JPS6214732Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a power amplifier bias circuit suitable for use in a class A push-pull power amplifier.

In general, in a class A push-pull power amplifier, when it is operated with a large amplitude such that the output signal clips under no load conditions, the idling current in the output stage becomes zero and the amplification element in the output stage is cut off. This causes problems such as large fluctuations in the power supply voltage. For example, FIG. 1 is a diagram showing an example of a conventional class A push-pull power amplifier that has such problems. That is, the power amplifier shown in FIG. 1 includes transistors 1 and 2 configured as a differential amplifier and a current mirror circuit 3 that constitute a driver stage, and an output stage that is driven by transistors 1 and 2 and is connected in a complementary SEPP. The transistors 4 and 5 of the driver stage are inserted in series between the transistor 2 of the driver stage and the transistor 6 of the current mirror circuit 3, and the current flowing through these transistors 2 and 6 is used to connect the transistors 4 and 5 of the output stage. The bias voltage source (constant voltage circuit) 10 includes a transistor 7 and resistors 8 and 9 to supply a bias to the circuit. However, when this power amplifier is used in these no-load conditions and the input signal changes with a large amplitude that causes the output signal to clip, the following problems occur. That is, when a large amplitude input signal is applied to the input terminals 11a and 11b of this power amplifier, during a period when the output signal obtained at the output terminal 12 is clipped, for example, when transistor 2 is in the on state, transistor 1 is cut off, the transistor 6 is turned off, and the current flowing through the bias voltage source 10 becomes zero, so that the bias voltage of the output stage transistors 4 and 5 becomes zero, and the idling current flows through these transistors 4 and 5. becomes zero. Figure 2 a-c
is the waveform of the output signal _E0 obtained at the output terminal 12 in the above operation and the output stage transistor 4,
5 is a diagram showing the waveforms of the idling currents Ii ₁ and Ii ₂ flowing through the motor. Therefore, during the above operation, as the average value of the power supply current decreases, the power supply voltage increases, causing the risk that each element may be destroyed due to overvoltage resistance, and the level of the input signal decreases. In some cases, the cut-off transistor will not return to normal operation quickly (a certain rise time is required to return to normal operation), and if this amplifier is a negative feedback amplifier, problems such as oscillation will occur. .

In view of the above-mentioned circumstances, this idea was developed to ensure idling current in the output stage and prevent fluctuations in power supply voltage, oscillation, etc. even when the power amplifier operates with a large amplitude under no-load conditions and the output signal clips. This circuit provides a bias circuit for a power amplifier that allows push-pull connection of output stage transistors, a driver stage transistor that drives these output stage transistors, and a driver stage transistor connected in series with the driver stage transistor. A power amplifier having a bias voltage source that supplies bias to the transistors in the output stage, wherein auxiliary transistors are connected in cascode to the transistors in the output stage, and the bases of these auxiliary transistors are connected to the emitters of the transistors in the output stage. A constant voltage source is inserted between them, current path forming transistors are connected in series to the bias voltage source, and each collector voltage of the output stage transistor is applied to each base of these current path forming transistors. , the bias voltage source is characterized in that a constant current is caused to flow through the bias voltage source regardless of the value of the current flowing through the transistor of the driver stage.

An embodiment of this invention will be described below with reference to FIGS. 3 and 4.

FIG. 3 is a diagram showing the configuration of the first embodiment of this invention, which is a class A power amplifier PA ₁ according to this invention.
FIG. As shown in this figure, the power amplifier PA ₁ includes transistors 21 and 22 and a current mirror circuit 23 constituting a driver stage, transistors 24 and 25 in an output stage connected in a complementary SEPP, a transistor 22, and a current mirror circuit 23. A bias voltage source 30 is inserted in series with the transistor 26 of the circuit 23 and includes a transistor 27 and resistors 28 and 29 for supplying bias to the output stage transistors 24 and 25. In this case, the above configuration is the first
The configuration is the same as that of the power amplifier shown in the figure. The difference between this power amplifier PA ₁ and the power amplifier shown in FIG. The points connected in cascode, and the bases of transistors 50 and 51 and transistors 24 and 2
At this point, a constant voltage was applied between each emitter of 5 by power supplies 52 and 53 (voltage values are V ₅₂ and V ₅₃ respectively).

According to the above-described configuration, the power supply 52 → the base of the transistor 50 → the emitter of the transistor 50 → the base of the transistor 54 → the transistor 5
4 emitter → resistor Rc → base of transistor 24 → emitter of transistor 24 → power supply 52
The value of the current I ₁ flowing through the loop is I ₁ = (V ₅₂ −3・VBE)/Rc...(1) (VBE is the voltage drop between the base and emitter of the transistor), and from equation (1) above, It can be seen that a constant current flows through the base of transistor 54, and that transistor 54 operates as a constant current source. That is, the constant current supply circuit 31 includes a transistor 54, a resistor Rc,
It is composed of a transistor 50 and a power supply 52, with the transistor 54 and resistor Rc forming a constant current generation side, and the transistor 50 and power supply 52 forming a constant voltage generation side. As is well known, when configuring a constant current circuit, a constant voltage generating means is required.
In this circuit, as described above, the power supply 52 and the transistor 50 connected in cascode form a floating constant voltage generation source. Further, in the same manner as above, the constant current supply circuit 32
Transistor 55, resistor Rd, transistor 5
1, and a power supply 53.

The power amplifier PA ₁ with the above configuration is used under no-load conditions, and even when the input signal changes with a large amplitude that causes the output signal to clip, the power amplifier PA 1 shown in Fig. No inconvenience will occur.

That is, even if a large amplitude input signal is applied to the input terminals 41a and 41b and, for example, the transistor 21 is cut off, the bias voltage source 3
0 is supplied with a constant current by the constant current supply circuits 31 and 32, the bias voltage generated by the bias voltage source 30 is secured, and thereby,
The idling current of the output stage transistors 24 and 25 is ensured. Therefore, this power amplifier
In PA ₁ , the power supply current does not change significantly even in the above operation, and the positive and negative power supply terminals 3
3, 37, the power supply voltage obtained does not change. 4a to 4c are diagrams showing the waveform of the output signal F ₀₁ obtained at the output terminal 42 and the waveforms of the idling currents Ii ₁₁ and Ii ₂₁ flowing through the output stage transistors 24 and 25 in the above-mentioned no-load operation. It is.

Furthermore, in the power amplifier PA ₁ , the emitter-collector voltage of each of the output stage transistors 24, 25 is made constant by the cascode-connected transistors 50, 51, and as a result,
Deterioration of high frequency characteristics caused by feedback capacitance and stray capacitance of transistors 24 and 25 is prevented.

In this way, the transistors 50 and 51 in the above embodiment have the function of improving the high-frequency characteristics of the transistors 24 and 25, respectively, and also function as constant voltage generation sources for the constant current supply circuits 31 and 32.

As explained above, according to this invention, there are push-pull connected transistors in the output stage, transistors in the driver stage that drive these transistors in the output stage, and transistors in the output stage connected in series with the transistors in the driver stage. A power amplifier having a bias voltage source that supplies bias to the transistors, wherein auxiliary transistors are connected in cascode to each of the transistors in the output stage, and a constant voltage is applied between the bases of these auxiliary transistors and the emitters of the transistors in the output stage. A current path forming transistor is connected in series to the bias voltage source, and each collector voltage of the output stage transistor is applied to each base of the current path forming transistor, and the bias voltage source is connected in series to the bias voltage source. Since a constant current is caused to flow through the source regardless of the value of the current flowing through the transistor in the driver stage, even if the power amplifier is operated with a large amplitude under no load and the output signal is clipped, the idling of the output stage will be reduced. The advantage is that it is possible to secure current, prevent the power supply voltage from rising, and prevent overvoltage of electrical components, and also shorten the response time of the output stage transistor to the signal and prevent oscillation. is obtained. In addition, the cascode-connected auxiliary transistor stabilizes the emitter-collector voltage of each output stage transistor, thereby preventing deterioration of high-frequency characteristics caused by feedback capacitance and stray capacitance of the output stage transistor. Ru. Moreover, since the auxiliary transistor that improves the high-frequency characteristics of the output stage transistor also serves as a constant voltage generation source for driving the current path forming transistor at a constant current, there is no need to provide a separate constant voltage generation means. This effect has also been achieved.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an example of a conventional class A power amplifier, FIGS. 2 a to c are diagrams showing the output signal waveform and idling current waveform of the power amplifier shown in FIG. 1, and FIG. Circuit diagram showing the configuration of a class A power amplifier PA ₁ with a bias circuit according to
Figures a to c are diagrams showing the output signal waveform and idling current waveform of the class A power amplifier _PA1 . PA ₁ ... Class A power amplifier, 21, 22... Driver stage transistor, 24, 25... Output stage transistor, 30... Bias voltage source, 50,
51...transistor (auxiliary transistor), 5
2, 53... Power supply (constant voltage source), 54, 55...
Transistor (current path forming transistor).

Claims (1)

[Claims for Utility Model Registration] A push-pull connected output stage transistor, a driver stage transistor that drives these output stage transistors, and a driver stage transistor connected in series with the driver stage transistor to supply a bias to the output stage transistor. In a power amplifier having a bias voltage source, auxiliary transistors are connected in cascode to each of the output stage transistors, and a constant voltage source is inserted between each base of these auxiliary transistors and each emitter of the output stage transistor. Further, current path forming transistors are connected in series to the bias voltage source, and each collector voltage of the output stage transistor is applied to each base of these current path forming transistors, and the driver stage is connected to the bias voltage source. A power amplifier bias circuit characterized in that a constant current is caused to flow regardless of the value of the current flowing through the transistor.