JPH0635537Y2 - Operational amplifier protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a protection circuit for an operational amplifier, and particularly when used in a circuit in which circuit power supplies having a voltage higher than the power supply voltage used for this operational amplifier are mixed. In addition, the present invention relates to a protection circuit for an operational amplifier which avoids damage caused by short circuit of the circuit power supply to the output terminal of the operational amplifier.

<Prior Art> FIG. 2 is a circuit diagram showing a configuration of a circuit of a conventional operational amplifier.

Q ₁ is an operational amplifier, the inverting input terminal (−) of which is connected to the input terminal T ₁ via the input resistor R _i and the circuit output terminal T ₂
Is connected via a feedback resistor R _f . The output terminal of the operational amplifier Q ₁ is connected to the circuit output terminal T ₂ via the limiting resistor R ₀ , and its non-inverting input terminal (+) is connected to the common potential point COM.
It is connected to the.

With such a configuration, for example, even if the circuit output terminal T ₂ is short-circuited by the circuit voltage E _L higher than the power supply voltage ± Vcc of the operational amplifier Q ₁ , if the value of the limiting resistor R ₀ is selected large,
With E _L , the excessive current flowing to the output terminal of the operational amplifier Q ₁ can be sufficiently limited by the limiting resistor R ₀ . Also in this case, the amplification degree between the input terminal T ₁ and the circuit output terminal T ₂ is not impaired.

However, in this case, if the value of the limiting resistor R ₀ is selected to be large in order to prevent the amplifier Q _{1 from} being damaged by the short circuit of the high circuit voltage E _L to the circuit output terminal T ₂ , the amplification factor of the circuit is changed. However, the amplitude of the output voltage and output current that can be used at the circuit output end T ₂ is limited.

Therefore, a circuit protection configuration as shown in FIG. 3 can be considered.

The configuration shown in FIG. 3 is different from the configuration shown in FIG. 2 in that a diode D ₁ is provided between the output terminal of the operational amplifier Q ₁ and the power supply terminal + Vcc.
However, the difference is that the diode D ₂ is connected between the power supply terminal and Vcc.

With such a configuration, when the circuit output terminal T ₂ is short-circuited by the high-voltage circuit voltage E _L , the current flowing by the circuit voltage E _L passes through the limiting resistor R ₀ ′ and the diodes D ₁ , D ₂ are generated. Therefore, if the voltage drop in the quasi-direction of the diodes D ₁ and D ₂ is ± 0.6V, the output voltage level is (+ Vcc-0.6V) to (-Vcc
Clamped to + 0.6V).

Therefore, the output voltage amplitude is (+ Vcc-0.6V) to (-Vcc +
The value of the limiting resistance R ₀ ′ can be selected smaller than that of the limiting resistance R ₀ shown in FIG.

By the way, the output stage of the operational amplifier Q ₁ is constructed, for example, as shown in FIG.

Between the power supply terminal + Vcc and -Vcc, resistor R ₁ , diode D ₃ ,
D ₄ and resistor R ₂ are connected in series, and the emitter and collector of transistor Q ₂ and resistor R ₃ and emitter and collector of transistor Q ₃ are connected in series in parallel with this series circuit.

In addition, a bias voltage is applied to the base of transistor Q ₂ from the connection point of resistor R ₁ and diode D ₃ , and
A bias voltage is applied to the base of the transistor Q ₃ from the connection point of D ₄ and the resistor R ₂ . And operational amplifier
The output of Q ₁ is drawn from the collector of transistor Q ₃ . A resistor that limits the source current in such an output stage
An example of an operational amplifier that has only R ₃ and does not have a resistor that limits the sink current is a model LM2904 manufactured by National Semiconductor.

However, in this way, only the resistor R ₃ that limits the source current is provided, and the resistor that limits the sink current is connected to the transistor Q ₃
In the case of an operational amplifier not provided on the side, even if clamped by diodes D ₁ and D ₂ as shown in FIG. 3,
The transistor Q ₃ is destroyed by the current I ′ resulting from the circuit voltage E _L flowing into the transistor Q ₃ from the output terminal of the operational amplifier.

Therefore, as shown in FIG. 5, a resistor that limits the sink current.
It is also conceivable to adopt an operational amplifier having a mechanism of inserting R ₄ on the collector side of the transistor Q _{3 as} the operational amplifier Q ₁ shown in FIG.

However, the operational amplifier Q ₁
Even if the output stage of Q ₁ is protected, limiting resistor R ₀ ′, diode D ₁ , power supply terminal + Vcc or limiting resistor R ₀ ′,
High-voltage circuit voltage E via diode D ₂ and power supply terminal −Vcc
_The power supply that supplies the power supply voltage ± Vcc to the operational amplifier Q ₁ is destroyed by the excessive current that flows in by shorting _L to the circuit output terminal T ₂ .

<Problems to be Solved by the Invention> Therefore, in the present invention, when a circuit voltage that is a voltage higher than the power supply voltage of the operational amplifier is mixed in the circuit, even if this circuit voltage is short-circuited to the circuit output end. An object of the present invention is to provide a protection circuit for an operational amplifier, which does not limit the output amplitude of the operational amplifier and does not destroy the operational amplifier or the power supply for supplying the power supply voltage of the operational amplifier.

<Means for Solving the Problems> In order to solve the above problems, the present invention provides a voltage supply terminal in which a current limiting resistor and a zener diode are connected in series and a circuit voltage is applied, and a previous current limiting resistor. Power supply voltage generating means for generating a power supply voltage corresponding to the voltage between both ends, a first limiting resistor connected to the output terminal of the operational amplifier energized by the power source voltage, the first limiting resistor and the circuit output terminal. A second limiting resistor connected in series with and one end of the first limiting resistor and the second limiting resistor connected at one end and the other of the current limiting resistor and the Zener diode at the other end. A diode connected to the connection point to limit the voltage at the output of the operational amplifier, a feedback resistor connected between the previous circuit output and the inverting input of the previous operational amplifier, and one end of this feedback input Connected to the input resistor and the input signal is applied to the other end. It is equipped with a resistance.

<Operation> When the circuit output terminal is short-circuited at a high voltage, the first limiting resistor limits the short-circuit current flowing into the output terminal of the operational amplifier, and the diode limits the amplitude of the output voltage of the operational amplifier. The short-circuit current flowing in the diode is discharged to a predetermined circuit element, and further, the short-circuit current flowing into the output terminal of the operational amplifier and the short-circuit current flowing in the diode are both limited by the second limiting resistor to ensure the operation of the operational amplifier. .

<Embodiment> An embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 is a circuit diagram showing the configuration of an embodiment of the present invention.

A circuit voltage E _L is applied between terminals T ₃ and T ₄ , and this circuit voltage E _L
By applying _L series circuit of a resistor R ₅ and the Zener diode D _z is obtained a Zener voltage V _z across the Zener diode D _z. The base and collector of the transistor Q ₄ are connected to both ends of the resistor R ₅ , and the power supply voltage Vcc which is the power supply of the operational amplifier Q ₁ is obtained between the emitter and the terminal T ₄ of the transistor Q ₄ .

The inverting input terminal (−) of the operational amplifier Q ₁ is connected to the other end of the input resistor R _i , one end of which is connected to the input end T ₁ , and the feedback resistor R ₁ is connected between the inverting input end (−) and the circuit output end T _{2. f} is connected. A limiting resistor R ₆ is placed between the output terminal of the operational amplifier Q ₁ and the circuit output terminal T _2.
And R ₇ are connected in series. These limiting resistors R ₆ and R ₇
The anode of the diode D ₅ is connected to the connection point K with and the cathode thereof is connected to the cathode of the zener diode Dz. The non-inverting input terminal (+) of the operational amplifier Q ₁
Are connected to a common potential point COM.

Next, the operation of the embodiment shown in FIG. 1 configured as above will be described.

The operational amplifier Q ₁ has an amplification degree determined by the input resistance R _i and the feedback resistance R _f .

When the circuit voltage E _L, which is higher than the power supply voltage Vcc, contacts the circuit output terminal T ₂ and is short-circuited, the voltage V _{K at the} connection point K is limited to (V _z −0.6 V) by the diode D _5. It At this time, the short-circuit current I ₁ flowing in the diode D ₅ is limited by the limiting resistor R ₇ that absorbs excess voltage, and its value is
D ₅ and Zener diode D _z are selected so as not to exceed the power dissipation.

On the other hand, the short-circuit current flowing into the output of the remaining operational amplifier Q ₁
I ₂ is limited by limiting resistor R ₆ so as not to exceed the allowable loss of operational amplifier Q ₁ .

By limiting resistor R _6, R ₇ and diode D ₅ above, to ensure the amplitude of the near to the output voltage of the power supply voltage Vcc of the operational amplifier Q _1, and while protecting the output stage of the operational amplifier Q _1, power supply voltage The operation of the operational amplifier Q ₁ is ensured even if a voltage higher than Vcc is short-circuited to the circuit output terminal T ₂ .

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, even when the circuit voltage, which is higher than the power supply voltage, contacts the circuit output terminal, the first limiting resistor and the second limiting resistor are connected. Since the voltage at the connection point with the resistor is fixed to the clamp voltage that is lower than the zener voltage of the zener diode by the forward voltage drop of the diode, which is a small value, first, it flows into the output terminal of the operational amplifier. The short-circuit current that occurs is current limited by the first limiting resistor, and the safety of the operational amplifier is ensured.
In addition, the shunt current shunted to the diode is limited by the second limiting resistor, so that the safety of the diode can be secured.

In this way, the first and second limiting resistors and the diode ensure the output voltage amplitude close to the power supply voltage of the operational amplifier, and even if a voltage higher than the power supply voltage contacts the circuit output terminal, Destruction can be prevented.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of one embodiment of the present invention, FIG. 2 is a circuit diagram showing a circuit configuration of a conventional first operational amplifier, and FIG. 3 is a circuit configuration of a conventional second operational amplifier. 4 is a partial circuit diagram showing the first configuration of the output stage of the operational amplifier shown in FIG. 3, and FIG. 5 is the second configuration of the output stage of the operational amplifier shown in FIG. It is a partial circuit diagram shown. Q ₁ …… Operational amplifier, Q _{2 to} Q ₄ …… Transistor, R _i …… Input resistance, R _f …… Feedback resistance, D _z …… Zener diode,
Vcc: Power supply voltage, E _L: Circuit voltage.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Bibliography Sho 55-148209 (JP, U) Steady Sho 61-8319 (JP, U) Opamp IC utilization know-how, CQ Publishing Co., Ltd. November 1984 1st, P. 240

Claims (1)

[Scope of utility model registration request] 1. A voltage supply terminal to which a circuit voltage is applied, in which a current limiting resistor and a zener diode are connected in series, and a power source voltage generating means for generating a power source voltage corresponding to the voltage across the current limiting resistor. A first limiting resistor connected to the output terminal of the operational amplifier energized by the power supply voltage, a second limiting resistor connected in series to the first limiting resistor and the circuit output terminal, and one end of the first limiting resistor. A diode connected to a connection point between the limiting resistance and the second limiting resistance and having the other end connected to a connection point between the current limiting resistance and the zener diode to limit the voltage at the output end of the operational amplifier; and the circuit output end. And an inverting input terminal of the operational amplifier, and a feedback resistor connected between the inverting input terminal and the inverting input terminal of the operational amplifier. Protection circuit.