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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Features of UMZCH with high output impedance. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers In many operational parameters, the superiority of UMZCH on integrated circuits over UMZCH on lamps is now beyond doubt. However, their superiority is not always confirmed by expert assessments of sound quality. It is no coincidence that our readers show an increased interest in measures that reduce intermodulation distortion in transistor amplifiers. This article discusses the structures of amplifiers with high output impedance that reduce these distortions. We also recommend that you familiarize yourself with the contents of the first two articles cited in the literature. An analysis of the articles published in [1] and [2] shows the complete coincidence of the conclusions made in them about the possibility of improving the quality of sound reproduction when an electrodynamic loudspeaker (EDG) is excited from an UMZCH with a high output impedance (Rout) by reducing intermodulation distortion (II). However, the sources of the emergence of AI described in the mentioned articles are fundamentally different. If in the first of them the main cause of distortions is assumed to be changes in the EDG impedance, then in the second it is stated that the source of IS is also the UMZCH, where there is intermodulation of the amplified signal and the response from the EDG coming through the common feedback circuit from the output of the amplifier. Consider the possible principles for constructing an UMZCH with a high output impedance in accordance with the recommendations in [1] and [2]. The simplest modification of the amplifier to reduce the sensitivity of the UMZCH system - a loudspeaker to a change in impedance, as indicated in [1], is possible by replacing the general OOS in voltage with the OOS in current in the UMZCH. Since, in this case, the required value of Rout is achieved due to deep current feedback, the output resistance of the UMZCH without feedback can be quite small. This creates ample opportunities for refining the most common UMZCH both in integrated design and on bipolar or field-effect transistors with an emitter (source) follower in the output stage. A simplified version of the structure of the chain of the general OOS is shown in fig. 1, where the Rocz resistor serves as a current feedback sensor. connected in series with the EDG In this circuit, an increase in the impedance of the EDG with increasing frequency leads to a decrease in the depth of the OOS and a corresponding rise in the frequency response with a steepness not exceeding 6 dB per octave. In this case, the necessary correction of the frequency response is achieved by using the simplest circuit Rk1Sk1, shown in Fig. 1 dotted line.
The features of UMZCH with a high output impedance include the need to increase the supply voltage by 20...30% to provide an increased output voltage due to changes in the EDG impedance [1]. Let us evaluate the applicability of UMZCH with the structure according to the scheme in fig. 1 to reduce the IS considered in [2], where the main requirement is to exclude the conditions for the effect of the response from the EDD on the amplification of other signals in the FOS loop. As follows from the properties of the output stage A1 (see Fig. 1), this requirement is not met due to the penetration of the EDH response (in the form of EMF) into the CNF circuit through the low output impedance of the original amplifier. An analysis of various UMZCH circuit solutions shows that meeting the requirements set out in [2] for UMZCH with low intermodulation distortion is possible only when using the UMZCH output stage with a high value of its own output impedance (without a common OOS). This is usually achieved in UMZCH with an output stage on transistors connected according to a common base (OB) or common emitter (OE) circuit. The same applies to cascades on field-effect transistors - respectively, for circuits with a common gate (CG) and a common source (OI). It is known that the switching circuit of transistors with OB (OZ) provides the highest value of Rout of the cascade. At the same time, however, the small value of its input resistance and the absence of current amplification significantly limit the possibilities of its application. An example of such an output stage structure was proposed in [3]. On fig. 2 shows a fragment of the output stage of the amplifier. Here, powerful transistors VT1, VT2 amplify the signal only in voltage. Transistors VT4, VT5, together with the emitter resistors of the cascade, stabilize its quiescent current in the temperature range, and transistors VT3, VT6 limit the maximum base current of the output transistors. A significant disadvantage of this option is a two-channel power supply that is not connected to a common wire.
The use of transistors in the output stage, connected according to the OE (OI) circuit, is more widespread due to the rather large values of the input resistance of the cascade and the current and voltage amplification. If it is necessary to increase the Rout of the cascade, it is possible to use additional local serial current feedback by including resistors in the emitter (source) circuits, as, for example, in [4] and [5]. The use of the original UMZCH with a high Rout does not exclude the possibility of simultaneous application of the general current feedback with a corresponding increase in Rout and an increase in the accuracy of converting the input voltage to the output current. In this case, a simplified version of the UMZCH circuit that meets the conditions set out in [2] coincides with the UMZCH circuit in Fig. 1. Thus, the fundamental difference in the UMZCH parameters for versions according to [1] and [2] is only in the value of Rout of the original amplifier and the need to increase the supply voltage by 20...30%. This is necessary to exclude distortions in the EDD response. If there is a lack of supply voltage, there is a deterioration in the quality of sound reproduction. In the practical implementation of UMZCH to solve the problems described in [2], some of its features should be taken into account. For example, the best stability in operation is achieved in the UMZCH with an inverting input with a minimum number of amplification stages with local OOS. If possible, the use of integrated op amps or their use for voltage amplification of no more than 20 dB should be excluded. There is no need to strive to obtain thousandths of a percent of non-linear distortions, it is enough to limit them to a value of about 0,1 ... 0,2%. The main efforts should be directed to reducing the spectrum and frequency dependence of harmonic distortions, their monotonous decrease with decreasing output signal level. It is advisable to limit the depth of the overall current feedback to 20 ... 30 dB, since sufficiently high UMZCH parameters are usually achieved with an input voltage to output current conversion factor of no more than 1 ... 1,5 A / V for an output power of 25 ... 40 W at an EDG impedance of 8 ohms. To reduce power losses, the resistance of the ROC3 resistor is recommended to be chosen relatively small. In this case, it may be necessary to include an additional amplifier in the general OOS circuit with appropriate amplification. Then it is better to correct the frequency response in the high-frequency region in the circuit of its local OOS. In order to reduce distortion when reproducing the attack of an audio signal, the parameters of the correction elements Rk2, Sk2 should be selected based on the required stability margin, and the frequency response correction in active loudspeakers should be carried out by other means. The choice of the UMZCH scheme is made based on the results of a comparison of the achieved qualitative characteristics, the objective measurements of which are performed by standard methods. In this case, the output current measurements can be replaced by voltage measurements proportional to the output current, for example, on the ROC3 resistor. If it is necessary to predict the results of a subjective quality assessment (SQA) of sound reproduction, the measurement of non-linear distortions of the UMZCH should be performed on a noise signal [6], using a real EDD as a load. It is more convenient to assess the change in the quality of sound reproduction using the UMZCH operational mode switch - with a high or low value of Rout. With this switching, current feedback is replaced by voltage feedback, and the frequency response correction elements are disabled. Literature
Author: A.Syritso
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Comments on the article: Sergei The diagram in Fig. 2 is incorrect. A bunch of transistors vt2, vt5, vt6 is drawn incorrectly. The final stage must fit the bases of transistors vt4, vt5. A two-channel power supply can and should be connected to a common wire and acoustics should be connected to vt1, vt2 emitters. Victor Sergey, if acoustics were connected to emitters, then the output impedance would be low. The circuit is correct, the transistors are out. switched on with a common base, the signal is fed to the emitters.
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