JPH01147907A - Power amplifier circuit - Google Patents

Abstract

PURPOSE:To isolate the input and output electrically while the efficiency and the frequency characteristic are improved by applying PWM modulation to a full wave rectified waveform of an input signal so as to attain current amplification and rectifying and smoothing the result thereby using it as a power voltage of a class B push-pull amplifier. CONSTITUTION:An input signal Si is subjected to full wave rectification by amplifiers 32, 34 and diodes 37, 38 via a terminal 30. The signal and a bias from a voltage divider comprising resistors 39-41 are applied and the signal is subject to PWM modulation by a comparator 43 by using a triangle wave from a triangle wave generator 42 and subjected to current amplification by an FET 44. The signal is extracted from a secondary side of a transformer 45, rectified and smoothed by diodes 48-51 and used as a power voltage of the class B push-pull amplifier 60. The push-pull amplifier 60 is driven by the input signal Si via the amplifier 32 and a transformer 70. Since the amplifier 60 has an excellent frequency characteristic and the collector-emitter voltage of transistors 63,64 is kept nearly constant regardless of the quantity of the input signal, excellent efficiency is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a power amplifier circuit using a class B push-pull amplifier.

(Prior art) Conventionally, as a technology in this field, what is described in "Basic Electronic Circuits Analog Edition" by Ken Yanagisawa (March 30, 1980), Marukubi Co., Ltd., P, 146-153 is known. there were.

The configuration will be explained below using figures.

2 and 3 show an example of the configuration of a conventional power amplifier circuit described in the above-mentioned document, and FIG. 2 is a circuit diagram of a transformer type B class push-pull power amplifier circuit, and FIG. is a circuit diagram of a class B single-ended push-pull power amplifier circuit.

The power amplifier circuit in Fig. 2 has an input transformer 1, PNPi-
It has a transistor 2 , an NPN transistor 3 , and an output transformer 4 , and a power supply VCC is connected between the emitters of the PNP transistor 2 and the NPN transistor 3 and the primary side of the output transformer 4 . A load 5 is connected to the side.

In this power amplifier circuit, when an input signal 3i is supplied to the primary side of the input transformer 1, transistors 2 and 3 operate for half a cycle depending on the polarity of the input signal 3i, and the remaining half cycle is a cutoff period. become a state. The collector current flowing from the power supply VCC to the transistor 2 and the collector current flowing from the current CC to the transistor 3 are added together at the output transformer 4, and a sine wave current flows through the load 5.

Although this type of power amplifier circuit has good amplification characteristics, the operation of the transistors 2 and 3, which are amplification elements, is a so-called active operation in which the transistors 2 and 3 act as resistors.

The power amplification circuit shown in FIG. 3 consists of a common emitter NPN transistor 10 with a voltage amplification function, an emitter follower NPN transistor 11 and a PNP transistor 11 with a power amplification function.
NP transistor 12 and the emitters of the NPN transistor 11 and PNP transistor 12 and the power supply v
A load 13 is connected between the CC and the CC. A bias diode 14 that also serves as temperature compensation is connected between the bases of the NPN transistor 11 and the PNP transistor 12, and the diode 14 supplies an appropriate static current to the NPN transistor 11 and the PNP transistor 12. In addition, the NPN transistor 11 and the PNP
Emitter of transistor 12 and NPN transistor 10
A feedback resistor 15 is connected between the base of the .

In this power amplifier circuit, when an input signal 3i is supplied to the base-emitter open side of the transistor 10, the collector potential of the transistor 10 reciprocates between O and Vcc. When the collector potential of the transistor 10 rises above Vcc/2, the transistor 11 becomes forward biased and the transistor 12 becomes reverse biased, so that a current flows through the load 13 through the route of power source CC→transistor 11→load 13. Also, the collector potential of the transistor 10 is VC
When the voltage drops below C/2, transistor 12 becomes forward biased.
Transistor 11 becomes reverse biased, and the load 13 goes through the route of power supply Vcc → load 13 → transistor 12.
A current flows through. Note that in this circuit, DC-like parallel voltage feedback is applied by the resistor 15 to stabilize the DC operating point, lower the output impedance, and reduce distortion. Similar to the one shown in FIG. 2, this power amplification circuit also had good amplification characteristics, but the operation of the amplification element was so-called active operation in which it acted as a resistor.

Conventionally, in addition to the circuits shown in FIGS. 2 and 3, a pulse width modulation (hereinafter referred to as PWM) switching power amplifier circuit has been proposed.

FIG. 4 is a conceptual diagram of a conventional PWM type switching power amplifier circuit. In this power amplifier circuit, the comparator 20 uses a triangular wave a for sending an input signal 5ielffl to PW
The PWM wave is converted into an M wave, the PWM wave is amplified by a current amplifying section 22 that performs a switching operation, and the amplified PWM wave B is passed through an LG filter 22 to obtain an output signal SO.

(Problems to be Solved by the Invention) However, the circuit with the above configuration has the following problems.

In the class B push-pull power amplifier circuits shown in Figures 2 and 3, the transistor 2.3°10~12, which is the amplifying element, generates a large amount of heat, and the efficiency is approximately 70% or less in practice, so the efficiency is relatively low. There was a problem with it being bad.

On the other hand, in the case of the PWM type switching power amplifier circuit shown in FIG. 4, the signal/noise ratio (hereinafter referred to as S/N
In order to improve the ratio, it is necessary to increase the time constant of the LC filter 22, but increasing the time constant attenuates high frequency signal components and deteriorates the frequency characteristics of the amplifier circuit. Furthermore, if the switching frequency in the current amplification section 21 is increased in order to increase the attenuation interval of the PWM wave component, the switching loss of the amplification element will increase, and unnecessary radiation to the outside will also increase. Furthermore, since the PWM wave B contains a DC component, it is not possible to electrically separate the amplifier circuit and the load by transformer coupling.

The present invention solves the problems that the prior art had, B@
Inefficiency in push-pull power amplifier circuits and PW
The present invention provides a power amplifier circuit that solves problems such as deterioration of frequency characteristics and difficulty in electrically separating the M-switching power amplifier circuit from the load.

(Means for Solving the Problems) In order to solve the above problems, the present invention provides a power amplification circuit for amplifying an input signal. A first circuit that pulse width modulates the applied signal with a triangular wave (including a sawtooth wave), and a current amplification of the output signal of this first circuit, which is taken out from the secondary side of the first transformer and rectified. a second circuit that smoothes and outputs a positive and negative DC voltage proportional to the absolute value of the input signal; a second transformer that takes out the input signal that is in phase with the positive and negative DC voltage; and a second transformer that outputs the positive and negative DC voltage. and a class B push-pull amplifier that amplifies the input signal taken out from the second transformer by using the power supply voltage as the power supply voltage.

(Function) According to the present invention, since the power amplifier circuit is configured as described above, the first circuit pulse width modulates the input signal, and the second circuit modulates the pulse width of the input signal in proportion to the absolute value of the input signal. This DC voltage is then supplied to a class B push-pull amplifier as a power supply voltage. The B-type push-pull amplifier, to which the minimum power supply voltage to obtain the required output amplitude is applied, is driven by the input signal supplied via the second transformer. This makes it possible to improve efficiency and frequency characteristics, and to electrically separate the power supply side, the input signal side, and the load side. Therefore, the above-mentioned problem can be eliminated.

(Embodiment) FIG. 1 is a circuit diagram of a power amplifier circuit showing an embodiment of the present invention.

This power amplification circuit has an input terminal 30 for an input signal 3i, and a grounded resistor 31, a voltage follower amplifier 32, and a resistor 33 are connected to the input terminal 30.

The amplifier 32 is a circuit that outputs a signal having the same waveform as the signal on the node N1 on the input terminal side to the node N2 on the output terminal side.

A (-) input terminal of an amplifier 34 with a gain of -1 is connected to the resistor 33, and a feedback resistor 35 is connected between the (-) input terminal and a node N3 on the output terminal side.
The (+) side input terminal of the amplifier 34 is grounded via a resistor 36. Resistors 33 and 35 have the same resistance value.

Rectifying diodes 37 are connected to nodes N2 and N3, respectively.
．． 38 is connected, and the node N4, which is the connection point of the diode 37.38, is grounded via a bias voltage dividing resistor 39. It is connected to the.

The voltage generated on node N4 and the output signal of triangular wave generator 42 are connected to the input terminal of comparator 43, and node N5 on the output terminal side of comparator 42 is connected to a switching field effect transistor (hereinafter referred to as FET). ) 44 gates. The FET 44 has its source grounded, and its drain connected through the primary side of the first transformer 45 to the power supply Vcc and a diode 46 for blocking reverse current. A smoothing electrolytic capacitor 47 is connected between the power supply VCC and ground.

On the secondary side of the transformer 45, rectifying diodes 4B, 4
9.50,5L smoothing coils 52 and 53 and smoothing capacitors 54 and 55 are connected, and the capacitors 54.
The positive and negative DC voltages generated on each of the nodes N6 and N7 on the 55 side are connected to the power supply line of the class B push-pull amplifier 60.

Note that each node N6. N7 is grounded via capacitors 54 and 55. The circuit elements shown above with reference numerals 30 to 43 constitute a first circuit, the circuit elements shown with reference numerals 44 to 55 constitute a second circuit, and the first and second circuits form a power supply section. There is.

Further, the output terminal side node N2 of the amplifier 32 is connected to the primary side of a second transformer 70 for signal transmission, and the secondary side node N8 is connected to the signal input terminal of the class B push-pull amplifier 60. There is.

The second transformer 70 has one end on its negative side and one end on its secondary side grounded in separate systems.

The class B push-pull amplifier 60 includes a preamplifier 61 that amplifies the signal on the node N8, a common emitter NPN transistor 62 that has a voltage amplification function, and an emitter follower NPN transistor 63 and a PNP that has a power amplification function. A transistor 64 is provided. The amplifier 6
1, its (+) side input terminal is connected to the resistor 65 and the node N8.
, its (-) side input terminal is connected to the resistor 66 and the feedback resistor 6.
7, one of the two power supply terminals is connected to a node N6 via a resistor 68 and a smoothing electrolytic capacitor 69, and the other power supply terminal is connected to a node N7. NPN transistor 62 has its emitter connected to node N.
7, its collector is connected to a node N6 via a bias diode 70.degree. 71 which also serves as temperature compensation and a resistor 72. The emitters of the NPN transistor 63 and the PNP transistor 64 are commonly connected to the output node N9, the base of the NPN transistor 63 is connected to the resistor 72, the collector thereof is connected to the node N6, and the collector of the PNP transistor 64 is connected to the node N9. N7, the base of which is an NPN transistor 62
are connected to their respective collectors. Output side node N
A load 80 is connected to 9.

In FIG. 1, the input side and power supply side grounding and the output side grounding are separated into separate systems.

FIG. 5 is a waveform diagram on each node in FIG. 1, and the operation of FIG. 1 will be explained with reference to this diagram.

First, when the input signal 3i is supplied to the input terminal 30, the same signal waveform as the signal waveform on the node N1 on the input terminal 30 side is output to the output side node N2 of one amplifier 32, and the signal waveform on the other side A signal waveform obtained by inverting the signal waveform of the node N1 is output to the output side node N3 of the amplifier 34.

Each node N2. The signal waveform on N3 is full-wave rectified by diodes 37 and 38, and a full-wave rectified waveform appears on node N4 on the resistor 39 side. A resistor 40 is placed on this node N4.
．． A DC bias is applied by 41, and the voltage necessary to operate the amplifier 60 is supplied to the secondary side of the transformer 45 even in a no-signal state.

The full-wave rectified waveform on the node N4 is compared with the triangular wave output from the triangular wave generator 42 by the comparator 43, and a PWM wave is output to the output side node N5 of the comparator 43. Normally, the frequency of the triangular wave is 10 times or more that of the input signal 3i, approximately 1
OOKH2 level is selected. The PWM wave on node N5 drives the gate of FET44, and its FET4
4, current amplification of the PWM wave is performed by the FET 44 and the transformer 45, and the same voltage waveform as the PWM wave is output from the secondary side of the transformer 45.

The secondary voltage of this transformer 45 is applied to the diodes 48 to 5.
0, the coils 52.53 and the capacitors 54.
After smoothing at node N6.55, a positive and negative voltage proportional to the absolute value of input signal Si is applied to node N6. It is output on N7 and is supplied as the power source for amplifier 60. At this time, the input signal 3i is supplied to the input side node N8 of the amplifier 60 through the transformer 70. The magnitude of the signal on node N8 is equal to the magnitude of the signal on node N6. It is proportional to the voltage on N7.

When the power supply voltage and the signal are supplied to the amplifier 60, the signal on the node N8 is amplified by the preamplifier 61 in the amplifier 60. When the output signal of the preamplifier 61 is positive polarity,
The NPN transistor 62 turns on, its connector potential drops, and the NPN transistor 63 turns off.
PNP transistor 64 is turned on. Further, when the output signal of the preamplifier 61 is of negative polarity, the NPN transistor 62 is turned off, its collector potential rises, the NPN transistor 63 is turned on, and the PNP transistor 64 is turned off.

When the NPN transistor 63 is turned on, the node N
6→NPN transistor 63→node N9→load 80→
A current flows through a route called ground, and when the PNP transistor 64 is turned on, a current flows through a route from ground to load 80 to node N9 to PNP transistor 64 to node N7.

Here, the magnitude of the signal on node N8 and the magnitude of the signal on node N6. N
Since there is a proportional relationship with the power supply voltage on 7, the transformer 45
By appropriately selecting the turns ratio of , it is possible to maintain the voltage between the collector and emitter of the transistors 63 and 64 at a constant value of approximately several volts, regardless of the magnitude of the input signal. It can provide the necessary output.

In addition, in this amplifier 60, similar to the conventional one shown in FIG. 3, DC-like parallel voltage feedback is applied by a feedback resistor 67, which stabilizes the DC operating point, lowers the output impedance, and reduces distortion. It is being

This embodiment has the following advantages.

(i) Since a positive and negative voltage proportional to the absolute value of the input signal 3i is generated and the class B push-pull amplifier 60 is operated with this voltage, regardless of the magnitude of the input signal 5i,
High efficiency.

(i> As the power supply for the class B push-pull amplifier 60, even if there is some switching ripple, it will not affect the output waveform. Therefore, the output of the filter section can respond even to high frequency input signals.On the other hand, B
Since the frequency characteristics of the class push-pull amplifier 60 are sufficiently good, as a result, the frequency characteristics of the entire power amplifier circuit are similar to those of the conventional P class push-pull amplifier 60.
Improved than WM switching power amplifier circuit.

(iii) Input signal @Si side and power supply VCC side,
Since the output side with the load 80 is separated by the transformer 45, 70, electrical isolation between the two is possible.

Note that the present invention is not limited to the illustrated embodiment, and various modifications are possible. Examples of such modifications include the following.

(a) The triangular wave used in pulse width modulation may be a sawtooth wave. a first circuit that performs this pulse width conversion;
The second circuit that performs rectification, smoothing, etc. may be configured with a circuit other than that shown in the drawings.

(b) E[The push-pull amplifier 60 is
It may also be configured with other circuits as shown in the figure.

(Effects of the Invention) As described above in detail, according to the present invention, the first circuit pulse-width modulates the input signal, and the second circuit modulates the input signal with a positive or negative signal proportional to the absolute value of the input signal. This DC voltage is converted into a DC voltage and supplied as the power supply voltage for the class B push-pull amplifier, and the class B push-pull amplifier is driven with an input signal that is in phase with the power supply voltage, which improves efficiency. It can be expected to have good frequency characteristics, and to be able to be electrically isolated from the load.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a power amplifier circuit showing an embodiment of the present invention;
Figures 2 and 3 are circuit diagrams of conventional power amplifier circuits;
The figure is a conceptual diagram of another conventional power amplifier circuit, and FIG. 5 is a waveform diagram of FIG. 1. 32.34.61...Amplifier, 37,38°4
8~50...Diode, 39~41...
...Resistance, 42...Triangular wave generator, 43...
...Comparator, 44...FET, 45.70.
...Transformer, 52.53...Coil,
54.55... Capacitor, 60...
Class B push-pull amplifier, 62.63...PN
P transistor, 64...NPN transistor, 80...load, 3i...input signal,
VGC...Power supply.

Claims (1)

[Claims] A first circuit that pulse-width modulates a signal obtained by adding a full-wave rectified waveform of an input signal and a DC bias to a triangular wave; A second voltage is taken out from the secondary side of the transformer, rectified and smoothed, and outputs a positive and negative DC voltage proportional to the absolute value of the input signal.
a second transformer that takes out the input signal that is in phase with the positive and negative DC voltage; and a class B push-pull that amplifies the input signal taken out from the second transformer using the positive and negative DC voltage as a power supply voltage. A power amplification circuit characterized by comprising an amplifier.