JPS62144411A - Power amplifier circuit - Google Patents

Abstract

PURPOSE:To obtain a low frequency power amplifier driven efficiently without using a matching circuit by detecting a current flowing to a load and using a value being the result of comparison between the detected current and an input signal so as to control a load current to follow the input signal. CONSTITUTION:The current flowing to a load 16 is detected by a current transformer 15, converted into a voltage by a current/voltage conversion circuit comprising an OP amplifier 17 and a resistor 18, inputted to an inverting input of a comparator 2, where the voltage is compared with an input signal 1. The result of comparison is latched in a DFF 3 by using an output of an oscillator 4 and when the result is positive, a transistor (TR) 7 is conducted through a buffer 5. When the result is negative, a TR 8 is conducted through a buffer 6. A power fed to the primary winding of a transformer 11 is outputted at the secondary winding as a rectangular wave voltage by using a power supply 9 and a smoothing capacitor 10 depending on the conduction/nonconduction of the TRs 7, 8. Harmonics are eliminated from the rectangular voltage output by an LPF comprising coils 12, 14 and a capacitor 13 and the result is fed to the load 16.

Description

[Detailed description of the invention] [Field of application of the invention] The present invention relates to a power amplifier circuit, and more particularly, to a power amplifier circuit.

The present invention relates to a low frequency power amplifier that can efficiently drive a large capacitive or inductive load such as an ultrasonic transducer without using a matching circuit.

[Background of the invention]

In conventional power amplifier circuits, when driving a load having large capacitance or inductive properties, it is necessary to insert a matching circuit between the output and the load to cancel out the capacitive or inductive components. Such a circuit system requires a low frequency transformer, a low frequency coil, a capacitor, etc. as voltage conversion and phase shifting elements constituting the matching circuit. These elements have disadvantages in that the frequency used is low, and as the frequency used is low and the power used increases, the device becomes large and has high loss, leading to a decrease in power efficiency and an increase in the size of the device.

An example of a method for miniaturizing the elements used in low-frequency power amplifiers and further improving power efficiency is Chapter 11 Pulse Communication in Power Circuit Basic Course 4 "Pulses and Digital Circuits" (2nd edition) written by Noriyoshi Koshiba (Ohmsha). As described in the section, Pulse Width Modulation; (
PWM) method has been proposed and put into practical use. In such a system, in order to reduce output distortion, a method is used in which the output voltage is detected and compared with the input voltage to control the output voltage.

However, with this method of detecting the output voltage, if a phase difference exists between the voltage and current of the load, unstable phenomena such as oscillation occur, or effective power transfer becomes impossible. For this reason, an impedance matching circuit is indispensable, which has the disadvantage of resulting in a decrease in power efficiency and an increase in the size of the device.

[Purpose of the invention]

It is an object of the present invention to eliminate the above-mentioned conventional drawbacks. The object of the present invention is to provide a power amplifier capable of driving large capacitive or inductive loads using a filter.

[Summary of the invention]

In order to achieve the above-mentioned object, the present invention switches the final stage at a frequency sufficiently higher than the operating frequency, and configures the final stage to supply power to the load via a transformer and a low-pass circuit. We decided to detect the current and use the value obtained by comparing this with the input signal to control the load current so that it follows the input signal. The high switching frequency allows the use of small, highly efficient transformers and coils, making it possible to construct highly efficient, compact power amplifiers that can drive capacitive and inductive loads.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention shown in the drawings will be described in detail.

The power amplifier circuit of the present invention is composed of a differential input type OP amplifier, and an input signal is input to a non-inverting input terminal from a current transformer (CT) 15, which will be described later, and is composed of an operational amplifier 17 and a return resistor 18. Current/voltage conversion circuit IVCON
A pulse width modulation circuit PWM includes a comparator 2 configured such that the output of the D-FF is input to an inverting input terminal, and a D-FF3 that operates using the output of the comparator 2 and the output of the oscillator 4; A switching circuit SW consisting of a transistor 7.8 that takes in the Q and Q outputs of the FF via a buffer circuit 5.6, a power supply 9, a smoothing capacitor 10, and a transformer 11, and a coil 12 connected to the output of the transformer. .14 and a capacitor 13, and the output of the filter and the load 1.
A current transformer (CT
) 15.

The power amplifier of the present invention is constructed as described above, and the operation of the present amplifier will be explained below.

The current flowing through the load 16 is detected by the current transformer 15, converted to voltage by the current/voltage (I/V) conversion circuit I/VCON constituted by the OP amplifier 17 and the resistor 18, and then sent to the inverting input of the comparator 2. It is compared with the input signal 1 inputted to the terminal and inputted to the non-inverting input terminal. This comparison result is latched into D-FF3 with the output of oscillator 4,
When the latch result is positive, transistor 7 is made conductive through buffer 5, and when it is negative, transistor 8 is made conductive through buffer 6. Depending on whether each transistor 7.8 is conductive or non-conductive, the power supply 9. Power supplied from the smoothing capacitor 10 to the primary side of the transformer 11 is output as a rectangular wave voltage to the secondary side. After high frequency components are removed from this rectangular wave voltage output by a low-pass filter composed of a capacitor 13 and a coil 14,
applied to load 16.

As described above, according to this embodiment, by selecting the frequency of the oscillator 4 to an appropriate value, the transformer 11. Coil 12.
14. A small capacitor 13 can be used, and therefore switching can be achieved with high efficiency.

〔Effect of the invention〕

According to the present invention, the current flowing through the load is converted into a current/voltage, the input voltage is compared and detected, and the switching of the transistors constituting the final stage amplifier is controlled so that the output current follows the input voltage. Low-frequency power amplifiers that drive capacitive or inductive loads can be configured with switching circuits without using large transformers or matching circuits, which has the effect of downsizing equipment and improving power efficiency. .

[Brief explanation of drawings]

The figure is a circuit diagram of a power amplifier circuit showing one embodiment of the present invention. 1... Signal input, 2... Voltage comparator, 3... D-
FF, 4... Oscillator, 5, 6... Buffer, 7, 8
...transistor, 9...power supply, 10...smoothing capacitor + 11...transformer, 12...coil,
13... Capacitor, 14... Coil, 15...
Current transformer, 16...Load, 17...OP
Amplifier, 18...resistance.

Claims (1)

[Claims] A power amplifier characterized by comparing and detecting the input voltage obtained by converting the current flowing through the load into a current/voltage, and controlling the switching of the transistors forming the final stage amplifier so that the output current follows the input voltage. circuit.