Improving the parameters of the amplifier on the K174UN7 chip. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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The ever-expanding range of specialized microcircuits, it would seem, should limit the creativity of radio amateurs. Indeed, such microcircuits are usually oriented by their developers to solve one specific task in electronic equipment or, at best, a narrow range of tasks. That is why radio amateurs and radio designers seem to be left with only creative "games of cubes" - to combine nodes on microcircuits assembled according to typical switching schemes.

However, the spirit of the rubric "Radio amateur makes an experiment", which used to appear more or less regularly on the pages of our magazine, does not die in the hearts of our readers. Evidence of this is the article by V. Gromov and A. Radomsky published here, which, in our opinion, should be paid attention not only by radio amateurs, but also by professionals - both hardware developers and microcircuit creators. We are waiting for their responses to that publication - after all, the K174UN7 chip is very widely used in household radio equipment.

Well, we appeal to all readers with a proposal - to conduct experiments both on improving typical switching circuits for specialized ICs, and on their use in non-standard switching circuits (implementation of new functions, etc.). However, having received an interesting positive effect, do not rush to write to the editor: check its reproducibility on several copies of microcircuits.

Currently, audio frequency power amplifiers (UMZCH) of small-sized radio equipment are often built on the basis of a specialized integrated circuit (IC) K174UN7 [1]. However, its application, no doubt, would be even wider if it were not for large non-linear distortions (in a typical connection - up to 10% with an output power of 4,5 W at a frequency of 1 kHz and a supply voltage of 15 V) and not high enough in some cases input impedance (50 kOhm). It is not surprising, therefore, that radio amateurs are looking for ways to reduce non-linear distortions, suggesting, for example, replacing the voltage boost circuit with a current stabilizer based on a field-effect transistor [2]. Unfortunately, the verification of the recommendations proposed in [2] showed that their implementation leads not so much to a reduction in distortion as to a decrease in the maximum power delivered to the load.

When testing several copies of the IS K 174UN7, it turned out that the most characteristic distortions of its output voltage are manifested in the "rounding" or explicit limitation of the negative half-cycle of the signal. In this regard, the effectiveness of such a measure as the regulation of the IC mode for direct current used in some industrial devices by applying voltage to its output 7 (through a resistor with a resistance of 3 ... 6,8 kOhm) from an adjustable divider was tested. The test showed that this measure also practically does not reduce the harmonic coefficient and does not increase the undistorted output voltage, but only allows it to be symmetrically limited.

The UMZCH variant, assembled according to the scheme in Fig. 1 has significantly better characteristics than the typical one on the indicated IC. One of its differences from the typical one is an additional OOS through the resistor R6.

Connecting the latter directly to the speaker head reduces the uneven frequency response and non-linear distortion due to the presence of capacitor C9. With the resistance of the resistor R6 indicated in the diagram, the supply voltage is 15 V and the output power is 4 W (at a load with a resistance of 4 ohms), the nominal input voltage of the device is 120 mV.

In addition, to reduce the number of ratings, the capacitance of the oxide capacitor C3 in the OOS circuit is reduced to 100 μF (the uneven frequency response in the frequency range of 40 ... 20 Hz does not exceed 000 dB).

The main difference between this UMZCH is in the resistance of the resistor R2 (in a typical IC connection, it is 47 kOhm). During the experiments, it was noticed that this resistor has a very significant effect on distortion, and by selecting it, you can significantly increase the output voltage of the UMZCH. (Of the ten tested ICs, only two did not require the selection of the resistor R2, i.e., changing its resistance relative to the typical one; the resistance of the resistors for the rest of the ICs varied within 0,1 ... 1 MΩ).

On fig. Figure 2 shows the dependence of the maximum output power Pmax and the harmonic coefficient Kg on the supply voltage Upit (distortions were measured at Pmax corresponding to the given voltage Upit). The parameters were evaluated at a frequency of 1 kHz with two values ​​of the resistance of the resistor R2: typical (47 kOhm) and optimized for maximum power (750 kOhm). The power Pmax was determined by the maximum output voltage, on the oscillogram of which the distortions were not yet visible to the eye (what these distortions were in reality are shown by the Kr curves).

Fig. 2

As can be seen from fig. 2, at Upit = 15 V, by selecting the resistor R2, it was possible to increase Pmax by 1,5 W while reducing the harmonic coefficient by almost 3,5 times, and at Upit = 18 V - by about 3 W with a decrease in K,. almost three times. (Obviously, with the same distortions, the gain in power Pmax would be even greater). The result obtained speaks for itself, given that the tested IC was quite conditioned: at Upit = 15 V, R2 = 47 kOhm and output power Pout = 4,5 W, its harmonic coefficient did not exceed 7,2% (after selecting the resistor R2 it decreased to 1,1%.

Dependences Pmax (Upit) and Kg (Upit) of the UMZCH with the optimized resistance of the resistor R2 (750 kOhm) were also taken at frequencies of 60 Hz and 5 kHz (Fig. 3). The decrease in Pmax at lower frequencies is due to the influence of the capacitance of the capacitor C9 (1000 μF). With a load resistance Rn \u4d 2000 Ohm, it is desirable to increase its capacitance to at least XNUMX microfarads.

Fig. 3

The curves depicted in Figs. 4, illustrate the dependence of the efficiency and quiescent current Iо on the supply voltage Upit for the same two resistances of the resistor R2. It is easy to see that with R2 = 750 kOhm, the efficiency also increases, and a tangible gain is obtained at Upit> 10 V.

Fig. 4

To identify the real dependence of the harmonic coefficient Kg on the output power level Pout, an IC copy with average parameters was tested at Upit=15 V, Rn=4 Ohm, C9=4000 μF and R2=R2opt=510 kΩ (Fig. 5). As can be seen, at Рout=4 W, the harmonic coefficient of the UMZCH assembled on this IC copy according to the diagram in Fig. 1, in the frequency range of 60 ... 10 Hz does not exceed 000%.

Fig. 5

The input impedance of the K174UN7 IC itself was calculated based on the results of measuring the input impedance of the UMZCH (with the volume control turned off), performed on an instance for which R2opt = 750 kOhm. It turned out that in the frequency range of 50 ... 15 Hz, the input resistance of the IC exceeds 000 MΩ. In other words, the input resistance of the UMZCH is practically equal to the resistance of the resistor R30 and, if necessary, can be much more than 2 kOhm.

When designing a stereo UMZCH, it may happen that the optimal resistances of the resistors R2 in the left and right channels turn out to be different. To obtain identical frequency responses, the output resistance of the previous stage in this case should be less than the resistance of the resistor R2, and the capacitance of the isolation capacitor C2 should be such that in the channel with a lower resistance of the resistor there is no noticeable drop in the frequency response at lower frequencies (in most cases it is enough to take C2 == 0,47 ... 1 uF).

UMZCH works well when powered from an unstabilized source, however, if the main thing is to obtain maximum output power and, accordingly, minimum distortion at an average, it is necessary to use a stabilizer with an output voltage of 17 ... 18 V.

It should be noted that when working with increased (up to 5 ... 6 W) output power, it is necessary to ensure good heat removal from the IC, taking the necessary measures in such cases to reduce the thermal resistance between its plates and the heat sink. It is very valuable that since the potential of the IC plates (relative to the common wire) is close to 0, a metal chassis or other metal parts of the structure connected to the common (negative) wire and providing effective heat dissipation can be used as a common heat sink without insulating gaskets.

Literature

1. Integrated Circuits: A Handbook by B. V. Tarabrin, L. F. Lunin, Yu. I. Smirnov, and others; ed. B.V. Tarabrina.- M.: Radio and communication, 1983.
2. Filin S. Distortion reduction in power amplifiers based on ICs. - Radio, 1981, No. 12, p. 40.

Authors: V. Gromov, A. Radomskin, Lvov; Publication: N. Bolshakov, rf.atnn.ru

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