JPS6231523B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to power amplifiers, and more particularly to input circuits thereof.

Conventionally, in a power amplifier that is supplied with voltage from a single power supply, the base of an emitter-follower transistor is connected to the input signal application terminal, a signal is output from the emitter, and this output signal is applied to the subsequent amplifier. An input circuit is constructed in which the DC voltage of the signal application terminal is set to approximately the ground voltage, the DC voltage difference between the signal source and the input signal application terminal is almost zero, and there is a direct connection between them without a DC blocking capacitor. This technique reduces the number of components and is advantageous in terms of cost and high-density packaging.

Conventionally, in such a power amplifier, as shown in FIG. 1, an input signal applied to an input terminal 2 is amplified by a transistor 14 constituting a differential amplifier through an emitter follower transistor 12, and then amplified by a transistor 14 constituting a differential amplifier. Predrive transistor 2 through output terminal 3
After being amplified by transistor 22, it is output from a single-ended push-pull circuit consisting of complementary transistor arrangements 25 and 26 and appears at output terminal 5. transistor 1
The base of 2 is connected to the input terminal 2, and its collector and common ground terminal 6 are connected via a resistor 8, and the emitter of the transistor 14 is connected to the ground terminal 6.
It is connected via a resistor 9 to the connection point of a resistor 18 and a Zener diode 19, which constitute a constant voltage circuit between the power supply terminal 4 and the ground terminal 6.
3 constitute a differential amplifier, and the collector of the transistor 14 is connected to the ground terminal 6 via a load resistor 16 and also to the output terminal 3. The collector of the front drive transistor 22 is connected to the power supply terminal 4 through a level shift circuit 24 having a plurality of diodes (three in this case) connected in series in opposite directions and a resistor 23. are connected to terminals corresponding to the bases of complementary transistor structures 25 and 26 connected in a Darlington manner, and terminals corresponding to the emitters thereof are commonly connected to the output terminal 5. A constant current flows through the resistor 20 from the constant current source 17, and the DC voltage at the output terminal 5 is level-shifted and applied to the base of the transistor 15. A resistor and a capacitor are connected in series between the NF terminal 1 and the ground (not shown). (1) A negative feedback circuit is formed with the resistor 20. The DC voltage at the input terminal 2 is determined by the base current of the transistor 12 and the resistor 8, and its value is easily close to the ground voltage, so that the signal source ei and the input terminal 2 are directly connected.

In such a power amplifier, an input signal applied to the input terminal 2 from the signal source ei is transmitted to the transistor 1.
After being amplified by the differential amplifier composed of 4 and 15, the output signal output to output terminal 3 is normally in the opposite phase within the audio frequency range, and when the input signal is in a positive half cycle, the output signal of output terminal 3 is The output signal is a negative half cycle, and when the input signal is a negative half cycle, the output signal is a positive half cycle. This means that when a negative half-cycle input signal is applied, the base and emitter of transistor 12,
The base and emitter of the transistor 14 are swung to the negative side, and the output terminal 3, that is, the collector of the transistor 14, is swung to the positive side.As the input signal becomes larger, their amplitude also increases, and the voltage difference between the emitter and collector of the transistor 14 increases. becomes smaller. If the negative half-cycle input signal increases further, the transistor 14 will become saturated, and its collector voltage will not swing higher than its emitter voltage, but will be regulated by the emitter voltage, which will respond to the input signal. Therefore, if the input signal becomes excessive, the output signal of output terminal 3 during the negative half cycle will swing to the negative side during the positive half cycle, and the input signal will be in a conductive state during the negative half cycle. Predrive transistor 22 becomes non-conducting or underdriven, and complementary transistor arrangement 26 becomes non-conducting or
This results in a lack of drive. As described above, when the input signal is small, the complementary transistor configuration 25 or 26 responds to each positive and negative half cycle and repeats conduction and non-conduction, but when the input signal is large, the complementary transistor configuration 26 becomes non-conductive during the negative half cycle. It becomes conductive, and the output waveform output from the output terminal 5 is the fourth one.
In contrast to the normal waveform shown in Fig. 4(a), an abnormal waveform as shown in Fig. 4(b) appears.

In view of the above-mentioned drawbacks of the conventional art, it is an object of the present invention to provide a good power amplifier with a simple circuit configuration that prevents the lower complementary transistor structure 26 from becoming non-conductive when an excessively negative input signal is applied. .

According to the present invention, an emitter follower transistor driven by an input signal input to a base, an amplifier that amplifies the output of the transistor, a predrive amplifier and a predrive amplifier that amplify the output signal of the amplifier. In the power amplifier equipped with a push-pull circuit for current amplifying the output of the emitter-follower transistor, the base of the emitter-follower transistor is connected to an input application terminal through a first resistor to supply a base bias, and through a second resistor; A part of the excessive negative input signal current applied to the input terminal is converted into the collector-base forward current of the emitter follower transistor by the second transistor.
is supplied through the resistor, and the base potential of the emitter follower transistor is lowered from the collector potential to the ground potential.
Means is provided for level shifting the DC voltage between the emitter follower transistor and the input terminal of the amplifier so that the forward voltage between the bases is regulated to a lower potential and the predrive amplifier is made conductive at the base potential. The present invention provides a power amplifier characterized in that:

The present invention will be explained in detail below with reference to the drawings.

FIG. 2 shows an embodiment of the present invention, in which the same parts as in FIG.
is connected to the input terminal 2 through the transistor 14, and its emitter is connected to the transistor 14 through the reverse diode 10.
is connected to the base of the resistor 9 and the resistor 9.
When a small signal is applied to input terminal 2, it is applied to the base of transistor 12 through resistor 7, and is applied to the base of transistor 12 through resistor 7.
Since the impedance of the resistor 8 and the transistor 12 seen from the base side to the emitter side is sufficiently large, the signal loss can be ignored.
The signal passed through the emitter of No. 2 is applied to the base of the transistor 14 through the diode 10, and the impedance seen from the base side of the resistor 9 and the transistor 14 to the emitter side is sufficiently large compared to the dynamic resistance of the diode 10, and there is no signal loss. It operates similarly to the conventional power amplifier shown in the figure. A negative excessive signal is applied to input terminal 2, which causes transistor 1 to
2, the collector-base forward current of transistor 12 is supplied through resistor 7 as part of the signal current, and as a result, the base potential of transistor 12 becomes ground potential (collector potential of transistor 12). The potential is regulated to be lower by the collector-base forward voltage, and the potential is prevented from becoming lower than that. At this base potential, the base potential of the transistor 14 is approximately equal to the collector-base forward voltage, the emitter-base forward voltage of the transistor 12, and the forward voltage of the diode 10.
Only the forward junction voltage is higher than the ground potential, and normally the emitter-collector saturation voltage of a transistor is smaller than the forward junction voltage at a small current.
Transistor 14 does not saturate, predrive transistor 22 does not become non-conducting or under-driven, and complementary transistor configuration 26 does not become non-conducting or under-driven, so that a negative input signal is applied regardless of the magnitude of the input signal. Of course, when a positive input signal is applied to the complementary transistor structure 26, the complementary transistor structure 25 becomes conductive, and an abnormal waveform as in the conventional case can be prevented. Here, it is assumed that the emitter-collector saturation voltage of the transistor 14 is smaller than the forward junction voltage, but if it is larger than the forward junction voltage, the voltage between the base of the transistor 14 and the emitter of the transistor 12 is A diode connected in series with the diode 10 may be additionally connected to the diode 10.

FIG. 3 shows another embodiment of the present invention, in which the same parts as in the embodiment of FIG. 2 are given the same reference numerals.
The difference is that a forward diode 11 is connected between the diode 10 and the emitter of the transistor 12, and the pre-drive amplifier is connected to the transistors 21 and 22.
The present invention is implemented in a power amplifier which has a Darlington connection, and has a high impedance when viewed from the output terminal 3 of the input stage to the predrive amplifier side. By configuring the pre-drive amplifier with Darlington-connected transistors 21 and 22, it is necessary to increase the amplitude of the output signal at the output terminal 3, and compared to the embodiment shown in FIG. 2, the forward voltage between the base and emitter of the transistor 21 is the diode 11
It is obtained by level-shifting the base potential of the transistor 14 by a forward voltage of . In this embodiment as well, the same effects of the present invention as in the embodiment shown in FIG. 2 can be obtained. In the embodiments shown in FIGS. 2 and 3, a resistor 7 is connected between the input terminal 2 and the base of the transistor 12, but the input signal current is small and the collector of the transistor 12 is connected within the range of the signal current. - If the base-to-base forward voltage does not change significantly, even if the resistor 7 is removed, the base potential of the transistor 12 will be regulated to a potential lower than the ground potential by the collector-base forward voltage, and the above effect will not be obtained. It will be done.

As described above, according to the present invention, it is possible to realize a power amplifier with a very simple configuration in which the push-pull circuit in the output stage operates normally according to the input signal every positive and negative half cycle even when an excessive input signal is applied. can do.

Furthermore, it goes without saying that the present invention can be easily applied to semiconductor integrated circuits.

[Brief explanation of the drawing]

Figure 1 is a circuit connection diagram of a conventional power amplifier, Figure 2 is a circuit diagram of a conventional power amplifier.
The figure is a circuit connection diagram showing one embodiment of the present invention, and FIG. 3 is a circuit connection diagram showing another embodiment of the present invention.
FIG. 4a shows the output waveform of the output terminal 5 of the embodiment of the present invention, and FIG. 4b shows the output waveform of the output terminal 5 of the conventional example. 1...NF terminal, 2...Input terminal, 4...Power terminal, 5...Output terminal, 6...Ground terminal, 8,
9, 13, 16, 18, 20, 23...Resistance, 1
2... Emitsuta follower transistor, 14, 15
...Transistors forming a differential amplifier, 21, 2
2... Front drive transistor, 24... Level shift circuit, 25, 26... Complementary transistor configuration, 7... Signal current limiting resistor, 10, 11...
Diode for DC voltage level shift, 19... Zener diode.

Claims (1)

[Claims] 1: an input terminal, an output terminal, a feedback terminal, a power supply terminal, a ground terminal, an input circuit connected to the input terminal, a differential amplifier that amplifies the output signal of the input circuit, and the differential amplifier a pre-drive amplifier for amplifying an output signal of the pre-drive amplifier; and a single-ended push-pull circuit comprising a complementary transistor configuration for amplifying the output signal of the pre-drive amplifier and outputting the amplified signal to the output terminal, and biasing the output terminal. In the power amplifier connected to the feedback terminal via a resistor, the input circuit includes an emitter follower transistor, the base of the emitter follower transistor is connected to the input terminal via the resistor, and the emitter is connected to level shifting means. A power amplifier, characterized in that the power amplifier is connected to the input terminal of the differential amplifier via the input terminal of the differential amplifier.