JP2008131630A - Power amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power amplifier capable of making band frequency broad without increasing a distortion rate. <P>SOLUTION: A positive/negative drive current for driving a final output element is partially charged and supplied by a first drive means including a current amplifying function and a second drive means 2 for transmitting a signal voltage, respectively and further, a bias current of the drive means 2 is stabilized so that the output element is accurately driven even during high-speed operation. Thus, broadbanding and distortion reduction of the power amplifier can be performed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a drive circuit that drives an output current amplifying element at high speed in a power amplifier that mainly handles audio signals.

  For example, circuits as shown in FIGS. 4 and 5 are generally used as conventional power amplifiers. In the example of FIG. 4, the output stage operating with push-pull and the drive stage for driving it are “Darlington connected”, and the base current of the output transistor is supplied from the emitter of the drive transistor Q1 or Q2.

  In FIG. 5, MOS / FET is used as an output element, and Q5 and Q6 operate complementarily to supply a necessary current to the load. The drive circuit controls the gate voltage of the output element by the emitter follower circuit of the transistor Q1 or Q2.

  The circuit shown in FIG. 6 is a push-pull configuration of the drive circuit shown in FIG. 4 (see, for example, Patent Document 1), and is used in a drive circuit of a digital amplifier.

Patent Publication 2000-106508

  In the conventional circuit example shown in FIGS. 4 and 5, when the reproduction band is to be expanded, it does not have a sufficient drive capacity corresponding to the expansion. In general, a relatively large input capacitance (Cin) is present as an output element, whether it is a bipolar transistor or a MOS / FET. In order to obtain an accurate output voltage, at least the time variation (dv / dt) of this signal voltage. It is necessary to supply a drive current component i (i = Cin × dv / dt) proportional to. In any of the circuit examples, the current driving capability is not symmetrical in the discharge direction (source direction) and the suction direction (sink direction), and signal time delay or distortion occurs. As a result, there is a limit to widening the band, and there is a drawback that adjustment of the phase or the like is necessary with the voltage amplifier in the previous stage.

  On the other hand, the circuit example of FIG. 6 is an improvement in the asymmetry of the drive capability, but there is a difficulty in the stability of the bias, and the output generated by the temperature change that causes the crossover distortion or the drive element Fluctuation of bias current was a problem.

  For this reason, this type of circuit is mainly used in a switching output circuit (class D) which does not particularly cause distortion, and is often used in an output stage of a digital amplifier or a switching regulator.

  The present invention provides means for expanding the frequency band of a power amplifier without sacrificing distortion characteristics.

  According to the power amplifier of the present invention, the control terminal (base or gate) of the output element is driven by a pair of drive means as described in claim 1, and is necessary in both directions of inflow and outflow current to the control terminal. While ensuring current supply capability, one of the drive means has a voltage transmission function, thereby providing a stable bias state to the output stage constituting the push-pull circuit.

  The drive means 2 having a voltage transmission function, the current comparator 3 that compares the current output from 2 with a preset reference current, and the drive means 1 that amplifies the difference signal are biased by the drive means 2. Since the control system is configured to keep the current constant, the output impedance of the drive circuit can be kept low, and the input signal voltage can be transmitted to the output terminal of the power amplifier with low distortion.

  As described above, according to the present invention, the output element operating in push-pull is driven by the low-impedance drive means 1 and 2 in a complementary manner, so that a wide-band and low-distortion power amplifier can be obtained. provide.

  Hereinafter, a specific embodiment of the present invention will be described with reference to FIG. FIG. 2 is a specific example of a circuit configuration using a bipolar transistor.

  Transistors Q5 and Q6 are push-pull output circuits composed of NPN and PNP transistors, respectively. The drive circuit for driving the output transistor has a symmetric configuration with respect to each output element as shown in FIG. 1, and only the drive circuit for the NPN transistor Q5 is shown here for the sake of simplicity. Yes.

  The output signal of the voltage amplifier 4 is input to the base of the drive transistor Q2. The collector of the transistor is connected to a current comparator 3, and the current comparator 3 outputs a signal (Id) proportional to a difference between a preset reference current (Ir) and the collector current (Ic2). To do. Id is supplied to the drive means “1” having a current amplification function, and its output current (I1) is supplied to the emitter of Q2 and the base of the output transistor Q5.

  If the base current of Q5 and the emitter and collector currents of Q2 are Ib5, Ie2, and Ic2, respectively, the current gain of drive means 1 is A2, the gain of current comparator 3 is A1, and the base current of Q2 is negligible. , Ib5 is expressed by Ib5≈ (A1 + A2) × (Ir−Ic2) −Ie2, Ie2≈Ic2. By transforming the equation, Ie2 becomes Ie2≈ (A × Ir−Ib5) / (A + 1), A = (A1 × A2). If the total current gain A is sufficiently large, Ie2≈ Ir is established. Therefore, the series of circuit operations described above constitutes a kind of servo system (control system) in which Ie2 is controlled to be a constant value (Ir) and Ib5 is regarded as a disturbance element.

  On the other hand, the transistor Q2 has the voltage transmission function described in claim 1, and its output impedance is such that when the emitter output resistance is re (= 1 / gm), the base current is controlled under the condition that the emitter current is controlled to a constant value. Since the AC voltage drop (Ie2 × re) between the emitters can be regarded as a constant voltage source having a constant voltage, the AC voltage is kept at a very low value.

  On the other hand, for a load whose phase relationship between current and voltage is not always constant, even if the load current fluctuates without correlation with the output voltage, Ib5 is a disturbance factor for the drive circuit. In effect, the base current required by the output transistor is always properly traced. As a result, the power amplifier can always maintain good distortion characteristics.

  Further, for a transient response at the rise or fall when a square wave is input, the drive means 1 supplies in the base current injection direction, and the transistor Q2 shares the collector minority carrier discharge direction. . By designing each current supply capability equally (symmetrically), the linearity of the circuit is less disturbed even in a transient operation, and the output transistor can be driven with a low impedance. In particular, even when the drive means 2 supplies a drive current exceeding the control range of the servo system, the output current of the drive means 1 is quickly cut off, and the transistor Q2 operates as a normal emitter follower. The signal voltage is properly transmitted to the output element.

  For example, assuming that the collector output capacity of Q5 is Cob and the emitter-base capacity is Cib, in order to secure a slew rate of ± V (dv / dt), at least for charging the equivalent input capacity, | Ib5 | ≧ | It is necessary to supply a positive and negative drive current represented by (Cob + Cib) × V |. The present invention provides means for raising the positive and negative drive currents symmetrically and at high speed, and the signal delay time of the power amplifier is kept extremely small as long as necessary current supply performance is ensured.

  On the other hand, when the output voltage of the power amplifier is fed back to the voltage amplifier 4, the phase characteristic of the high frequency is an important factor in considering stability, but the group delay of the power amplifier is within the control band of the servo system. Since the phase delay is almost negligible, the bare characteristic of the voltage amplifier becomes dominant. In other words, according to the present invention, by setting the bandwidth of the voltage amplifier within the bandwidth of the servo circuit, it is possible to provide a low distortion amplifier that is not affected by the load, and the design flexibility of the voltage amplifier can be expanded. In addition, widening the bandwidth of the amplifier can be achieved by expanding the control band of the servo system.

  FIG. 3 shows another embodiment of the present invention in which a power MOS • FET is used as an output element. The basic operation is the same as that of the bipolar transistor described above, but a constant voltage source 7 for setting a bias voltage between the gates of the push-pull output circuit is added, and the class A is adjusted by adjusting this voltage source. The bias state of the output stage can be arbitrarily set from A to D.

  Of course, the power amplifier of the present invention can also be configured by using a MOS FET for the drive means 1 and 2. Further, in this example, an example in which the voltage comparator 9 is used for the current comparator 3 is shown.

  When the output element is a MOS / FET, there is no need to supply a drive current corresponding to the output load current unlike a bipolar transistor, and there is no concept of a transition frequency, but the gate input capacitance is relatively large. There are advantages and disadvantages in expanding the frequency bandwidth. Needless to say, in any case, the effect of the present invention is not changed.

Configuration diagram of the present invention FIG. 3 is a circuit configuration diagram for explaining the operation of the embodiment of the present invention. Circuit configuration diagram showing another embodiment of the present invention Circuit configuration diagram showing an embodiment of a conventional power amplifier Circuit configuration diagram showing another embodiment of a conventional power amplifier Circuit configuration diagram showing another embodiment of a conventional power amplifier

Explanation of symbols

1 Drive means 1 of the present invention
2 Drive means 2 of the present invention
3 Current comparator 4 Voltage amplifier 5 Output element (Q5)
6 Output element (Q6)
7 Constant voltage source (Bias voltage circuit)
8 Load of power amplifier 9 Voltage comparator Q, Q1 to Q7 Transistors Q1a, Q2a Transistors R, R5 to R6 Resistor D Diode IN Signal input terminal OUT Output terminal of power amplifier + B + Power supply terminal −B −Power supply terminal Is1 , Is2 constant current source

Claims (1)

  A power amplifier drive circuit in which a current for driving an output element is supplied by two independent drive means in a source direction and a sink direction, a first drive means having a current amplification function, and a signal voltage transmission function And a current comparator that compares the bias current of the second drive means with a preset reference current value, and the output signal of the current comparator is the first drive. A power amplifier characterized in that the bias current of the second drive means is kept constant by controlling the output current of the means.

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