JP3243947B2 - Operational amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an operational amplifier having a clamp circuit, and more particularly, to an operational amplifier having a fast response to a recovery from a limited output voltage.

[0002]

2. Description of the Related Art An operational amplifier having a clamp circuit keeps an output voltage constant when an input voltage value changes from a linear operation state to, for example, prevent the output voltage from swinging to a power supply voltage. It operates to limit the value.

FIG. 3 is a circuit diagram showing an example of an operational amplifier having such a conventional clamp circuit. In FIG. 3, 1, 2, 3, 4 and 5 are transistors, 6 and 7 are constant current sources, 8 and 9 are resistors, and 10 and 11 are diodes. 100 and 101 are input voltages, 102 is an output voltage, and 103 is a clamp reference voltage.

[0004] Also, 1, 2, and 6 represent a differential circuit 50,
3, 4, 8 and 9 constitute an active load circuit 51, 5 and 7 constitute an output stage circuit 52, and 10 and 11 constitute a clamp circuit 53, respectively.

[0005] Input voltages 100 and 101 are connected to the bases of transistors 1 and 2, respectively.
Are the emitter of the transistor 2 and the constant current source 6
To one end.

The collector of transistor 1 is connected to the collector and base of transistor 3 and the base of transistor 4 respectively. The collector of the transistor 2 is connected to the collector of the transistor 4, the base of the transistor 5, the cathode of the diode 10, and the anode of the diode 11, respectively.

The emitters of the transistors 3 and 4 are connected to a resistor 8
9 is connected to one end of the constant current source 7 while the emitter of the transistor 5 outputs the output voltage 102 and is connected to one end of the constant current source 7. Further, the clamp reference voltage 10 is applied to the anode of the diode 10 and the cathode of the diode 11.
3 is applied.

Further, the other ends of the resistors 8 and 9 and the collector of the transistor 5 are connected to a positive voltage source, and the other ends of the constant current sources 6 and 7 are connected to a negative voltage source, respectively.

Now, the operation of the conventional example shown in FIG. 3 will be described. When the input voltages 100 and 101 are at the same potential, the conventional example enters a linear operation state, and the output current of the constant current source 6 becomes 2
It is equally divided and flows to transistors 1 and 2.

Further, since the transistors 3 and 4 constitute a current mirror circuit, the same current as the current flowing through the transistors 1 and 2 flows. At this time, the diodes 10 and 11 constituting the clamp circuit 53 are each "OFF".
It has become.

When the input voltages 100 and 101 change, FIG.
When the potential of the middle "A" becomes high, the diode 11 becomes "ON".
, And accordingly, the transistor 2 is turned “OFF”.

When the clamp reference voltage 103 is set to "V _CLAMP ",
The base-emitter voltage of transistor 5
_{Assuming that} V _BE5 ”and the forward voltage of the diode 11 are“ V ₁₁ ”, the output voltage 102“ V _OUT ”becomes V _OUT = V _CLAMP + V ₁₁ −V _BE5 (1).

On the other hand, when the potential "A" in FIG. 3 becomes low, the diode 10 turns "ON", and accordingly, the transistors 1, 3 and 4 turn "OFF". Diode 10
Is "V ₁₀ ", the output voltage is 102 ".
V ′ _OUT ″ becomes V ′ _OUT = V _CLAMP −V ₁₀ −V _BE5 (2)

As a result, the conventional example shown in FIG. 3 can operate to limit the output voltage 102 to a constant voltage value in the clamp operation state.

[0015]

However, in the conventional example shown in FIG. 3, the operating current in the clamp operation state is different from the operation current in the linear operation state. That is, when the diode 11
At the time of “N”, since the transistor 2 is “OFF”, the output current of the constant current source 6 flows through the transistor 1 as it is.

Further, since the transistors 3 and 4 constitute a current mirror circuit, the same current flows through the transistors 3 and 4. Therefore, the operating current of the transistors 1, 3 and 4 is twice that of the linear operating state.

On the other hand, when the diode 10 is "ON", the transistors 1, 3 and 4 are "OFF", so that the output current of the constant current source 6 flows through the transistor 2 as it is. Therefore, the operating current of the transistor 2 is twice that of the linear operating state.

Here, in order to recover from the clamp operation state to the linear operation state, the operating current of each transistor needs to return to the value of the linear operation state.

However, as can be seen from FIG. 3, the transistors 3 and 4 are PNP transistors, and generally have a lower operation speed than NPN transistors, and also have a slow response due to a thermal effect in addition to an electrical response. However, there is a problem that the response from the clamp operation state is limited by the operation speed of the transistors 3 and 4. Therefore, an object of the present invention is to realize an operational amplifier having a fast recovery response from the clamp operation state.

[0020]

According to the present invention, there is provided an operational amplifier having a clamp circuit for limiting an output voltage to a constant voltage value based on an operation state caused by a change in an input voltage. a differential circuit input voltage is input, and an active load circuit two outputs of the differential circuit is connected, an output stage circuit for outputting one of the outputs of the differential circuit as the output voltage, the difference The other side of the motion circuit
And the active load circuit have a cathode and an anode.
A first diode respectively connected to the differential circuit;
The cathode and anode are connected to the other output and one output of
A second diode respectively connected to the differential circuit;
One output and an emitter at the base of the active load circuit
And collector are connected respectively, and the base is clamped.
And a transistor to which a quasi-voltage is applied.
And a pump circuit .

[0021]

[Operation] Of the two transistors constituting the differential circuit,
The current flowing through the transistor operating alone in the clamp operation state is applied to the two transistors constituting the active load circuit.
By flowing the current equally, the current flowing through the two transistors constituting the active load circuit becomes constant regardless of the linear operation state or the clamp operation state.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings.
FIG. 1 is a circuit diagram showing one embodiment of the operational amplifier according to the present invention. Here, 1 to 9, 50, 51, 52, 100,
Reference numerals 101 and 103 have the same reference numerals as in FIG. FIG.
, 12 and 13 are diodes, 14 is a transistor, and 102a is an output voltage. 12 to 14 constitute a clamp circuit 53a.

Input voltages 100 and 101 are connected to the bases of transistors 1 and 2, respectively,
Are the emitter of the transistor 2 and the constant current source 6
To one end.

The collector of the transistor 1 is a diode 1
The collector of the transistor 2 is connected to the collector of the transistor 4, the base of the transistor 5, the anode of the diode 13, the transistor 14
Are connected to the respective emitters.

The anode of the diode 12 is connected to the collector and base of the transistor 3, the base of the transistor 4, and the collector of the transistor 14, respectively.

The emitters of the transistors 3 and 4 are connected to a resistor 8
9 is connected to one end of the transistor 5, and the emitter of the transistor 5 outputs the output voltage 102a and outputs the output voltage 102a.
To one end. Further, a clamp reference voltage 103 is applied to the base of the transistor 14.

Further, the other ends of the resistors 8 and 9 and the collector of the transistor 5 are connected to a positive voltage source, and the other ends of the constant current sources 6 and 7 are connected to a negative voltage source, respectively.

Here, the operation of the embodiment shown in FIG. 1 will be described. However, it is assumed that a non-inverting amplifier is configured using the operational amplifier according to the embodiment, and the output voltage 102a is fed back with an appropriate feedback rate and is input as the input voltage 101.

First, when the operational amplifier is in a linear operation state, the input voltages 100 and 101 have the same potential, and the output current of the constant current source 6 is divided into two equal parts and flows to the transistors 1 and 2 as described above.

At this time, the diode 12 constituting the clamp circuit 53a is "ON", the diode 13 and the transistor 14 are "OFF".
2 flows through the transistors 3 and 4 as they are.

Therefore, the output current of the constant current source 6 is "I",
The currents flowing through the transistors 3 and 4 are referred to as “I ₃ ” and “I
₄ ", then I ₃ = I ₄ = I / 2 (3).

Here, when the output voltage 102a rises with the rise of the input voltage 100 and the diode 13 is turned "ON", the current flowing from the collector of the transistor 4 to the transistor 2 flows to the transistor 1 via the diode 13. Start flowing.

That is, the open loop gain of the operational amplifier decreases, and the output voltage 102a does not follow the rise of the input voltage 100, and changes to the clamp operation state.

The input voltage 101 is equal to the output voltage 102a.
Is feedback, and cannot follow the rise of the input voltage 100. Therefore, the input voltage 101 becomes the input voltage 10
It becomes smaller than 0, and the transistor 2 is turned "OFF".

Therefore, the output current of the constant current source 6 flows through the transistor 1 as it is, is divided by the diodes 12 and 13, and flows through the transistors 3 and 4. However,
Since the transistors 3 and 4 form a current mirror circuit, the currents flowing through the transistors 3 and 4 become equal, and the relationship shown in Expression (3) is maintained.

At this time, the positive power supply voltage is set to “V”._CC", Transi
The base-emitter voltage of the_BE3”
And "V_BE5", Forward voltage of diodes 12 and 13
To “V ₁₂"And" V₁₃", The resistance value of the resistor 8 is" R ".
Output voltage 102a "V_{OU Ta}"Is V_OUTa= V_CC-IR / 2-V_BE3-V₁₂ + V₁₃-V_BE5 (4).

On the other hand, when the output voltage 102a falls with the fall of the input voltage 100 and the transistor 14 is turned "ON", the current flowing through the transistor 2 flows through the transistor 14 as well as the transistor 4.

That is, the open loop gain of the operational amplifier decreases, and the output voltage 102a does not follow the fall of the input voltage 100, and changes to the clamp operation state.

The input voltage 101 is equal to the output voltage 102a.
Is a feedback, and cannot follow the drop of the input voltage 100. Therefore, the input voltage 101 becomes the input voltage 10
It becomes larger than 0, and the transistor 1 is turned "OFF".

Therefore, the output current of the constant current source 6 flows through the transistor 2 as it is, and is divided into the transistor 4 and the transistor 3 via the transistor 14 to flow. However, since the transistors 3 and 4 constitute a current mirror circuit, the currents flowing through the transistors 3 and 4 become equal, and the relationship shown in Expression (3) is maintained.

At this time, the output voltage 102a "_V'OUTa"
Sets the base-emitter voltage of the transistor 14 to "V
If the _{BE14 ", V 'OUTa = V} CLAMP -V BE14 -V BE5 (5)

As a result, of the two transistors 1 or 2 constituting the differential circuit 50, the current flowing through the transistor 1 or 2 which operates alone in the clamp operation state is transferred to the two transistors 3 and 4 constituting the active load circuit 51.
The current flowing through the transistors 3 and 4 becomes constant regardless of the linear operation state or the clamp operation state.

Therefore, the recovery response from the clamp operation state is not limited by the operation speed of the transistors 3 and 4.

FIG. 2 is a structural block diagram showing another embodiment of the operational amplifier according to the present invention. Where 1,
2,4-7,9,12-14,50,52,53a, 1
Reference numerals 00, 101 and 103 have the same reference numerals as in FIG.

In FIG. 2, 3a is a transistor, 8a is a resistor, 102b is an output voltage, and 3a, 4, 8a and 9 constitute an active load circuit 51a.

The connection relationship is almost the same as that of the embodiment shown in FIG. 1, except for the transistor 3a and the resistor 8a.
Is the opposite of the connection relationship.

That is, one end of the resistor 8a is connected to the base of the transistor 4, the anode of the diode 12, and the transistor 1
4 and the other end of the resistor 8a is connected to the collector and base of the transistor 3a. The emitter of the transistor 3a is connected to a positive voltage source.

The operation of the embodiment shown in FIG. 2 operates similarly to the embodiment shown in FIG. Further, the resistor 8a functions to suppress the influence of the collector parasitic capacitance of the transistor 3a on the base of the transistor 4 in particular.

That is, since the collector parasitic capacitance of the transistor 3a is not directly connected to the base of the high-impedance transistor 4 through the resistor 8a, factors that limit the operation speed are reduced, and the recovery response is further increased.

[0050]

As is apparent from the above description,
According to the present invention, the following effects can be obtained. Of the two transistors constituting the differential circuit, the current flowing through the transistor which operates alone in the clamp operation state is divided into two equal parts and flows into the two transistors constituting the active load circuit, whereby the recovery response from the clamp state is improved. A fast operational amplifier can be realized.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of an operational amplifier according to the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the operational amplifier according to the present invention.

FIG. 3 is a circuit diagram showing an example of a conventional operational amplifier.

[Explanation of symbols]

 1, 2, 3, 3a, 4, 5, 14 Transistor 6, 7 Constant current source 8, 8a, 9 Resistance 10, 11, 12, 13 Diode 50 Differential circuit 51, 51a Active load circuit 52 Output stage circuit 53, 53a Clamp circuit 100, 101 Input voltage 102, 102a, 102b Output voltage 103 Clamp reference voltage

Claims (1)

(57) [Claims] An operational amplifier having a clamp circuit for limiting an output voltage to a constant voltage value based on an operation state caused by a change in an input voltage, wherein: a differential circuit to which the input voltage is input; An active load circuit to which an output is connected; an output stage circuit that outputs one output of the differential circuit as the output voltage; and a cathode connected to the other output of the differential circuit and the active load circuit.
The first diode to which the anode and anode are connected respectively
To the other output and one output of the differential circuit.
Second diode to which the anode and the anode are connected, respectively.
And one output of the differential circuit and the active load circuit.
The emitter and collector are connected to the base of
Transistor with clamp reference voltage applied to base
And a clamp circuit comprising: