JPH04127602A - Output circuit for operational amplifier - Google Patents

Abstract

PURPOSE:To realize the output circuit for an operational amplifier with a low output impedance up to a set maximum current by selecting a current switch comprising two transistors(TRs) of a same characteristic to protect a over current. CONSTITUTION:Bases of TRs Q5 and Q9 connect to a collector of a TR Q3 and an emitter of a TR Q4, and bases of TRs Q6 and Q10 connect to a collector of a TR Q2 and an emitter of a TR Q1. Moreover, let an IS of the TRs Q9, Q10 be smaller than an IS of the TRs Q5, Q6, and since no series resistor is inserted to emitters of the TRs Q5, Q6, the output impedance is low and the circuit is suitable for an operational amplifier at a high processing speed with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an operational amplifier output circuit having an overcurrent protection circuit, and more particularly to an operational amplifier output circuit with improved characteristics.

(Prior Art) An operational amplifier is an amplifier used as a calculation element of an analog electronic computer. An overcurrent protection circuit is provided to prevent overcurrent from flowing through the second operational amplifier and damaging it during operation. FIG. 3 is a diagram showing an example of an output circuit of an operational amplifier to which a conventionally used overcurrent protection circuit is added. In the figure, the emitter of NPN transistor Q1 and the collector of NPN h transistor Q2 are connected to resistor R1.
The emitter of transistor Q2 is connected to power supply V through resistor R2. It is connected to the.

Also, the power supply V is connected to the emitter through the resistor R3. The collector of the PNP transistor Q3 connected to C is the resistor R4.
, and the collector of transistor Q4 is connected to VEE. Voltage V11V2 is constant current source transistor Q11
It is supplied to the base as the bias voltage of Q2. Further, input terminal 1 is connected to the base of transistor Q11Q4.

The collector of the NPN l-transistor Q is connected to VCC, the emitter is connected to the output terminal 2 through the resistor R9, the emitter of the PNP transistor Q6 is connected to the output terminal 2 through the resistor R6, and the collector is connected to the connected to. Further, the collector of transistor Q3 and the base of transistor Q5 are connected, and the collector of transistor Q3 is connected to the base of transistor Q5.
The collector of Q6 and the base of transistor Q6 are connected.

The collector of NPN I-transistor Q7 is connected to the collector of transistor Q3 and the base of transistor Q5, the emitter to output terminal 2, and the base to the emitter of transistor Q. The emitter of transistor Q8 is connected to output terminal 2, the base is connected to the emitter of transistor Q6,
The collector is connected to the base of transistor Q6 and the collector of transistor Q2. Transistor Q+”
Q6 and resistors R4 to R6 constitute an operational amplifier as a pull-up amplifier, and transistors Q7Q8 and resistors R1 and R6 operate as an overcurrent protection circuit.

Next, the operation of this operational amplifier will be explained. When there is no input signal at input terminal 1, transistors Q3 and Q2 have bias voltages 1 and V2 applied to their bases.
It is turned on by

The collector current IC3 of the transistor Q3 supplies the emitter current of the transistor Q4, and also supplies the emitter current of the transistor Q4.
, and the emitter current IE'i of the transistor Q, flows through the resistor R5.
. flow. Similarly, the collector voltage of transistor Q2 is 1yl.
tlc2 supplies the emitter current of the transistor Q1 and is also inputted as the base current of the transistor Q6, and by causing the emitter current IE6 of the transistor Q6 to flow, a current 2 is applied to the resistor R6. flow. At this point, resistor R5
, R6 are equal, and the output current at output terminal 2 is zero.

Next, a case where a signal is input manually to the input terminal 1 will be explained. Transistors Q11Q4 are transistors Q2. Q
Since it is biased with a constant current by B, its pace emitter voltage VBE is constant (0.7V). Furthermore, a transistor Q.

Since collector current always flows through either transistor Q6. The base-emitter voltage VBE of either Q6 is approximately 0. 7V (typical value of VBH). Therefore, input terminal 1 and output terminal 2 have almost the same potential,
A current appears at the output terminal 2 according to the potential of the output terminal 2 and the load.

Now, when an overcurrent flows during the positive cycle of the input signal, current I flows through R9. becomes larger, IoR1≧0
, 4■, the base-emitter voltage of transistor Q7 increases, so transistor Q7 turns on, and the collector current of transistor Q7 pulls out the base current of transistor Q, and the emitter current of transistor Q5 decreases. Shi I. prevents the increase in When Io increases to a value that satisfies IoR1 = 0.7V (a typical value of the transistor's pace-emitter voltage VB), transistor Q7 becomes completely on, and transistor Q
The base current of transistor Q becomes the minimum value, the emitter current of transistor Q becomes the minimum value, and the output current is limited to this value. The same applies to the negative cycle of the human input signal.

(Problem to be Solved by the Invention) By the way, since the overcurrent protection circuit sets the maximum current based on the VBB of the transistors Q7 and Q8, the temperature dependence of the maximum current is about -0.3%/°C, which is large. There is a problem. Also, the transition period from when the overcurrent protection function starts working until it is completely restricted is long. This state is the fourth
It is shown in the figure. The figure shows the relationship curve between load resistance and output current. As is clear from the figure, the output current fluctuates greatly due to temperature changes, and the transition period is long. In the transition section, the output impedance of the amplifier increases, leading to deterioration of frequency characteristics and linearity. Also, if the resistance value of resistor R9R6 is large, the output current will be ■. Due to the slight increase in transistor Q7. Since the base voltage of Q8 rises and the overcurrent protection function is activated, it is not possible to increase the resistors R6 and R6, and it is necessary to make them 100Ω or less. This protection circuit cannot be used in processes where low resistance cannot be used. do not have.

The present invention has been made in view of the above points, and its object is to realize an operational amplifier output circuit with low output impedance up to a set maximum current.

Another object of the present invention is to realize an operational amplifier output circuit that has a small temperature coefficient at a set maximum current.

Still another object is to realize an operational amplifier output circuit having an overcurrent protection function even in a process in which low resistance cannot be used.

(Means for Solving the Problems) The present invention for solving the above-mentioned problems consists of a first transistor and a second transistor that serve as input signal transistors, and a constant current source with a constant bias voltage applied to the base. In the output circuit of an operational amplifier, the output circuit of an operational amplifier includes a third transistor and a fourth transistor, which serve as transistors for use, and a fifth transistor and a sixth transistor, which serve as output transistors. A seventh transistor is connected in parallel and forms a current switch with the third transistor, and the emitter of the fourth transistor is connected in parallel, and the '! an eighth transistor constituting the J4 transistor and a current switch;
its base is connected to a connection between the collector of the third transistor and the emitter of the second transistor;
a collector is connected to the base of the seventh transistor and connected to VCC via an output current detection resistor;
Its base and emitter are connected in parallel with the fifth transistor, which is an output transistor, and its emitter is connected to the output terminal, and the fifth transistor Is
9th for output current detection with IS value smaller than
The base of the transistor is connected to the junction between the collector of the fourth transistor and the emitter of the first transistor, and the collector is connected to the base of the eighth transistor and the output current detection resistor through the output current detection resistor. , whose base and emitter are output transistors.
a tenth transistor for output current detection, which is connected in parallel with the transistor, has an emitter connected to the output terminal, and has a value of 15, which is smaller than 15 of the sixth transistor. That is.

(Operation) In the positive cycle of the input signal, the output current is supplied by the fifth transistor. When the output current becomes an overcurrent, the collector current of the ninth transistor increases and the collector voltage decreases. Therefore, the base voltage of the seventh transistor decreases, switching it to the third transistor, turning off the third transistor, cutting off the base current supplied to the fifth transistor, and turning off the fifth transistor, which is the output transistor. Turn off the transistor to prevent overcurrent.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention. In the figure, parts equivalent to those in FIG. 3 are given the same reference numerals. Therefore, transistors Q1-Q6 and resistors R2 and R3 are operational amplifiers as conventional push-pull amplifiers. In the figure, Q is an output current detection NPN transistor whose base and emitter are connected in parallel to the base and emitter of an output transistor, NPN l-transistor Q.
to is a PNP transistor for output current detection whose base and emitter are connected in parallel to the base and emitter of a PNP transistor Q6, which is an output transistor. The bases of transistor Q and transistor Q are connected to the collector of transistor Q3 and the emitter of transistor Q4, and the bases of transistor Q6 and transistor Qll'l are connected to the collector of transistor Q2 and the emitter of transistor Q1. Also, I of transistors Q9 and Qlo are transistors Q and Ql.
, is smaller than Is of . Transistor Q11
is connected to the emitter of transistor Q3, which acts as a constant current source for transistor Q4, is connected to VCC through resistor R3, is connected to collector VEE, and is connected to the base or collector of transistor Q. The base of the transformer 7 is connected to V through resistor R7. Connected to C.

The emitter of the transistor Q12 is connected to the emitter of the transistor Q2, which operates as a constant current source for the transistor Q, and is connected to the VER via the resistor R2, the collector is connected to Vcc, and the base is connected to the collector of the transistor Qto. The base of the transistor shown in FIG. 1 is connected to VER through a resistor R8.

Next, the operation of the embodiment configured as described above will be explained.

In the embodiment of FIG. 1, the operation is performed complementary to the positive cycle and the negative cycle of the input signal, so only the positive cycle will be described. Since the operations of the parts explained in FIG. 3 of the conventional example are the same, they will be omitted, and only the operations of the parts unique to this embodiment will be explained.

In the positive cycle of the signal input to the input terminal,
When a positive current is output from the output terminal 2, the collector current of the transistor Q is designed not to exceed a certain set value. Now, the ratio of the saturation current IS5 of transistor Q and the saturation current IS9 of transistor Q9 is a(a)]5
).

] IS. -aIs9- (1) Since the transistors Q and Q5 have the same VBE, the ratio of the collector currents IC9 and IC9 is a. That is, I cq-a I c9 − (
2) The collector potential V3 of the transistor Q is given by the following equation.

... (3) As is clear from equation (3), when it increases by 1°, V3
becomes smaller.

In the current switch consisting of transistor Q3 and transistor Qz, its threshold is the bias voltage ■
1, and under normal conditions, transistor Q3 is on and transistor Ch+ is off, but as shown in equation (3), as IC'i increases, ■ and decreases, as shown in the following equation. Become a relationship.

V3-V, ...(4)(
From equations 3) and 4, V = Vcc IC5-(5)■ becomes (5
), the current switch consisting of transistor Q3 and transistor Q+1 switches,
Transistor Q3 turns off and transistor Q++ turns on. Is transistor Q1 working as a constant current source for emitter follower transistor Q?
Transistor Q is turned off, so transistor Q
4 also turns off, the base current of transistor Q5 disappears, transistor Q also turns off, and the output current becomes 0.
becomes. However, in reality, transistor Q and Q4 are balanced so that they do not become completely zero, and transistor Q
The emitter current of , will never become O, and therefore, the maximum value of the output current will be as shown in the following equation.

IC5-(VCCV5) ・= (6)(
6) I of formula. , or the maximum current, and if this value is exceeded, the overcurrent protection circuit will operate.

As explained above, this embodiment has the following effects.

(a) As a current detection transistor, a transistor Q9 has an Is value of 1/a of the Is of the output transistors Q11 and Q6.
．． Q. 'C9+' because they are connected in parallel.
The CIO also becomes IS, /a, and it is possible to use a resistor with a higher resistance value than Rq and R6 in the conventional circuit for the current-voltage conversion resistors R7 and R.

(b) Since no resistor is inserted in series with the emitters of the output transistors Q and Q6, the output impedance is low, making it suitable for high-speed, high-precision operational amplifiers.

(c) Since the current switch is used to switch the bias current during the overcurrent protection operation, the transition period can be smaller than the conventional switching method using the VBE of the transistor. FIG. 2 shows the output current characteristics of this example. It can be seen that the shift is clearly small when compared to the conventional characteristics shown in FIG.

(D) Overcurrent protection is achieved by switching current switches made up of two transistors with the same characteristics, so VBl, : changes with temperature, but two transistors change in the same direction. So, in the end, V
The relationship between 1 and V3 and the relationship between 2 and v4 are always the same, and are theoretically not subject to fluctuations due to temperature coefficients. The value is -0.3%/℃ in the conventional method, but in this method, the value is -0.01%/℃.
It becomes about 7%/℃.

(Threshold 2.5V, transition interval ±5VTVT-k
T/q) (e) The circuit of this embodiment only has two more transistors than the conventional circuit, and does not require a complicated circuit.

(Effects of the Invention) As explained in detail above, according to the present invention, since there is no series resistance in the output current circuit, the operational amplifier has a low output impedance in the output current range below the set maximum current at which the overcurrent protection function operates. can be realized. Furthermore, since the set maximum current is detected by a current switch composed of two transistors having the same characteristics, it is essentially independent of temperature, and an operational amplifier output circuit with a small temperature coefficient at the maximum current can be realized. Furthermore, a high resistance can be used as the current-voltage conversion resistor, and even if a high resistance is used, an operational amplifier output circuit having an overcurrent protection function can be realized, which has a great practical effect.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of an embodiment of the present invention, Fig. 2 is an output current characteristic diagram of the embodiment, Fig. 3 is a circuit diagram of a conventional device, and Fig. 4 is an output current characteristic of the circuit shown in Fig. 3. It is a diagram. 1...Input terminal 2...Output terminal Q11Q4
Human power transistor Q2. Q3. Constant current transistor Q6. Q6 ・Output transistor Q11Q, o・Output current detection transistor QllllQ
12. Current switch transistors R1 to R8...
resistance

Claims (1)

[Claims] A first transistor (Q_1) and a second transistor (Q_4) act as input signal transistors, and a third transistor whose base is supplied with a constant bias voltage acts as a constant current source transistor. In an output circuit of an operational amplifier including a transistor (Q_3), a fourth transistor (Q_2), and a fifth transistor (Q_5) and a sixth transistor (Q_6) which are output transistors, the third transistor ( The emitter of the third transistor (Q_3) is connected in parallel with the emitter of the third transistor (Q_3).
_3), a seventh transistor (Q_1_1) forming a current switch, and the emitter of the fourth transistor (Q_2) are connected in parallel, and the emitter of the fourth transistor (Q_2) is connected in parallel with the emitter of the fourth transistor (Q_2).
_2), an eighth transistor (Q_1_2) constituting a current switch, and its base is connected to the connection between the collector of the third transistor (Q_3) and the emitter of the second transistor (Q_4), and the collector is It is connected to the base of the seventh transistor (Q_1_1) and connected to V_C_C via the output current detection resistor (R_7), and its base and emitter are in parallel with the fifth transistor (Q_5), which is an output transistor. a ninth transistor (Q_9) for output current detection, whose emitter is connected to the output terminal (2) and has an I_S value smaller than the I_S of the fifth transistor (Q_5);
), its base is connected to the junction between the collector of the fourth transistor (Q_2) and the emitter of the first transistor (Q_1), and its collector is connected to the base of the eighth transistor (Q_1_2) for output current detection. Resistance (R
_8) to V_E_E, its base and emitter are connected in parallel with the sixth transistor (Q_6), which is an output transistor, and its emitter is connected to the output terminal (2
) and the sixth transistor (Q_6
) A tenth transistor (Q_1_0) for detecting an output current having an I_S value smaller than the I_S value of the operational amplifier.