure the necessary margin of stability, it may be necessary to clarify the value of the capacitor C6.

In the practical implementation of the UMZCH according to the above scheme, it is advisable to supplement it with a load protection device from the constant voltage component, fuses in the power circuits and an overload indicator.

To increase the output power (or output voltage), the TDA7294 microcircuits can be switched on according to the bridge circuit recommended by the manufacturer (Fig. 13).

The frequency response of the bridge amplifier is shown in fig. 14, and the dependence of the harmonic coefficient on the output power - in fig. 15 and 16.

The same characteristics are also provided when the amplifiers are bridged according to modified circuits (Fig. 17). For their optimal operation in this mode, the following equalities must be fulfilled: R6.1 = R6.2 = Radd, and R1.1 = R1.2. where R6.1. R6.2, R1.1. R1.2 is the resistance of the resistors in the OOS circuits UMZCH1 and UMZCH2 (see Fig. 12), and Rdop is the resistance of the resistor connected between points C1 and B2 (Fig. 17).

These conditions are met when using resistors with a resistance deviation of ± 1% from the nominal value.

Author: A.Syritso, Moscow

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