CONCEPTUAL ERRORS FOUND IN A FEW PROMINENT TEXTBOOKS ON INTEGRATED CIRCUITS

EDITOR: B. SOMANATHAN NAIR

Abstract

It is highly essential that textbooks must be free from errors of all types, especially errors of a conceptual nature. If errors are detected in the first edition of a textbook, they must be corrected in its second and subsequent editions. If such a practice does not take place, then the authors of this article feel that responsible journals may spare a few pages as a regular practice to bring out such errors so that the students and teachers are not mislead.

      In this article, certain conceptual errors in the description of the internal circuitry of LM380 IC chip, appearing in three major textbooks are pointed out. These books are in their second or subsequent editions.

1. INTRODUCTION

The output circuit of any audio-power amplifier will usually be in the class-B or class-AB push-pull mode. Class-B push-pull mode of operation was used in amplifiers constructed using vacuum pentodes and is obsolete now with the advent of semiconductor devices. Instead of class-B, class-AB mode of operation has been more popular for audio-power amplifiers constructed using BJTs to avoid cross-over distortion that can occur in class-B operation. LM380 is an integrated-circuit chip used as audio-power amplifier for consumer applications. The output stage of LM380 is shown in Fig. 1.

     Referring to Fig. 1 and the datasheet of Texas Instruments¹, the output stage of LM380 is made up of transistors Q₇, Q₈, and Q₉, as well as biasing diodes D₁ and D₂. From the figure, it is clear that the output stage of LM380 is a complementary-symmetry, Class-AB, emitter-follower, push-pull, power amplifier. It can be seen that this is a very lengthy name. However, this lengthy name will reveal all the characteristic aspects of the output stage of LM380. The name highlights the following:

1.    The output stage is a complementary-symmetry push-pull power amplifier consisting of a matched pair of transistors Q₇ and Q₈, respectively.

2.    Current gain (β) of the NPN transistor Q₇ is typically about 250. However, current gain of the PNP transistor Q₈ is only about 50. So, to increase the gain of this transistor to 250, an NPN transistor Q₉ is connected to Q₈ in a composite mode.

3.     Q₇ and Q₈ form an emitter-follower configuration, which can drive low-resistance loads such as loudspeakers directly.

4.    Diodes D₁ and D₂ are used to bias Q₇ and Q₈, respectively. This makes the output a class-AB push-pull power-amplifier stage. It may be noted that class-AB configuration is used to avoid cross-over distortion (that will occur if class-B configuration is used).

2. ERRORS DETECTED IN SOME PROMINENT TEXTBOOKS ON INTEGRATED CIRCUITS

Regarding the description of this IC, conceptual errors detected in three major textbooks are described below.

2.1 Error found in Reference 2

In this text, on page 114, fourth paragraph, the following explanation appears:

     “The single-ended output of the second gain stage drives the third, or output stage. This is a class-B, complementary push-pull emitter-follower output stage, comprised of transistors Q₇, Q₈, and Q₉, and D₁ and D₂.”

            In the explanation given above, the output stage is mentioned as class-B, whereas it is clear that it is indeed class-AB with biasing diodes D₁ and D₂ attached to bases of transistors Q₇ and Q₈, respectively. It would become class-B only if the diodes are removed and the bases Q₇ and Q₈ are shorted together as shown in Fig. 2. Thus the explanation given in the text by Soclof regarding the output stage of LM380 IC is really misleading.

2.2  Error observed in Reference 2

       In the textbook by Sedra and Smith³, on page 1262, Paragraph 3, the following explanation appears:

      “The output stage is class AB, utilizing a compound pnp transistor (Q₈ and Q₉).”

The explanation given above conveys two wrong messages to the reader of this text: first, the output stage is made-up of a compound PNP transistor Q₈ and Q₉. It can be easily seen that Q₉ is an NPN transistor and the statement would have been non-confusing, if it were modified as “the compound pair of  PNP transistor Q₈ and NPN transistor Q₉ together will effectively act as a PNP transistor of high gain (about 250, the same as that of transistor Q₇).

      The second major error can again be found in the same statement: “The   output stage is class AB, utilizing a compound pnp transistor (Q₈ and Q₉).” Nobody can find fault in a person who assumes that these two transistors (Q₈ and Q₉) form the push-pull class-AB complementary-symmetry output power amplifier stage of LM380. Figure 3 highlights this confusion.

2.3 Incomplete Explanation found in Reference 4

     The textbook by Gaykwad gives the explanation of LM380 output stage (Fig. 4) in the following form:

      “The output stage is a quasi (false)-complementary pair emitter follower formed by NPN transistors Q₁₀ and Q₁₂. In fact, the combination of PNP transistor Q₁₁ and NPN transistor Q₁₂ has the power capability of an NPN transistor but the characteristic of a PNP transistor”

       .

In the explanation cited above, the author has used the term ‘quasi (false)-complementary pair emitter follower NPN transistors Q₁₀ and Q₁₂’ instead of specifying the class of the amplifier. Further, he also suggests that Q₁₀ and Q₁₂’, which are NPN transistors form quasi (false)-complementary pair, whereas in reality, Q₁₀ and Q₁₁ form the actual complementary pair, as explained in Section 1. Thus the explanation given above is a little bit confusing. 

3.  CONCLUSION

It can be seen from the above arguments that the output stage of LM380 IC amplifier is described in three different forms in three major textbooks published in this area. It is easy to note that an instructor who teaches the circuit from Reference 1 will describe the circuit as class-B, whereas another teacher who follows  Reference 2 will teach that the output stage is made up of transistors Q₈ and Q₉. From an examination point of view, those who write answers based on these texts are sure to go wrong. In conclusion, it may be stated that textbooks must be free from conceptual errors. It is also earnestly requested that description of a certain topic must follow the same or similar in all the texts published on this topic.

4. REFERENCES

1. Texas Instruments, Datasheet on LM380, Oct 2004.

2. Soclof, S., Applications of Analog Integrated Circuits, pp 111-114., Prentice-  Hall, 1985.

3. Sedra, A.S, and Smith, K.C., Microelectronic Circuits, 5^th Ed.   pp 1262, Oxford University Press, 2009.

4.  Gaykwad, R.A., Op-Amps and Linear Integrated Circuits, 4^th Ed. pp 431-433, Pearso Education Inc., 2000.