JPH0160790B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an AC level detection circuit that detects an AC input level and extracts a period in which the input level is higher than a desired value as an output pulse.

FIG. 1 shows an example of a conventional configuration. In the figure, 1 is an input terminal, 2 is an AC amplifier, 3 is a rectifier, and D1, C1, and R1 are diodes, capacitors, and resistors that constitute the rectifier 3. 4 is a voltage comparator, 5 is a reference voltage source for providing a comparison reference voltage to the voltage comparator, and 6 is an output terminal.

Next, the operation will be explained.

An AC input signal whose level changes discontinuously is applied to the input terminal 1. The signal amplified to a desired level by the AC amplifier 2 is exchanged into a DC voltage by the rectifier 3. Needless to say, this DC voltage changes in proportion to the AC level of the input. When the rectified DC voltage exceeds the voltage set by the reference voltage source 5, the operation of the voltage comparator 4 results in an output signal at 6. That is, the output signal is obtained as a pulse signal only during the period when the AC input level is equal to or higher than the desired level.

However, in the conventional configuration, it is necessary to use a relatively large capacity capacitor C1 in the rectifier.
In addition, since the capacitor C1 is discharged through the resistor R1, it takes a long time to discharge the capacitor C1, resulting in a slow response speed.

The present invention provides an AC level detection circuit that eliminates the drawbacks of the conventional circuit, uses a small capacitor, and has a high response speed and is suitable for integration into an integrated circuit.

FIG. 2 shows an embodiment of the present invention. In FIG. 2, 4 and 5 are voltage comparators equivalent to the conventional example, respectively;
Although it is a reference voltage source, it is shown here as a specific circuit example. Reference numeral 7 denotes a limiter amplifier that amplifies the input to a desired level and outputs it as square waves having mutually opposite phases. 8 is the reverse phase A of the limiter amplifier
and a peak adder for adding the peak levels of the positive phase B outputs. Further, 9 is a low pass filter. In the figure, Q1 to Q8 are transistors, R1 to R10 are resistors, and C2 is a capacitor.

FIG. 3 shows waveforms at various parts in FIG. 2.

The operation will be explained below with reference to FIGS. 2 and 3. The signal applied to the input terminal 1 is amplified to a desired level by the limiter amplifier 7,
Square wave outputs with mutually opposite phases are obtained at points A and B.
Signals below the desired level will not become a square wave. This level setting can be arbitrarily set by changing the degree of amplification of the limiter amplifier 7. Outputs A and B having opposite phases to each other are added by a peak adder 8, and an output is obtained at point C. If the outputs at points A and B are ideal square waves, the added waveform will be a DC voltage, but in reality it will be an output with spikes S as shown in FIG. When the output C is applied to a voltage comparator 4 having a low-pass filter 9, the spike component is removed by the filter 9, and the desired pulse waveform is obtained at the output 6. Furthermore, it goes without saying that the voltage V _ref of the reference voltage source 5 should be set so that an input at a level that does not result in a square wave does not appear as an output.

By the way, since the spike component is a very high frequency component compared to the input, R7 and C2 can be set to considerably small values.

Further, since the spike component is a high frequency component, the resistor R7 of the filter can be omitted by appropriately selecting the current I1 of the voltage comparator 4.
If the input frequency is 1MHz or more, C2 can also be omitted by utilizing the collector-emitter capacitance of transistor Q7 and the base-emitter capacitance of transistor Q8, that is, omit low-pass filter 9. It is possible to do so.

As described above, according to the present invention, a circuit can be constructed using a very small capacitance, and a circuit that is optimal for an integrated circuit that cannot incorporate a large capacitance can be provided. A fast AC level detection circuit can be realized.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a conventional example, FIG. 2 is a circuit diagram showing an embodiment of the present invention, and FIG. 3 is an explanatory diagram showing signal waveforms at various parts in FIG. In the figure, 1 is an input terminal, 2 is an amplifier, 3 is a rectifier, 4 is a voltage comparator, 5 is a reference voltage source, 6 is an output terminal, 7 is a limiter amplifier, 8 is a peak adder,
9 is a low pass filter. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1 Amplify the AC input signal to the desired level,
A limiter amplifier that outputs square waves with mutually opposite phases, a peak adder that adds the positive and negative phase outputs of this limiter amplifier and extracts only the maximum level of each, and the output of this peak adder. An alternating current level detection circuit comprising: a voltage comparator that compares the output level with a reference voltage source and inverts the output according to the output level of the peak adder.