JP6111712B2 - Amplifier circuit - Google Patents

Description

  The present invention relates to an amplifier circuit for amplifying a signal.

  A conventional general class B amplifier circuit is provided with a bias circuit in order to reduce crossover distortion. (For example, refer to Patent Document 1).

  FIG. 1 is a circuit diagram showing the main configuration of a conventional class B amplifier circuit. The class B amplifier circuit shown in FIG. 1 includes a voltage amplifier circuit 20 and a current amplifier circuit 10. The current amplifier circuit 10 includes four transistors (transistors Q11, Q12, Q13, and Q14), three resistors (resistors R11, R12, and R13), and a bias circuit 100.

  The npn transistor Q11 and the transistor Q13 are Darlington connected. Similarly, the pnp transistors Q12 and Q14 are also connected in Darlington. The collectors of the transistors Q11 and Q31 are connected to the positive power supply, and the collectors of the transistors Q12 and Q14 are connected to the negative power supply. A resistor R11 is connected between the emitters of the transistors Q11 and Q12. The emitters of the final stage transistor Q13 and transistor Q14 are connected to the output terminal via a resistor R12 and a resistor R13, respectively.

  The bias circuit 100 is a circuit that maintains a constant voltage between the bases of the transistors Q11 and Q12. The bias circuit 100 sets the voltage between the bases of the transistor Q11 and the transistor Q12 so that a voltage for turning on each transistor (hereinafter referred to as Vbe) is applied between the base and emitter of the transistors Q1 to Q4. Has been. As a result, even when there is no input or when the input is at a low level (hereinafter referred to as idling), it is possible to eliminate the state where current flows in the final stage and the load cannot be driven, and crossover distortion can be reduced. it can.

JP-A-7-74551

  However, since there is a variation in Vbe for each transistor, it is necessary to adjust the bias circuit itself for each device so that each transistor is not cut off.

  Moreover, since Vbe changes with heat generation in each transistor, temperature compensation is required for the bias circuit. That is, it is necessary to detect the heat generation of the final stage transistor and feed it back to the bias circuit (for example, it is necessary to thermally couple the transistor in the bias circuit with the final stage transistor).

  Further, when Vbe of the final stage transistor changes due to heat generation, the current value increases with temperature compensation, and thus the final stage transistor requires emitter resistors (resistors R1 and R2 shown in FIG. 1). In this case, the maximum output decreases or the efficiency decreases.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an amplifier circuit that suppresses efficiency reduction and does not require various adjustments and temperature compensation.

  The amplifier circuit of the present invention is an amplifier circuit that operates the Darlington connection transistors in the output stage in a complementary manner. The amplifier circuit according to the present invention further includes a bias circuit that applies a voltage to the Darlington connection transistor, and the bias circuit cuts off one of the transistors in the final stage among the Darlington connection transistors during idling. It is characterized by.

  Specifically, the bias circuit is a negative feedback circuit including a bias voltage application transistor and a voltage feedback resistor connected to the base of the bias voltage application transistor. One end of this voltage feedback resistor is connected between the emitter of the preceding transistor and the base of the succeeding transistor in the Darlington connection transistor. In addition, an idling current setting resistor is connected between the base and emitter of the subsequent transistor.

  By adjusting the value of the idling current setting resistor, the idling current is determined. Therefore, if the voltage feedback resistor of the bias circuit is set appropriately, the voltage between the base and the emitter of the final stage transistor (the rear stage transistor) can be maintained at a value that cuts off the rear stage transistor. In this case, at the time of idling, a current flows only through the previous stage transistor.

  Of the last-stage transistors in the Darlington connection, the transistors on the side that is not cut off at the time of idling can be inverted connection.

  According to the present invention, efficiency reduction is suppressed, and adjustment and temperature compensation become unnecessary.

It is a circuit diagram which shows the main structures of the conventional class B amplifier circuit. 1 is a circuit diagram showing a main configuration of a class B amplifier circuit according to a first embodiment. FIG. It is a figure which shows the electric current at the time of idling. It is a circuit diagram which shows the main structures of the class B amplifier circuit which concerns on 2nd Embodiment. It is a circuit diagram at the time of switching positive / negative.

  Hereinafter, an amplifier circuit of the present invention will be described with reference to the drawings. FIG. 2 is a circuit diagram showing a main configuration of the class B amplifier circuit according to the first embodiment.

  The amplifier circuit shown in FIG. 2 includes a current amplifier circuit 1 and a voltage amplifier circuit 2. In practice, there are other configurations such as a circuit for phase adjustment, but they are not shown in the present embodiment and will not be described.

  The amplifier circuit of this embodiment is used as an input signal, for example, as a power amplifier that amplifies an audio signal.

  The voltage amplifier circuit 2 includes three transistors (transistors Q1, Q2, and Q3), eight resistors (resistors R51 to R58), a capacitor C1, and a capacitor C2.

  The emitters of the pnp-type transistor Q1 and transistor Q2 are each connected to the positive power supply via a resistor R52. The collector of the transistor Q1 is connected to the negative power supply via the resistor R53, and is connected to the base of the npn transistor Q3. The base of the transistor Q1 is connected to the input terminal. The input terminal and the base of the transistor Q1 are connected to the ground via a resistor R51 and are grounded.

  The collector of the transistor Q2 is connected to the negative power supply. The base of the transistor Q2 is connected to the output terminal via the resistor R55. The output terminal and the base of the transistor Q2 are connected to the ground via a resistor R54 and are grounded.

  These transistor Q1, transistor Q2, resistor R54, and resistor R55 function as a circuit for stabilization (feedback).

  On the other hand, the collector of the transistor Q3 is connected to the positive power supply via the capacitor C2, the resistor R57, and the resistor R56. The emitter of the transistor Q3 is connected to the negative power source via the resistor R58.

  The current amplifier circuit 1 is connected between the positive power supply and the collector of the transistor Q3. Thereby, the voltage amplification circuit 2 amplifies the voltage of the input signal with a predetermined amplification factor by appropriately setting the values of the resistors R52 to R57, and applies the amplified voltage to the current amplification circuit 1. .

  A capacitor C1 is connected between the resistor R56 and the resistor R57 and between the input / output line (connection line between the input terminal and the output terminal). Capacitor C1 uses a capacitor having a large capacity, such as an electrolytic capacitor, cuts off the DC component from the power supply, and allows only the AC component corresponding to the input signal to pass. The capacitor C2 is connected to the positive electrode through the resistor R56 and the resistor R57 and is connected to the collector of the capacitor Q3. The capacitor C2 has a smaller capacity than the capacitor C1, and will be described later. It functions as a capacitor for operation of the bias circuit 50.

  Next, the current amplifier circuit 1 will be described. The current amplifier circuit 1 includes four transistors (transistors Q5, Q6, Q7, and Q8), a resistor R5, a resistor R6, and a bias circuit 50.

  The npn transistor Q5 and the transistor Q7 are Darlington connected. Similarly, the pnp transistors Q6 and Q8 are also connected in Darlington.

  The collectors of the transistors Q5 and Q7 are connected to the positive power supply, and the collectors of the transistors Q6 and Q8 are connected to the negative power supply.

  A resistor R5 and a resistor R6 are connected between the emitters of the transistor Q5 and the transistor Q6. Further, the resistor R5 and the resistor R6 are connected to the output terminal. These resistors R5 correspond to the idling current setting resistors of the present invention.

  The emitters of the final stage transistor Q7 and transistor Q8 are each connected to an output terminal.

  With these circuit configurations, the current amplifying circuit 1 causes the Darlington-connected transistor made up of the transistors Q5 and Q7 and the Darlington-connected transistor made up of the transistors Q6 and Q8 to operate in a complementary manner, and the input signal is amplified at a predetermined amplification factor. Amplifies current. The amplified current is output from the output terminal. For example, if the input signal is an audio signal, the amplified audio signal is output from the output terminal and supplied to the speaker.

  Next, the bias circuit 50 will be described. The bias circuit 50 is a negative feedback circuit, and includes a transistor Q4 and a resistor R1 and a resistor R2 connected to the base of the transistor Q4. The transistor Q4 corresponds to the bias voltage applying transistor of the present invention. The resistors R1 and R2 correspond to the voltage feedback resistors of the present invention.

  Here, one end of the resistor R1 is connected to the base of the transistor Q7 (emitter of the transistor Q5), and one end of the resistor R2 is connected to the base of the transistor Q6 (emitter of the transistor Q4).

  Therefore, the bias circuit 50 functions as a negative feedback circuit that keeps the voltage between the bases of the transistors Q7 and Q6 constant during idling.

  For example, assuming that the voltage between the bases of the transistors Q7 and Q6 is set to 1.5 V by the voltage feedback resistors R1 and R2, the transistors Q6 and Q8 are turned on. Vbe of each transistor is about 0.6V, and is about 1.2V in total. The remaining voltage of about 0.3 V is applied between the base and emitter of Q7. Since this voltage is sufficiently lower than Vbe of the transistor Q7, the transistor Q7 is cut off. Therefore, the idling current is determined by the resistance value of the resistor R5 (for example, the current I = 0.3 / R5). The resistor R6 is set to a sufficiently large value (for example, R5: R6 = 1: 2) compared to the resistor R5.

  In this case, at the time of idling, the transistor Q7 remains cut off, so that the positive power supply is switched from the negative power supply to the negative power supply, as shown by the solid line arrow in FIG. The current will flow through.

  As a result, no idling current flows through the transistor Q7 in the final stage, so that the transistor Q7 does not generate heat during idling. Further, since the heat generation of the transistors Q6 and Q8 is slight in the idling state, the decrease in Vbe of these transistors Q6 and Q8 has almost no effect on the cutoff of the transistor Q7. Therefore, the bias circuit 50 does not require temperature compensation, and it is not necessary to thermally couple the transistor Q7 in the final stage and the transistor Q4 in the bias circuit.

  As described above, Vbe of the final stage transistor changes due to heat generation, but the sensitivity to the idling current is low, and under the condition that the transistor Q7 is cut off at the time of idling, no emitter resistance is required in the final stage transistor. There is no decrease in output or efficiency.

  Furthermore, if the ratio of the resistors R1 and R2 is set so that the voltage applied to the transistor Q7 in the final stage is sufficiently lower than the operation Vbe of the transistor Q7, the Vbe due to the individual difference of the transistors. Even if there is some variation, adjustment of the bias circuit for each device is not necessary because the transistor Q7 is cut off. For example, in the above-described example, since the voltage of 0.3 V, which is sufficiently lower than the variation of Vbe, is set to be applied to the base of the transistor Q7, Vbe varies by, for example, about ± 10 mV. However, the transistor Q7 maintains the cutoff.

  Even if the transistor Q7 itself generates heat when driving the load, the voltage applied to the base of the transistor Q7 is sufficiently low so that the transistor does not turn on during idling. For example, even if the Vbe of the transistor Q7 is 0.6 V and the temperature characteristic is about 3 mV / ° C., the transistor Q7 remains cut off until the temperature rises to about 100 ° C. Note that the voltage applied to the base of the transistor Q7 can be set to be lower (for example, 0.2 V). In this case, the margin for temperature rise is further increased.

  Next, FIG. 4 is a circuit diagram showing a main configuration of the class B amplifier circuit according to the second embodiment. The components common to those in FIG. 2 are denoted by the same reference numerals and description thereof is omitted.

  The amplifier circuit shown in FIG. 4 is one in which the negative-side final stage transistor (transistor Q8 in FIG. 2) of the complementary Darlington-connected transistors is changed to an npn-type transistor, and the negative side is inverted Darlington connection. It is.

  Also in this case, if a voltage (for example, 0.3 V) is applied to both ends of the resistor R5 so that the transistor Q7 is cut off, the positive power supply is connected to the negative power supply from the transistor Q5 to the input / output line and A current flows through transistor Q80.

  In this case, the voltage between the base and the emitter of the transistor Q6 is halved (about 0.6 V) compared to the circuit of FIG.

  In the amplifier circuit shown in FIG. 4, Vbe of the negative-side final stage transistor Q80 is applied to the outside of the bias circuit 50. Therefore, even if the temperature of the transistor Q80 rises and Vbe falls, There is no effect and thermal stability is further improved.

  In both the first and second embodiments, if the transistor Q4 and the transistor Q6 are mounted on the same substrate, the heat generated by the transistor Q6 is transmitted to the transistor Q4 to some extent, so that it functions as temperature compensation for the transistor Q6. Can be expected to do.

  In the first embodiment and the second embodiment, the two-stage Darlington connection transistor has been described. However, the present invention can be applied to, for example, a three-stage Darlington connection transistor.

  Further, as shown in FIG. 5A, the same effect can be obtained even in a circuit in which the polarity of the transistors after the transistor Q4 in the circuit shown in FIG. it can. That is, the ratio of the resistance R1 connected to the base of the transistor Q6 and the resistance R2 connected to the base of the transistor Q7 is set so that the voltage applied to the transistor Q7 in the final stage is sufficiently lower than the operation Vbe of the transistor Q7. Set to voltage. For example, if the voltage between the bases of the transistors Q6 and Q7 is set to about 1.5V, for example, the transistors Q6 and Q8 are turned on, and the voltage applied to the transistor Q7 is the remaining voltage of about 0.3V. Transistor Q7 can be cut off. Also in this case, at the time of idling, current flows from the transistor Q8 through the input / output line and the transistor Q5 as shown by the thick solid arrow in FIG. 5A, which is the same as the circuit shown in FIG. An effect can be obtained. Also in this case, as shown in FIG. 5B, by replacing the final stage transistor Q8 with a pnp transistor Q80, the voltage between the base and the emitter in the transistor Q6 is halved (about 0.6V). And Vbe of the transistor Q80 can be applied outside the bias circuit 50.

Q4, Q5, Q6, Q7, Q8 ... Transistors R1, R2, R5, R6 ... Resistor 1 ... Current amplifier circuit 2 ... Voltage amplifier circuit 50 ... Bias circuit

Claims (2)

An amplifier circuit that operates the Darlington connection transistor of the output stage in a complementary manner,
A bias circuit for applying a voltage to the Darlington connection transistor;
The bias circuit is a voltage feedback circuit including a bias voltage application transistor and a voltage feedback resistor connected to the base of the bias voltage application transistor.
In the Darlington connection transistor, one end of the voltage feedback resistor is connected between the emitter of the preceding transistor and the base of the succeeding transistor, and an idling current setting resistor is connected between the base and emitter of the succeeding transistor. Thus, at the time of idling, one of the transistors in the final stage among the Darlington connection transistors is cut off. The amplifier circuit according to claim 1 ,
Of the Darlington-connected transistors, the final-stage transistor that is not cut off at the time of idling is an inverted circuit.

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