JPH07120935B2 - Switching circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a switching circuit using a field effect transistor.

2. Description of the Related Art In recent years, a transistor switch circuit and a field effect transistor switch circuit have been increasingly used as an electronic switch circuit by using a mechanical switch for a signal switching circuit. This is because the electronic switch circuit has easier switching timing, the cost of the semiconductor element has become cheaper in recent years, and there are many advantages such as high reliability.
However, electronic switching circuits have narrow operating conditions, and there are still some inferiors to mechanical switches when considering the maximum operating voltage and ON resistance.

For example, when the input signal is cut off by the conventional field effect transistor switching circuit shown in FIG. 3, the input electrode 4 and the output electrode 5 of the P channel field effect transistor 2 are inserted between the input terminal 1 and the output terminal 3. Therefore, the control electrode 6 of the field effect transistor 2 has a +
The control voltage of V _G is applied through the resistor 10 and the diode 8 to cut off the input electrode 4 and the output electrode 5.

However, the input signal that can be handled by the field effect transistor switching circuit of FIG. 3 is controlled by the control voltage + V _G. It is shown in FIG.

FIG. 4 shows the relationship between the control voltage + V _G and the input signal 19 when the field effect transistor switching circuit of FIG. 3 is cut off. Only the cutoff voltage V _P of the field effect transistor is always higher than the peak value of the input signal. Unless the high control voltage + V _G is applied to the control electrode of the field effect transistor, the input signal cannot be completely cut off. That is, a control voltage of 5V cannot cut off an input signal with a peak value of 30V.

The present invention has been made in view of the above points, and an object thereof is to completely cut off a field effect transistor switching circuit with a control voltage lower than the peak value of an input signal handled by a simple circuit configuration.

Means for Solving the Problems In order to solve the above problems, the present invention provides a capacitor between a control electrode and an output electrode of a field effect transistor, and a control current blocking diode and a resistor between the control electrode and a control terminal. By connecting an impedance element between the input circuit and the series connection circuit of the diode and the resistor,
A circuit for cutting off a field effect transistor with a control voltage smaller than the peak value of an input signal is obtained.

Action According to the present invention, the input signal coming from the input electrode through the impedance element and the control voltage applied to the control terminal through the resistor are superimposed on the diode-resistor connection between the control electrode and the control terminal. , The electric charge can be stored in the capacitor between the control electrode and the output electrode through the diode, and the potential difference between the control electrode and the input electrode or the output electrode can always be kept above the cut-off voltage of the field effect transistor. The field effect transistor can be turned off with a smaller control voltage.

Practical Example FIG. 1 is a circuit diagram showing an example of a switching circuit of the present invention. In FIG. 1, the input electrode 4 and the output electrode 5 of the P-channel field effect transistor 2 are inserted between the input terminal 1 and the output terminal 3, and the load resistor 13 is connected between the output terminal 3 and the ground. There is. A capacitor 7 is inserted between the control electrode 6 and the output electrode 5 of the field effect transistor 2, and a series circuit of a control current blocking diode 8 and a resistor 10 is inserted between the control electrode 6 and the control terminal 11.
An impedance element 12 is inserted between the junction 9 of the resistor 10 and the resistor 10 and the input electrode 4. With this configuration, the field-effect transistor 2 is controlled to be turned off and on by applying a control voltage above or below the cutoff voltage of the field-effect transistor 2 to the control terminal 11.

FIG. 2 is an operation explanatory diagram of the circuit configuration of FIG. Now, the control voltage + V _P shown by 15 in FIG. 2 is applied to the control terminal 11,
When the input signal shown in FIG. 2 is applied to the input electrode 4 from the input terminal 1, the input signal from the input electrode 4 through the impedance element 12 and the control voltage + V _P coming from the control terminal 11 through the resistor 11 are joined. The control voltage waveform superimposed on the point 9 becomes the waveform shown in FIG. At this time, the impedance of the impedance element 12 is set low and the resistance 10 is set to a high resistance so that the signal at the junction point 9 has the same amplitude as the input signal applied to the input electrode 4, and the impedance element 12 has a DC cut capacitor. Is used, the DC potential at the junction point 9 can be made the control voltage + V _P applied to the control terminal 11. Also, there is an interelectrode capacitance of C ₁ and C ₂ shown in Fig. 5 between the input electrode or output electrode of the field effect transistor and the control electrode, and the value is less than several P _F , and C ₁ and C ₂ The capacities of are almost equal. Therefore, in the combined capacitance C ₇ + C ₂₁ of the output electrode 5 and the control electrode 6 of FIG. 1, the electric charge of Q ₁ = (V _1n + V _P ) (C ₇ + C ₂₁ ) is stored due to the positive potential of the junction 9. The electric charge of Q ₂ = V _P C ₂₀ is stored in the capacitance C ₂₀ of the input electrode 4 and the control electrode 6. When the junction 9 becomes negative potential, the charge Q ₁
Since the diode 8 is prevented from flowing out to the junction 9 by the diode 8, it only moves to the capacitance C ₂₀ of the input electrode 4 and the control electrode 6, and the sum of the charge amounts of Q ₁ and Q ₂ does not change. Therefore, when the potential of the control electrode 6 is the lowest and the potential at that time is V _x Becomes At this time, if there is no leakage current from the control electrode 6 (junction capacitance of the diode 8 or reverse leakage current, stray capacitance on the printed circuit board, etc.) and C ₂₀ = C ₂₁ , then even if the capacitor C ₇ is omitted, V _x = V _It becomes _{P and} the potential of the control electrode 6 is the second
As shown in FIG. 17, in order to maintain the cut-off voltage V _P of the field effect transistor 2 at all times, the input electrode 4 and the output electrode 5 are
The OFF state can be maintained. However, due to the junction capacitance of 1 to 2 PF of the diode 8 and the stray capacitance of the printed circuit board, the cut-off voltage V _P of the field effect transistor cannot be maintained at the time of a negative input signal unless the capacitor C ₇ is provided. Distorts the signal. Next, when turning on the field effect transistor 2, by applying a control voltage equal to or lower than the cutoff voltage V _P to the control terminal 11, the input electrode 4,
The potentials of the output electrode 5 and the control electrode 6 are always the cutoff voltage V _P.
Field effect transistor 2 is always ON because it is kept below
It becomes a state.

In this embodiment, the field effect transistor is shown as a P channel field effect transistor, but the same effect can be obtained by connecting the polarity of the control current blocking diode in reverse using an N channel field effect transistor.

As described above, according to the switching circuit of the present invention, the field effect transistor can be controlled to be turned on and off with an extremely simple circuit configuration at a very low control voltage for an excessive input signal. Very useful to see.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram showing a switching circuit in an embodiment of the present invention, FIG. 2 is a waveform diagram for explaining the operation of FIG. 1, and FIG. 3 is a conventional switching circuit. Circuit diagram of the fourth
FIG. 5 is a waveform diagram for explaining the operation of FIG. 3, and FIG. 5 is a circuit diagram showing the electrode capacitance of the field effect transistor. 1 ... Input terminal, 2 ... Field-effect transistor, 3 ...
… Output terminal, 4 …… input electrode, 5 …… output electrode, 6 ……
Control electrode, 11 ... Control terminal, 12 ... Impedance element.

Claims (1)

[Claims] 1. An input electrode and an output electrode of a field-effect transistor are inserted between an input terminal and an output terminal, and a capacitor is connected between the control electrode and the output electrode of the field-effect transistor. -Type transistor and a control terminal, a series circuit composed of a control current blocking diode connected to the control electrode of the field-effect transistor and a resistor connected to the control terminal is connected, and a connecting portion between the diode and the resistance is connected. A switching circuit, wherein an impedance element is connected between the input electrode and the input electrode.