Power amplifier

Abstract

To provide a power amplifier for amplifying an audible frequency band input signal with improved various characteristics such as a distortion characteristic at a low cost.SOLUTION: A power amplifier 1a includes: an amplifier 11 for amplifying an input signal and outputting it from an output Ot1 as a first signal; an amplifier 12 for amplifying the first signal from the amplifier 11 and outputting it from an output Ot2 as a second signal; an amplifier 13 for amplifying the second signal from the amplifier 12 and outputting it from an output Ot3 as a third signal; a capacitor C connected between the output Ot1 and a mixing node Mxd; a resistor R1 connected between the output Ot2 and the mixing node Mxd; an inductor L1 connected between the output Ot3 and the mixing node Mxd; an inductor L2 connected between the mixing node Mxd and a load resistor RL; and a circuit for negatively feeding back a mixed signal at the mixing node Mxd to an input of the amplifier 11.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to, for example, a power amplifier.

Background Art

[0002] In a power amplifier for audio, when amplifying an input signal in an audible band with a frequency of 20 Hz to 20 kHz and outputting it to a load such as a speaker, it is required to amplify with a low distortion rate. As a technique for amplifying with such a low distortion rate, for example, the technique described in Patent Document 1 is known. Patent Document 1 discloses a power amplifier including an amplification stage for voltage amplification and an output stage for current amplification, and among these, the output stage is composed of a plurality of stages. In this configuration, impedances (Z1 to Z4) are provided to cancel the distortion of the output signal. Specifically, the connection point with the load is connected to the output of the amplification stage via an impedance (Z3) and to the output of the output stage via an impedance (Z1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the power amplifier described in Patent Document 1, since the distortion characteristics are improved by feedforward, high precision is required for circuit elements. Such high-precision circuit elements are generally expensive, resulting in a problem of high cost. In view of the above circumstances, one aspect of the present disclosure aims to provide a technique for improving various characteristics such as distortion characteristics in a power amplifier that amplifies an input signal in an audible band while suppressing costs.

Means for Solving the Problems

[0005] A power amplifier according to one aspect of the present disclosure includes: a first amplifier that amplifies an input signal and outputs it as a first signal from a first output; a second amplifier that amplifies the first signal and outputs it as a second signal from a second output; a third amplifier that amplifies the second signal and outputs it as a third signal from a third output; a capacitor connected between the first output and a mixing node; a first resistor connected between the second output and the mixing node; a first inductor connected between the third output and the mixing node; a second inductor connected between the mixing node and a load; and a feedback circuit that negatively feeds back the mixed signal of the mixing node to the input of the first amplifier. [Brief explanation of the drawing]

[0006] [Figure 1] This is a circuit diagram showing the configuration of a power amplifier according to an embodiment. [Figure 2] This figure shows the frequency-gain characteristics of the power amplifier according to the embodiment. [Figure 3] This diagram shows a specific example of a power amplifier. [Figure 4] This diagram shows the first modified example of a power amplifier. [Figure 5] This figure shows a second modified example of the power amplifier. [Modes for carrying out the invention]

[0007] In audio power amplifiers, negative feedback is used to reduce distortion when amplifying input signals in the audible range. Generally, open-loop gain attenuates as the frequency increases, especially at high frequencies. To apply negative feedback stably, the open-loop gain must be adjusted to be below 0 dB at frequencies lower than the frequency at which the high-frequency phase may rotate by nearly 180 degrees and cause oscillation.

[0008] In power amplifiers, the distortion is greatest at the output stage, and to reduce this distortion, a significant amount of negative feedback must be applied to this output stage. However, the output stage, which handles large currents, has inferior responsiveness compared to output stages that handle small currents, and the upper limit of the frequency at which negative feedback can be applied is limited to about 1 MHz. For this reason, power amplifiers that handle large currents at the final stage are at a disadvantage in terms of distortion compared to small-signal amplifiers that handle small currents at the final stage and can apply feedback up to about 20 MHz, or high-performance operational amplifiers composed of integrated circuits.

[0009] Generally, the impedance of a speaker, which is the load of a power amplifier, is 4 or 8 ohms. The cable connecting the output of the power amplifier to the input of the speaker has stray capacitance. To prevent oscillation caused by the capacitance of the cable, an inductor (coil) of a few microhours is connected to the output of the power amplifier. Therefore, the actual load of the power amplifier is the speaker and the inductor, and if the impedance of the speaker is 4 ohms, it can be considered as (4 ohms + a few microhours). In the frequency range above 200 kHz, the load impedance of the power amplifier is dominated by the inductor's several microhours, and increases as the frequency increases.

[0010] Let's assume we extend the negative feedback bandwidth of a power amplifier from around 1 MHz to around 20 MHz. In the 20 MHz band, the load impedance is dominated by the coil inductance, ranging from tens to hundreds of ohms. Furthermore, the input signal contains almost no components in such high frequencies. Therefore, even if the maximum output current of such high-frequency components is very small compared to the output current in the audible band, it does not pose a practical problem.

[0011] Thus, audio power amplifiers have different characteristics from other power amplifiers, such as those for radio frequencies. Below, a power amplifier according to an embodiment, which is configured taking these characteristics into consideration, will be described with reference to the drawings.

[0012] Figure 1 is a circuit diagram showing the configuration of the power amplifier 1a according to the embodiment. Power amplifier 1a power-amplifies the input signal Ain output from circuit 30 and outputs the signal to the load resistor R. L It supplies power to the following: Power amplifier 1a has amplifiers 11, 12, 13, capacitor C, resistors R1, R2, and inductors L1 and L2. In this embodiment, amplifiers 11, 12, and 13 are each amplifiers capable of amplifying signals.

[0013] Amplifier 11 is an operational amplifier that amplifies the difference voltage between the input signal Ain supplied to its non-inverting input (+) and the feedback signal from the mixing node Mxd supplied to its inverting input (-), and outputs it as a first signal. The output Ot1 of amplifier 11 is connected to the non-inverting input (+) of amplifier 12 and to one of the two terminals of capacitor C. The other terminal of capacitor C is connected to the mixing node Mxd. This operational amplifier is composed of discrete circuits, integrated circuits, or a hybrid of both.

[0014] Amplifier 12 is a voltage buffer with a voltage gain of "1" that amplifies the first signal from amplifier 11 as a second signal by providing 100% negative feedback of its output. The output Ot2 of amplifier 12 is connected to the non-inverting input (+) of amplifier 13 and to one of the two terminals of resistor R1. The other terminal of resistor R1 is connected to the mixing node Mxd.

[0015] Amplifier 13 is a voltage buffer with a voltage gain of "1" that amplifies the second signal from amplifier 12 as a third signal by negative feedback of 100% of its output. The output Ot3 of amplifier 13 is connected to one of the two terminals of resistor R2. The other terminal of resistor R2 is connected to one of the two terminals of inductor L1. The other terminal of inductor L1 is connected to the mixing node Mxd and one of the two terminals of inductor L2. The inductor L1, resistor R1, and capacitor C form a crossover circuit that determines the mixing ratio of the first signal, second signal, and third signal at the mixing node Mxd. The inductor L2 is provided to prevent oscillation due to the stray capacitance of the cable up to the resistor R L up to the resistor R.

[0016] At the mixing node Mxd, the first signal from the amplifier 11 that has passed through the capacitor C, the second signal from the amplifier 12 that has passed through the resistor R1, and the third signal from the amplifier 13 that has passed through the resistor R2 and the inductor L1 are mixed at the mixing ratio, and the mixed signal is supplied to the inverting input (-) of the amplifier 11 and the inductor L2. In other words, the mixed signal is supplied to the resistor R via the inductor L2 L and is negatively fed back to the amplifier 11. That is, the path until the mixed signal at the mixing node Mxd is fed back to the inverting input (-) of the amplifier 11 is the feedback circuit.

[0017] In this embodiment, for example, the capacitor C is 2.2 μF, the resistor R1 is 45 Ω, the resistor R2 is 0.11 Ω, the inductors L1 and L2 are each 5 μH, and the resistor R L is 8 Ω.

[0018] In the power amplifier 1a, the amplifier 13 includes an output element designed to handle a large current, the amplifier 12 includes an output element designed to handle a smaller medium current, and the amplifier 11 includes an output element designed to handle an even smaller small current. Therefore, the amplifier 12 can operate faster than the amplifier 13, and the amplifier 11 can operate faster than the amplifier 12 and the amplifier 13.

[0019] In the power amplifier 1a, the low-speed amplifier 13 that outputs a large current, the high-speed amplifier 12 that outputs medium power, and the faster amplifier 11 that outputs a small current operate in parallel, and the third signal, second signal, and first signal output by them are crossovered according to the band by the crossover circuit and output.

[0020] In this configuration, slower amplifiers 12 and 13 are connected after the high-speed amplifier 11. Of these, the second signal from the preceding amplifier 12 is supplied to the mixing node Mxd via resistor R1, and the third signal from the subsequent amplifier 13 is supplied to the mixing node Mxd via resistor R2 and inductor L1.

[0021] In this embodiment, the values ​​of capacitor C, resistor R1, and inductor L1 are determined as follows: That is, the frequency at which the level of the second signal from amplifier 12 that has passed through resistor R1 and the level of the third signal from amplifier 13 that has passed through inductor L1 cross over (first crossover frequency) is such that the values ​​of inductor L2 and the load resistor R1 cross over. L The values ​​of resistor R1 and inductor L1 are determined so that the roll-off frequency is higher than that determined by the two components. Due to inductor L2, at the first crossover frequency, resistor R L The load current flowing through it becomes smaller than the maximum current that the output element of amplifier 12 can handle. Furthermore, the values ​​of capacitor C and resistor R1 are determined such that the frequency at which the level of the first signal from amplifier 11, which has passed through capacitor C, and the level of the second signal from amplifier 12, which has passed through resistor R1, cross (second crossover frequency) is higher than the first crossover frequency. Due to inductor L2, at the second crossover frequency, resistor R L The load current flowing through the circuit becomes smaller than the maximum current that the output element of amplifier 11 can handle. As specific values ​​for the three frequencies mentioned above, for example, the roll-off frequency may be approximately 250 kHz, the first crossover frequency approximately 1.4 MHz, and the second crossover frequency approximately 1.6 MHz when the load is 8 Ω.

[0022] Figure 2 illustrates the frequency characteristics of the open-loop gain in power amplifier 1a according to the embodiment. For comparison, Figure 2 also shows the open-loop gain of a conventional power amplifier. Conventional power amplifiers use slow-speed elements in the final stage output stage that can handle large currents. To prevent oscillation when negative feedback is applied, the frequency response of the open-loop gain is limited so that the frequency at which the gain, which attenuates at 6 dB / octave towards high frequencies, reaches 0 dB is approximately 1 MHz.

[0023] As shown in this figure, in the power amplifier 1a according to this embodiment, the frequency at which the open-loop gain drops to 0 dB is around 20 MHz, and when the closed-loop gain is set to 0 dB as in Figure 1, negative feedback is applied up to around 20 MHz. Comparing at the same frequency, the amount of feedback in the bandwidth of 1 kHz or higher in power amplifier 1a increases by approximately 26 dB, or about 20 times, compared to the conventional model. In this embodiment, since negative feedback can be applied more evenly and deeply than in the conventional model across high bandwidths, and the upper limit of the bandwidth to which negative feedback is applied is also expanded, voltage distortion of the output waveform can be reduced in power amplification in the audible range. Furthermore, in this embodiment, distortion characteristics are improved by normal negative feedback, so high precision is not required for the circuit elements as in conventional feedforward feedback. For this reason, in this embodiment, distortion characteristics and other characteristics can be improved in a power amplifier that amplifies input signals in the audible range while keeping costs down.

[0024] In power amplifier 1a shown in Figure 1, the closed-loop gain of the mixed signal with respect to the input signal Ain is 0 dB (1x) up to approximately 20 MHz, as shown in Figure 2. However, this is the voltage gain at the mixing node Mxd before the inductor L2, and does not include the load resistor R. L This is not the voltage gain of the output signal involved. The roll-off frequency is sufficiently far (more than 50kHz) from the audible range of 20Hz to 20kHz, so in terms of the audible range, the voltage gain of the output signal with respect to the input signal of power amplifier 1a is the same value as its closed-loop gain, i.e., 0dB (1x).

[0025] Figure 3 is a circuit diagram of a power amplifier 1b, which is a specific example of the power amplifier 1a according to the embodiment. In power amplifier 1a, buffer amplifiers 12 and 13, which have a voltage amplification factor of "1", are configured as emitter followers using complementary bipolar transistors, respectively, in power amplifier 1b, which is a specific example thereof. More specifically, in power amplifier 1b, amplifier 12 is configured as an emitter follower using npn transistor Q2p and pnp transistor Q2n, respectively, whose collectors are connected to the positive and negative power rails, and amplifier 13 is configured as an emitter follower using npn transistor Q3p and pnp transistor Q3n, respectively, whose collectors are connected to the positive and negative power rails.

[0026] In Figure 1, the output Ot2 from amplifier 11 to amplifier 12 corresponds to the emitter output Ot2p of the positive transistor Q2p and the emitter output Ot2p of the negative transistor Q2n in Figure 3. Resistors R11 and R12 are connected in series between the emitter outputs Ot2p and Ot2n. The connection point of resistors R11 and R12 is connected to the mixing node Mxd. In this specific example, resistors R11 and R12 are equivalent to resistor R1 in Figure 1. Similarly, the output Ot3 in Figure 1 corresponds to the emitter output Ot3p of the positive transistor Q3p and the emitter output Ot3n of the negative transistor Q3n in Figure 3. Resistors R21 and R22 are connected in series between the emitter outputs Ot3p and Ot3n. The connection point of resistors R21 and R22 is connected to one terminal of inductor L1. In this specific example, resistors R21 and R22 are equivalent to resistor R2 in Figure 1.

[0027] In this specific example, AC emitter outputs Ot2p and Otp2n are identical voltage sources and can be considered a single voltage source. By setting resistors R11 and R12 to 91Ω, a 45.5Ω resistor equivalent to resistor R1 is placed between this single voltage source and the mixing node Mxd. Similarly, AC emitter outputs Ot3p and Ot3n are identical voltage sources, and by setting resistors R21 and R22 to 0.22Ω and inductor L1 to 5mH, a series-connected resistor of 0.11 ohms equivalent to resistor R2 and a 5mH inductor are placed between the single voltage source and the mixing node Mxd. Thus, power amplifier 1b in Figure 3 is an equivalent specific example to power amplifier 1a in Figure 1.

[0028] In the specific example shown in Figure 3, amplifiers 12 and 13 are configured as emitter followers using bipolar transistors, but they may also be configured as source followers using field-effect transistors. Furthermore, Darlington-connected transistors may be used for the emitter followers and source followers, respectively.

[0029] Figure 4 is a circuit diagram of a power amplifier 1c according to a first modification of the power amplifier 1a in Figure 1. In the embodiment, power amplifier 1a was a voltage buffer with a closed-loop gain of "1" (=0dB) when 100% negative feedback was applied. However, the power amplifier 1c according to the first modification has a predetermined gain with less than 100% negative feedback by resistors R3 and R4. Specifically, by connecting resistor R3 between the inverting input (-) of amplifier 11 and ground, and resistor R4 between the inverting input (-) of amplifier 11 and the mixing node Mxd, power amplifier 1c has a closed-loop gain (voltage gain) of (R3+R4) / R3 at the mixing node Mxd. In the first modification, in order to keep the amount of feedback in each band on the decibel scale the same as the circuit in Figure 1, amplifier 11 is designed so that the open-loop gain before applying negative feedback is higher in the entire range than the open-loop gain shown in Figure 2 by its voltage gain (i.e., (R3+R4) / R3). By making improvements to amplifier 11 in Figure 1, such as using elements with higher gain but equivalent frequency characteristics and increasing the number of voltage amplification stages, the open-loop gain (embodiment) in Figure 2 can be shifted upward by the voltage gain without significantly changing its shape. As a result, the frequency response of the closed-loop gain of power amplifier 1b becomes almost the same as that of power amplifier 1a in Figure 1. For example, in the first modified example, if resistor R3 is 1kΩ and resistor R4 is 20kΩ, the closed-loop gain of power amplifier 1c becomes 21 (≒26dB). Note that the upper limit of the bandwidth in which the closed-loop gain is flat remains 20MHz.

[0030] Figure 5 is a circuit diagram of a second modified power amplifier 1d, which is a further modification of the power amplifier 1c having voltage gain. In the power amplifiers 1a and 1c according to the embodiment, amplifiers 12 and 13 were voltage buffers, but as shown in Figure 5, a voltage buffer 11b may be provided at the output of amplifier 11, resulting in the configuration of amplifiers 11a and 11b, thereby increasing the driving force of amplifier 11. Amplifier 11a is an operational amplifier with high gain, and the voltage buffer 11b has a voltage amplification factor of "1". In the second modified example, the values ​​of the passive elements are the same as in the first modified example in Figure 4, and the power amplifier 1d has a voltage gain of approximately 26 dB up to the upper frequency band of 20 MHz. The operational amplifier amplifier 11a is composed of discrete circuits, integrated circuits, or a hybrid of both, similar to amplifier 11. The voltage buffer 11b is composed of a complementary transistor emitter follower or source follower, similar to amplifiers 12 and 13.

[0031] From the above description, preferred embodiments of the present invention can be understood, for example, as follows.

[0032] A power amplifier according to one aspect of the present disclosure (Aspect 1) includes: a first amplifier that amplifies an input signal and outputs it as a first signal from a first output; a second amplifier that amplifies the first signal and outputs it as a second signal from a second output; a third amplifier that amplifies the second signal and outputs it as a third signal from a third output; a capacitor connected between the first output and a mixing node; a first resistor connected between the second output and the mixing node; a first inductor connected between the third output and the mixing node; a second inductor connected between the mixing node and a load; and a feedback circuit that negatively feeds back the mixed signal of the mixing node to the input of the first amplifier.

[0033] According to Embodiment 1, more negative feedback than conventional power amplifiers can be stably applied at high frequencies beyond the audible range, improving various characteristics such as distortion characteristics in the audible range. High-speed distortion that cannot be completely eliminated by negative feedback via the relatively slow third amplifier is reduced by negative feedback via the faster second amplifier, and even faster distortion that cannot be completely eliminated by the second amplifier is further reduced by negative feedback via the even faster first amplifier. Here, feedforward is not used, and high precision is not required for the circuit components. The audible range refers to the frequency range of sounds that humans can hear, specifically the frequency range of 20Hz to 20kHz. The second inductor is originally intended to prevent oscillation caused by stray capacitance in the cable from the power amplifier output to the load, but it also prevents damage to the first and second amplifiers due to overcurrent.

[0034] In a specific embodiment 2 of embodiment 1, the resistance of the first resistor and the inductance of the first inductor are determined such that the first crossover frequency between the second signal that has passed through the first resistor and the third signal that has passed through the first inductor is higher than the roll-off frequency determined by the inductance of the second inductor and the resistance of the load. In embodiment 2, at the first crossover frequency, the impedance of the second inductor becomes greater than that of the load resistance, and since there are almost no such high-frequency components in the input signal, the possibility of the load current exceeding the maximum current that the second amplifier can handle is extremely small. Furthermore, by leaving a sufficient gap between the roll-off frequency and the first crossover frequency, the load current can be reliably kept below the maximum current that the second amplifier can handle.

[0035] In a specific embodiment 3 of embodiment 2, the capacitance value of the capacitor and the resistance value of the first resistor are determined such that the second crossover frequency between the first signal that has passed through the capacitor and the second signal that has passed through the first resistor is higher than the first crossover frequency. According to embodiment 3, in which the second crossover frequency is higher than the first crossover frequency, the load current at the second crossover frequency becomes even smaller than at the first crossover frequency, and even with the first amplifier, which has a smaller maximum current capacity than the second amplifier, the possibility of the load current exceeding its maximum current is extremely small.

[0036] Another specific embodiment 4 of embodiment 1 is that the second amplifier and the third amplifier are either emitter followers or source followers, respectively. If the components of the second and third amplifiers are bipolar transistors, an emitter follower is preferred, and if the components are field-effect transistors, a source follower is preferred.

[0037] Another specific embodiment 5 of embodiment 1 is negative feedback by the feedback circuit, in which the closed-loop gain of the power amplifier in the audible range becomes a fixed value of 1 or more. According to embodiment 5, distortion can be reduced by more negative feedback in the high frequency band, including the band beyond the audible range, while the voltage gain can be kept constant across the entire band, including the audible range. [Explanation of Symbols]

[0038] 1a...Power amplifier, 11...Amplifier (1st amplifier), 12...Amplifier (2nd amplifier), 13...Amplifier (3rd amplifier), C...Capacitor, R1...Resistor (1st resistor), L1...Inductor (1st inductor), L2...Inductor (2nd inductor), 12...Amplifier (2nd amplifier), 13...Amplifier (3rd amplifier).

Claims

1. A first amplifier that amplifies the input signal and outputs it as a first signal from the first output, A second amplifier that amplifies the first signal and outputs it as a second signal from the second output, A third amplifier that amplifies the aforementioned second signal and outputs it as a third signal from the third output, A capacitor connected between the first output and the mixing node, A first resistor connected between the second output and the mixing node, A first inductor connected between the third output and the mixing node, A second inductor connected between the mixing node and the load, A feedback circuit that negatively feeds back the mixed signal of the mixing node to the input of the first amplifier, A power amplifier with the following features.

2. The resistance of the first resistor and the inductance of the first inductor are determined such that the first crossover frequency between the second signal that has passed through the first resistor and the third signal that has passed through the first inductor is higher than the roll-off frequency determined by the inductance of the second inductor and the resistance of the load. The power amplifier according to claim 1.

3. The capacitance value of the capacitor and the resistance value of the first resistor are determined such that the second crossover frequency between the first signal that has passed through the capacitor and the second signal that has passed through the first resistor is higher than the first crossover frequency. The power amplifier according to claim 2.

4. The second and third amplifiers are either emitter followers or source followers, respectively. The power amplifier according to claim 1.

5. The closed-loop gain of the output signal with respect to the input signal in the audible range, determined by the feedback circuit, is a fixed value of 1 or more. The power amplifier according to claim 1.

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