JP2850599B2 - Analog output circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an analog output circuit suitable for, for example, a constant-voltage power supply circuit, which uses a field-effect transistor as an output transistor to perform analog operation under a power supply voltage higher than its withstand voltage. .

[0002]

2. Description of the Related Art As is well known, a field effect transistor has a feature of high input impedance, and by utilizing this, an analog output can be taken out with a relatively low output impedance while controlling the gate with a weak input signal. Hereinafter, the configuration and operation of the analog output circuit according to the related art will be described with reference to FIGS.

As shown in FIG. 5, a pair of power supply potential points V and E
, A field effect output transistor 1 and a resistor 3 are connected in series, and while an output transistor 1 is gate-controlled by an input signal Si, an analog output signal So is extracted from an interconnection point between the output transistor 1 and the resistor 3. In a normal state, the output transistor 1 performs an analog control operation within a substantially linear characteristic range between the completely off state and the completely on state. In this state, not only the output signal So but also the power supply voltage V is applied to the channel of the output transistor 1. A value obtained by dividing the voltage by the resistance value and the resistance value of the resistor 3 is used. In the example of FIG. 5, since the output transistor 1 is a p-channel type, it is connected to the potential point side of the power supply voltage V, and its gate is connected to the resistor 4 for setting the control speed. The resistance value of the resistor 4 is set so that the time constant with the capacitance of the gate of the output transistor 1 becomes a desired value, usually several nS to several μS.

In this output circuit, the output signal is
In order to raise So as quickly as possible, the input signal Si is temporarily dropped to the ground side potential E to completely turn on the output transistor 1, and when stopped, the input signal Si is raised to the power supply side potential V to completely turn off the output transistor 1. State. Furthermore, not only at the time of starting and stopping, but also during normal operation, noise may be mixed into the input signal Si and the gate of the output transistor 1 may be broken down due to the spike voltage. Diodes 5 and 6 are for protecting this gate,
The diode 5 conducts when the gate potential rises above the power supply voltage V, and normally limits the forward voltage of about 0.5 to 0.6 V so as not to exceed it. The diode 6 has a gate potential that is less than the ground potential E. When the power supply voltage drops, the voltage is turned on to limit the voltage not to fall below the forward voltage.

The conventional example of FIG. 6 differs from that of FIG. 5 only in that an n-channel type field effect transistor is used as the output transistor 2, and is otherwise the same. In this example, the n-channel type output transistor 2 is connected to the ground potential point E side, and an input signal Si of a potential of the ground potential E is supplied to its gate when the output circuit is activated and when the output circuit is stopped. The protection effect of the diodes 5 and 6 against the dielectric breakdown of the gate of the output transistor 2 is the same as in FIG.

[0006]

However, in the conventional analog output circuits shown in FIGS. 5 and 6, when the power supply voltage V increases,
The gates of the output transistors 1 and 2 cannot be protected.
That is, in the circuits shown in FIGS. 5 and 6, when the forward voltage of the diodes 5 and 6 is Vf, the potential of the gates of the output transistors 1 and 2 can be limited to the range from -Vf to V + Vf as can be understood from the above description. If the value of V + Vf is output transistor 1 or 2
If the power supply voltage V becomes high enough to exceed the gate breakdown voltage of the gate, the gate cannot be protected.

For this reason, the conventional analog output circuits shown in FIGS. 5 and 6 are actually applicable only when the power supply voltage V is about half of the gate withstand voltage of the output transistors 1 and 2 and the output signal So of a desired voltage value is output. If the required power supply voltage V exceeds this limit, it cannot be applied. To solve this problem, the breakdown voltage may be increased by increasing the thickness of the gate oxide film of the output transistors 1 and 2. However, the control gain is easily reduced, which is very disadvantageous in performing the analog operation. Further, it is possible to limit the change in the gate potential to a narrow range as much as possible, but there is a disadvantage that the operation at the time of starting or stopping is slowed down.

An object of the present invention is to solve such a problem,
An object of the present invention is to make it possible to operate an analog output circuit under a higher power supply voltage than before, without being limited by a gate withstand voltage of a field-effect output transistor suitable for analog operation.

[0009]

According to the present invention, there is provided an analog output circuit comprising: a series connection of a field effect output transistor and a resistor for analog operation between a pair of power supply potential points; An input transistor for analog operation is connected between the gate of the transistor and the power supply potential point on the resistor side, and a voltage dividing resistor and a voltage limiting element are connected in parallel between the gate of the output transistor and the power supply potential point on the output transistor side. This is achieved by controlling the input transistor by the input signal and extracting the output signal from the interconnection point between the output transistor and the resistor.

It is advantageous to use a bipolar input transistor, connect the collector and the emitter between the gate of the output transistor and the power supply potential point on the resistance side, and control the base by an input signal. As the voltage limiting element, for example, a normal Zener diode may be used, but it is desirable that the voltage limiting element be provided with a bidirectional voltage limiting function. For this purpose, the voltage limiting elements are connected in series or in parallel in opposite directions. It is preferable to use a pair of connected zener diodes.

Further, in practical application of the present invention, protection between the source and drain of the output transistor is often required. For this reason, a Zener diode or the like is used as a voltage limiting element for a voltage in the same direction as the power supply voltage. It is desirable to connect between source and drain. Note that the present invention is particularly advantageous for application to a constant voltage power supply circuit. In this case, the input transistor is controlled using a difference signal between the output signal and the reference voltage value as an input signal, and the output signal of the analog output circuit is output as a constant voltage output. It should be taken out as.

[0012]

According to the present invention, in order to provide an optimum potential corresponding to the value of the power supply voltage, the desired output signal value and the gate withstand voltage to the gate of the output transistor, the input transistor described in the preceding section is connected to the gate and the power supply potential point on the resistance side. In between, a voltage dividing resistor is provided between the gate and the power supply potential point on the output transistor side, and the power supply voltage is divided by the input transistor and the voltage dividing resistor. , The analog output circuit can be easily configured for any power supply voltage and output signal voltage regardless of the withstand voltage value.

Further, according to the present invention, in order to rationally limit the voltage applied to the gate of the output transistor when the output circuit is started or stopped, the voltage limiting element referred to in the configuration of the preceding paragraph is replaced with the gate and the power supply potential point on the output transistor side. By connecting them in between, the starting and stopping operations of the analog output circuit are accelerated by using the output transistor to the fullest extent permitted by the withstand voltage of the gate.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. First, a basic embodiment of an analog output circuit according to the present invention using p-channel type and n-channel type output transistors will be described with reference to FIGS. 3 and 4, and FIG. 1 and FIG. Embodiments in which the inventive circuit is applied to a constant voltage power supply circuit will be described in the case of using p-channel and n-channel output transistors. In these embodiments, a bipolar transistor is used for the input transistor. However, the present invention can be applied to other embodiments as appropriate.

In the analog output circuit 20 of FIG. 3, the output transistor So and the resistor 3 are connected in series between a pair of power supply potential points V and E, and the output signal So is extracted from the interconnection point as in the prior art. However, the present invention is different in that the output transistor 1 can be operated under a power supply voltage that is at least half the gate withstand voltage. That is, when the guaranteed value of the gate breakdown voltage of the output transistor 1 is 8 V, the power supply voltage V is set to, for example, 10 to 15 V, which is higher than the half of 4 V. In the embodiment shown in FIG. 3, since the output transistor 1 is a p-channel type, it is connected to the power supply potential point side of the power supply voltage V. Conversely, the resistor 3 connected to the ground potential point E divides the power supply voltage V together with the on-resistance of the output transistor 1 for analog operation in a normal state except when the analog output circuit 20 is started or stopped, and 5
The resistance value is set to several hundred Ω so that an output signal So of a voltage of 1010 V is obtained.

In the present invention, the input transistor 11 for analog operation is connected between the gate of the output transistor 1 and the power supply potential point E on the resistor 3 side, and the voltage dividing resistor 13 and the voltage limiting element
14 is connected in parallel between the gate and the potential point V on the output transistor 1 side.
Analog control is performed by the input signal Si which gives 1 to the base. The voltage dividing resistor 13 is used to set the gate potential of the output transistor 1 in the above-described normal operation state of the analog output circuit 20.
Divides the power supply voltage V together with the resistance of the power supply and normally applies a voltage of several tens of kΩ so that only a voltage lower than the breakdown voltage is applied to the gate.
Is set to a high resistance value.

The voltage limiting element 14 is for protecting the gate of the output transistor 1. For example, a Zener diode may be used for this as shown in FIG. 3, and the Zener voltage is selected so as to be slightly lower than the gate breakdown voltage of the output transistor 10. Is done. For example, when the gate withstand voltage of the output transistor 1 is 8V, the voltage exceeding the withstand voltage value is selected even when the voltage limit of the voltage limiting element 14 is set to 6V and a forward voltage of, for example, 0.6V of the voltage limiting element 15 is applied. Avoid getting to the gate.

In the analog output circuit 20 of FIG. 3 configured as described above, the gate of the output transistor 1 is analog-controlled according to the input signal Si received by the input transistor 11, and the input signal is controlled by the analog operation of the output transistor 1. An output signal So corresponding to Si is output. In a normal analog operation state, the output transistor 1 operates near the gate potential set by the voltage dividing resistor 13, so that an excessive voltage exceeding the breakdown voltage is not applied to the gate. When the analog output circuit 20 is started, the input signal Si is controlled to increase the control voltage applied to the gate of the output transistor 1 by increasing the control voltage applied to the gate of the output transistor 1 in order to speed up the rise of the output signal So. Is limited to within the breakdown voltage value, and in this case also, there is no possibility that the gate will be broken down. When the analog output circuit is stopped, the input transistor 11 is turned off, the potential of the gate of the output transistor 1 is raised to the power supply potential point V, and the voltage applied to the gate is reduced, so that there is no problem.

In the analog output circuit 20 shown in FIG. 4, an n-channel output transistor 2 is used, and the output transistor 2 is connected to the power supply potential point E for grounding, and the resistor 3 is connected to the power supply potential point V for power supply voltage. , And the input transistor 12
The np transistor is used to connect between the gate of the output transistor 2 and the power supply potential point V, and the voltage dividing resistor 13 and the voltage limiting element 14 are connected in parallel between the gate and the power supply potential point E. The procedure for setting the resistance value of the voltage dividing resistor 13 and the limiting voltage of the voltage limiting element is the same as in the previous embodiment. This analog output circuit
The same applies to the case where an excessive voltage exceeding the breakdown voltage is not applied to the gate of the output transistor 2 during the normal operation of the transistor 20.

When the analog output circuit 20 shown in FIG. 4 is started, the input transistor 12 is turned off.
Is turned off and the output signal So is pulled up via the resistor 3, so that there is no problem in the direction in which the voltage applied to the gate decreases. Conversely, when stopped, the input transistor 12 is turned on to increase the gate potential of the output transistor 2. In addition, the voltage limiting element 14 limits the voltage applied to the gate to a value equal to or lower than the withstand voltage value, thereby preventing dielectric breakdown. In each of the embodiments shown in FIGS. 3 and 4, the input transistors 11 and 12 are connected to the voltage dividing resistor 13
In addition to the function of setting the gate potentials of the output transistors 1 and 2, the amplifier has an effect of increasing the control gain of the analog output circuit 20 by amplifying the input signal Si to improve the accuracy of analog control.

In the embodiment of FIG. 1, the analog output circuit of FIG.
20 applies to constant voltage power supply circuits. Therefore, a differential amplifier 30 is provided in a stage preceding the input transistor 11 to generate a signal having a difference between the output signal So and the reference voltage Vr, and to provide the input signal Si to the input transistor 11 as the input signal Si.
The output signal So is controlled to a value specified by the reference voltage Vr, and the output signal So is stabilized by the capacitor 31 and then taken out as a constant voltage output Vo. In this constant voltage power supply circuit, since the gain of the differential amplifier 30 in the feedback system of the output signal So is added to the gain of the analog output circuit 20, the output signal So is controlled to a high gain and equal to the reference voltage Vr. An extremely stable constant voltage output Vo can be obtained.

In the analog output circuit 20 of FIG. 1, a voltage limiting element 14 having the same limiting voltage in the opposite direction is provided in addition to the voltage limiting element 14, and both are connected in parallel in the illustrated example. Voltage control elements 14 and 15 are both output transistors 1
In the example shown in the figure, four diodes 16 are connected in series in opposite directions to each other for setting a zener current. When the gate withstand voltage of the output transistor 1 is 8 V, the forward voltage of each diode 16 is set to 0.5 V, and the Zener voltage of the Zener diodes 14 and 15 is set to 5 V, for example, and the voltage applied to the gate is limited to 7 V at the maximum. I do. The voltage limiting element 15 is for protecting the gate of the output transistor 1 when unexpected noise enters.

Further, in the embodiment of FIG. 1, a voltage limiting element 17 is connected between the source and the drain of the output transistor 1 in the same direction as the voltage control element 14 with respect to the power supply voltage V. This voltage limiting element 17 is useful for protecting the analog output circuit 20 from starting when the output signal So is at the ground potential E when the power supply voltage V is high, and the source-drain breakdown voltage of the output transistor 1 is, for example, 10 V In the example shown, a Zener diode with a Zener voltage of 4 V is used for
Connect them in series.

In the embodiment of FIG. 2, the analog output circuit 20 of FIG. 4 is applied to a constant voltage power supply circuit as in FIG. 1, but the output transistor 2 is an n-channel type and the input transistor is
12 is the same as FIG. 1 except that it is a pnp type, and the description is omitted. In the example of the figure, the voltage limiting element 17
As with the voltage limiting elements 14 and 15, an appropriate number of diodes 16 are connected in series in the reverse direction.

[0025]

As described above, according to the present invention, a field effect type output transistor and a resistor for analog operation are connected in series between a pair of power supply potential points, and the gate of the output transistor and the power supply potential point on the resistance side are connected. An analog output circuit is configured by connecting an input transistor for analog operation between the input transistor and a voltage dividing resistor and a voltage limiting element in parallel between the gate of the output transistor and the power supply potential point on the output transistor side. The following effects can be obtained by extracting the output signal from the interconnection point between the output transistor and the resistor while controlling the transistor by the input signal.

(A) By providing an input transistor and a voltage dividing resistor and dividing the power supply voltage by both, an appropriate potential according to a predetermined power supply voltage value and a desired output signal value is given to the gate of the output transistor. In addition, since the voltage applied to the gate during normal operation of the analog output circuit can be set to a value less than the withstand voltage value, the analog output circuit can be operated at any power supply voltage and output signal voltage without being restricted by the withstand voltage value of the output transistor gate. And can be easily configured. (b) By connecting the voltage limiting element between the gate and the power supply potential point on the output transistor side, the voltage applied to the gate of the output transistor during the start and stop of the analog output circuit is rationally limited, Can be used to the fullest extent permitted by the gate withstand voltage to speed up the operation of the analog output circuit when it is started or stopped. (c) Since the amplifying function of the input transistor with respect to the input signal can be used in addition to the amplifying function of the output transistor, the control gain of the analog output circuit can be increased as compared with the conventional case, and the control accuracy for the output signal can be improved. By applying such a high gain analog output circuit to, for example, a constant voltage power supply circuit, the output signal can be precisely controlled and taken out as a very stable constant voltage output.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment in which an analog output circuit of the present invention is applied to a constant-voltage power supply circuit when a p-channel output transistor is used.

FIG. 2 is a circuit diagram showing an embodiment in which the analog output circuit of the present invention is applied to a constant voltage power supply circuit, in a case where an n-channel output transistor is used.

FIG. 3 is a circuit diagram showing a basic example of an analog output circuit according to the present invention when a p-channel output transistor is used.

FIG. 4 is a circuit diagram showing a basic embodiment of an analog output circuit according to the present invention when an n-channel output transistor is used.

FIG. 5 is a circuit diagram showing a case where a conventional analog output circuit uses a p-channel output transistor.

FIG. 6 is a circuit diagram showing a case where a conventional analog output circuit uses an n-channel output transistor.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 p-channel output transistor 2 n-channel output transistor 3 resistor 11 input transistor 12 input transistor 13 voltage dividing resistor 14 voltage limiting element 16 voltage limiting element 17 voltage limiting element 20 analog output circuit 30 differential amplifier E ground side Power supply potential point Si input signal So output signal V Power supply voltage side power supply potential point or power supply voltage Vr Reference voltage Vo Constant voltage output

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H03K 17/567 H03K 17/10

Claims (3)

(57) [Claims] 1. An analog field effect output transistor and a resistor are connected in series between a pair of power supply potential points, and an analog operation field effect transistor is connected between the gate of the output transistor and a power supply potential point on the resistance side. Connect the input transistor, connect the voltage dividing resistor and the voltage limiting element in parallel between the gate of the output transistor and the power supply potential point on the output transistor side,
In an analog output circuit for extracting an output signal from an interconnection point between an output transistor and a resistor, an input transistor controls an input signal of a difference signal between the output signal and a reference voltage value, and extracts an output signal as a constant voltage output. An analog output circuit characterized by the above. 2. A circuit according to claim 1, wherein a bipolar transistor is used as the input transistor, the collector and the emitter are connected between the gate of the output transistor and the power supply potential point on the resistance side, and the base is connected to the input signal. An analog output circuit characterized by being controlled. 3. The circuit according to claim 1, wherein the voltage limiting element is a first voltage limiting element having a forward direction from a gate of the output transistor to a power supply potential point on the output transistor side, and a first voltage limiting element. An analog output circuit comprising a second voltage limiting element having a direction opposite to that of the second voltage limiting element and connected in parallel.