Semiconductor Integrated Circuit Device

This invention relates to a semiconductor integrated circuit device suitable for a low frequency amplifier circuit capable of being driven with a lower voltage power source.

It is an object of the present invention to provide a low frequency power amplifier circuit using a vertical type PNP transistor with a high current amplification factor h.sub.FE.

A further object of this invention is to provide a low frequency power amplifier circuit which is integrated in a common semiconductor substrate and which is capable of operating with a low voltage, thus, resulting in an enhanced voltage utilization efficiency of the power source voltage.

A still further object of this invention is to eliminate distortion which tends to occur in the signal output of a low frequency power amplifier, in the case where a complementary symmetrical circuit using vertical type PNP and NPN transistor provided at the output stage of the amplifier is formed in a common semiconductor substrate, due to the fact that these transistors differ from each other in terms of current amplification factor h.sub.FE.

Other objects, features and advantages of the present invention will become apparent from the ensuing description taken in conjunction with the accompanying drawings.

FIG. 1 is a circuit diagram showing an example of prior-art low frequency amplifier circuit.

FIG. 2 is a circuit diagram showing an example of conventional inverted Darlington circuit arrangement.

FIG. 3 is a circuit diagram showing the low frequency amplifier circuit according to an embodiment of the present invention.

FIG. 4 is a schematic cross-sectional view showing a semiconductor integrated circuit device including a vertical type PNP transistor and an NPN transistor in accordance with the present invention.

FIG. 5 is a circuit diagram showing the low frequency amplifier circuit according to another embodiment of the present invention.

FIG. 6 is a view illustrating an output current waveform.

FIG. 7 is a circuit diagram showing the low frequency amplifier circuit according to still another embodiment of the present invention.

FIG. 8 is a cross sectional view showing another semiconductor integrated circuit device including a vertical type PNP transistor and an NPN transistor according to the present invention.

In order to give a better understanding of the present invention, description will first be made of the prior-art low frequency amplifier circuit arrangements which have conventionally been employed in audio amplifiers, with reference to FIGS. 1 and 2.

In the low frequency amplifier circuit shown in FIG. 1, an NPN transistor 11 has its collector connected with a power source Vcc, the emitter of the NPN transistor 11 being connected with the emitter of a PNP transistor 12 the collector of which is grounded. Thus, the series connection of the NPN transistor 11 and PNP transistor 12 is connected between the power source Vcc and ground. Two diodes 17 and 18 are provided, the cathode of the diode 17 being connected with the anode of the diode 18. The anode of the diode 17 is tied to the base of the NPN transistor 11, and the cathode of the diode 18 is coupled to the base of the PNP transistor 12. Another NPN transistor 13 is provided which has its emitter grounded and its collector connected with the cathode of the diode 18. Furthermore, a constant current Io is provided which is connected between the power source Vcc and the anode of the diode 17, whereby the NPN transistor 13 and PNP transistor 12 are biased through the diodes 17 and 18. An input terminal 16 is led out of the base of the NPN transistor 13, and an output terminal 15 is led out of the connection point between the NPN transistor 11 and the PNP transistor 12.

With such a circuit arrangement as mentioned just above, however, difficulty has been encountered in an attempt to achieve an enhanced voltage utilization efficiency due to the fact that there is caused a remarkable power loss, in the case where the power source voltage in use is as low as about 3 V.

Explaining more specifically about the low frequency amplifier circuit exemplified in FIG. 1, the NPN transistor 11 and PNP transistor 12 are employed at the output stage of the low-frequency amplifier circuit shown in FIG. 1, and thus, a d.c. voltage (referred to as V.sub.BE hereinafter) occurs between the base and the emitter of each of these transistors, so that the voltage utilization efficiency is thereby lowered. This adverse effect becomes remarkable especially for the case where the power source voltage is as low as about 3 V.

More specifically, the V.sub.BE of a transistor which is formed in a silicon substrate is generally about 0.7-0.8 V; thus, when the power source voltage Vcc for the low frequency amplifier circuit is relatively high, the power loss due to V.sub.BE is negligible, whereas when the power source voltage is as low as about 3 V, such power loss cannot be neglected since the sum of the V.sub.BE voltages of these two transistors provided at the output stage reaches about 1.5 V so that the maximum amplitude (peak-to-peak value) of an audio signal is limited to about 1.5 V, as a result of which the efficiency of the power amplifier circuit is greatly reduced. Moreover, under a low voltage condition, there is the tendency that the audio output is subject to distortion so that the low frequency amplifier circuit fails to operate satisfactorily.

In view of what has been mentioned just above, by employing germanium transistors whose V.sub.BE is lower at the output stage of a low frequency circuit, it may be possible to reduce the power loss and thus achieve an enhanced efficiency.

Practically, however, it is impossible to integrate low frequency amplifier circuit comprising elements consisting of such different types of semiconductor material in the same semiconductor substrate. If it were attempted to obtain such an amplifier circuit in the form of a hybrid integrated circuit, an increased number of components as well as an increased number of manufacturing steps would be required.

Furthermore, various limitations are laid on the steps of fabricating a semiconductor device including integrated transistors, which are opposite in polarity to each other, in a common semiconductor substrate. Especially, when a complementary type circuit comprising a PNP transistor and an NPN transistor is formed at the output stage of a low frequency amplifier circuit, it is preferred that these transistors have an equal current amplification factor h.sub.FE. As well known in the art, a PNP transistor taking a monolithic form is commonly formed as a lateral type transistor, and the current amplification factor h.sub.FE thereof is lower than that of an NPN transistor; thus, in order to achieve a desired current amplification factor, such a PNP transistor should be combined with an NPN transistor to constitute a Darlington circuit.

Referring now to FIG. 2, there is shown a low frequency amplifier circuit using a Darlington circuit arrangement.

An inverted Darlington circuit such as shown in FIG. 2 has conventionally been employed at the output stage of a low frequency power amplifier. In FIG. 2, a PNP transistor 21 and an NPN transistor 22 are connected in series between a voltage source Vcc and another voltage source-Vcc; the base of the PNP transistor 21 is connected with the collector of an NPN transistor 23 to constitute a first Darlington circuit; and the base of the NPN transistor 22 is connected with the collector of a PNP transistor 24 to constitute a second Darlington circuit. The NPN transistor 23 and PNP transistor 24 have their emitters connected with each other. An input terminal 25 is led out of the interconnected bases of the transistors 23 and 24, and an output terminal 27 is led out of the interconnected collectors of the transistors 21 and 22.

A desired current amplification factor can be achieved with the Darlington circuit formed as mentioned just above. However, with the circuit arrangement of FIG. 2, too, for a power source voltage as low as about 3 V, power loss due to the V.sub.BE of the input NPN transistor 23 cannot be neglected, so that the voltage utilization efficiency becomes lower, as was the case with the prior art circuit arrangement shown in FIG. 1. Moreover, the circuit arrangement shown in FIG. 2, like the low frequency amplifier circuit shown in FIG. 1, is not necessarily suitable to be fabricated in the form of an integrated circuit device.

In the case where a complementary symmetrical circuit using PNP and NPN transistors is formed in a common semiconductor substrate as mentioned above, there is the tendency that the signal output is liable to be subject to distortion due to the fact that the individual transistors differ from ach other in terms of current amplification factor h.sub.FE.

The present invention is intended to overcome the aforementioned drawbacks of the prior-art techniques.

The present invention will now be described in greater detail with reference to FIGS. 3 and 8.

Referring to FIG. 3, there is shown the low frequency amplifier circuit according to an embodiment of the present invention, wherein a PNP transistor 31 and an NPN transistor 32 are connected in series between a power source Vcc and ground; and a resistor 38, an NPN transistor 33, a PNP transistor 34 and a resistor 39 are also connected in series between the power source Vcc and ground. The PNP transistor 31 has its base coupled to the connection point between the resistor 38 and the collector of the NPN transistor 33, and the NPN transistor 32 has its base tied to the connection point between the collector of the PNP transistor 34 and the resistor 39. Two diodes are connected between the bases of the transistors 33 and 34 and grounded via a transistor 35. There is provided a constant current source I.sub.0 which feeds to these two diodes so that the transistors 33 and 34 are biased thereby. An input terminal 37 is led out of the base of the NPN transistor 35, and an output terminal 36 is led out of the connection point A between the PNP transistor 31 and the NPN transistor 32. A d.c. voltage of Vcc/2 is imparted to the connection point B between the NPN transistor 33 and the PNP transistor 34. A loudspeaker S representing a load impedance Z.sub.1 is connected at one end with the output terminal 36 through a coupling capacitor C.sub.1 and grounded at the other end.

In a double power source system, that is, in the case where the transistors 31 and 32 are connected in series between the power source Vcc and a negative power source-Vcc, the connection point B between the emitters of the transistors 33 and 34 is grounded, as shown in FIG. 7.

Referring to FIG. 4, there is shown, in a schematic cross-sectional view, a semiconductor integrated-circuit device constituting the low frequency amplifier circuit according to one embodiment of the present invention, illustrating the region in which the PNP transistor 31 and NPN transistor 32 of the low frequency amplifier circuit are formed in a common semiconductor substrate.

To fabricate the semiconductor integrated circuit device according to the present invention, two, first and second, N type epitaxial layers 43 and 44 are first formed in superimposed relationship with each other on a P type semiconductor substrate 45; then a first P.sup.+ layer 47 is formed in the first epitaxial layer 43; and a second P.sup.+ type layer 48 is formed in the second epitaxial layer 44 in such a manner as to extend as far as the aforementioned first P.sup.+ type layer 47 so that a collector region is thereby defined. In that portion of an N type epitaxial layer 49 which is surrounded by the second P.sup.+ type layer 48 is formed a P type layer 410 so that a base region is formed by the surrounded portion of the N type epitaxial layer 49 and an emitter region is formed by the P type layer 410. Designated at 46 is a burried layer. In this way, a vertical type PNP transistor 31 is constituted. At the same time, an NPN transistor 32 is constituted in the second epitaxial layer 44 on the same semiconductor substrate, the NPN transistor 32 being isolated from the PNP transistor 31 by means of a P type isolation region. The vertical type PNP transistor 31 has its collector connected with the collector of the NPN transistor 32, and an output terminal 36 is led out of the connection point therebetween. The vertical type PNP transistor 34 is provided at its emitter with a power source voltage Vcc/2, and the NPN transistor 32 has its emitter grounded.

In a double power source system, the NPN transistor 32 is provided at its emitter with a negative power source voltage-V.sub.CC as shown in FIG. 8.

The vertical type PNP transistor 31 formed by the use of two epitaxial layers as mentioned above represents a sufficiently high d. c. current amplification factor h.sub.FE ; with such construction, therefore, it is possible to achieve a desired d. c. current amplification factor without establishing Darlington circuits such as shown in FIG. 2. This facilitates the manufacture of a low frequency power amplifier circuit in an integrated circuit form.

The low frequency power amplifier circuit according to the present invention is capable of operating with a lower power source voltage vy virtue of being constructed in the form of a semiconductor device whose output stage comprises a complementary circuit such as shown in FIG. 4, and in addition, it represents an increased power utilization efficiency.

The power utilization efficiency of the low frequency amplifier circuit according to the present invention as shown in FIG. 3 will now be numerically compared with that of the prior-art low frequency amplifier circuit shown in FIG. 1, assuming that the power source is 3 V. Such an efficiency is given by ##EQU1## In FIG. 1, the sum of the V.sub.BE voltages of the NPN transistor 11 and PNP transistor 12 constitutes power loss of about 1.5 V. The following equations hold true: ##EQU2## where Z is a load impedance. By using equations (1), (2) and (3), the efficiency is obtained as follows: ##EQU3##

In contrast, with the low frequency amplifier circuit according to the present invention wherein the PNP transistor 31 and NPN transistor 32 constitute the output stage thereof, the d. c. voltage V.sub.CE (sat) between the collector and the emitter of each transistor contributes to power loss. The d. c. voltage is in a range of 0.1 to 0.2 V, and thus the maximum amplitude of the output signal becomes about 2.6 V, which results in an increased power source voltage efficiency. By using equations (1), (2) and (3), the efficiency is calculated as follows: ##EQU4##

As will be readily appreciated from the foregoing explanation, in the case where the power source voltage is as low as about 3 V, the voltage utilization rate (efficiency) is increased from 25% to 75%. Thus, the semiconductor integrated circuit device which constitutes the low frequency power amplifier circuit according to the present invention can provide an output with a greatly increased efficiency.

Description will now be made of distortion which tends to be caused in the output because of the difference in current amplification factor h.sub.FE between the vertical type PNP transistor and the NPN transistor which are provided at the output stage to constitute a complementary type circuit in a semiconductor substrate.

In the low frequency amplifier circuit shown in FIG. 3, bias currents I.sub.1 and I.sub.2 are controlled by a current mirror circuit so that these bias currents are homologously related to each other. In the case where the PNP transistor 31 and NPN transistor 32 at the output stage are formed in the same semiconductor substrate, and assuming that the current amplification factors of these two transistors are h.sub.FE1 and h.sub.FE2 respectively, the following relationship generally holds: h.sub.FE1 <h.sub.FE2. The output current I.sub.OUT1 is given by I.sub.OUT1 =I.sub.1 .multidot.h.sub.FE1, and the output current I.sub.OUT2 is given by I.sub.OUT2 =I.sub.2 .multidot.h.sub.FE2. Thus, the relationship between the output currents turns out to be I.sub.OUT1 <I.sub.OUT2 as shown in FIG. 6. In this way, the output voltages which are produced across the load impedance Z.sub.1 of the speaker S will have the following relationship: I.sub.OUT1 .multidot.Z.sub.1 <I.sub.OUT2 .multidot.Z.sub.1. FIG. 6 shows the relationship between the output currents I.sub.OUT which occur in the low frequency amplifier circuit shown in FIG. 3. As shown in FIG. 6, in the case where vertical type PNP transistor and NPN transistor are provided at the output stage, sinusoidal waveform of the output signal is liable to be derived out as distortion in the output signal, and thus in the case of an audio signal, the audio output is subject to distortion, which provides an uncomfortable feeling.

Referring to FIG. 5, there is shown a still further embodiment of this invention, wherein the output stage comprises a PNP transistor 51 and an NPN transistor 52. The PNP transistor 51 has its emitter connected with a first power source Vcc, the collector of the PNP transistor 51 being connected with the collector of the NPN transistor 52 which has its emitter connected with a second power source--Vcc. Thus, there is formed a so-called complementary symmetrical circuit. At the output stage, the PNP transistor 51, together with a PNP transistor 501, constitutes a current mirror circuit, and also the NPN transistor 52, together with an NPN transistor 502, constitutes another current mirror circuit. Furthermore, the PNP transistor 51, together with an NPN transistor 53, forms an inverted Darlington circuit, and also the NPN transistor 52, together with a PNP transistor 54, forms another inverted Darlington circuit.

There is the tendency that there occurs a difference between output currents I.sub.OUT3 and I.sub.OUT4 because of the difference in current amplification factor between the PNP transistor 51 and the NPN transistor 52 which are provided at the output stage. However, by changing the emitter area of the transistors constituting the current mirror circuit, it is possible to control the collector current which flows through the collector of each transistor; thus, distortion which would otherwise be caused can be eliminated equivalently. In the case where the two transistors constituting the current mirror circuit have an equal emitter area, and an equal current can be caused to flow through the transistors, whereas in the case where these transistors are different from each other in terms of emitter area, currents which are in reverse proportion to the emitter areas respectively are caused to flow through the respective transistors. Thus, by making the emitter area ratio K.sub.1 of the PNP transistor 51 to the PNP transistor 501 equal to the emitter area ratio K.sub.2 of the NPN transistor 52 to the NPN transistor 502, it is possible to establish a homologous relation between the output currents I.sub.OUT3 and I.sub.OUT4. The aforementioned emitter area ratios K.sub.1 and K.sub.2 are given as follows:

where A.sub.51 and A.sub.501 denote the emitter areas of the PNP transistors 51 and 501 respectively, and A.sub.52 and A.sub.502 indicate the emitter areas of the NPN transistors 52 and 502 respectively.

By establishing between the emitter area ratios K.sub.1 and K.sub.2 a relationship such that K.sub.1 =K.sub.2, it is possible to make the output currents I.sub.OUT3 and I.sub.OUT4 equivalently equal to each other. In this way, it is possible to prevent distortion from being caused in the output voltage available across the load impedance of the speaker. Thus, application of the present invention to the audio field will result in the output signal being improved in terms of distortion, so that uncomfortable feeling which tends to be caused by distortion in the audio output, can be effectively avoided. As will be readily appreciated, the amplifier circuit according to the present invention is equally applicable to other fields than audio field, and in such cases, too, excellent effects can be produced. In addition, in accordance with the present invention, there is provided a power amplifier circuit with a reduced power loss.

Although the present invention has been described with respect to some specific embodiments, it is to be understood that the invention is not restricted thereto but covers any and all modifications and changes which may be made within the scope of the appended claims.