JPS6218103A - Limiter circuit - Google Patents

Abstract

PURPOSE:To set optionally an output voltage by connecting a transistor (TR) between a current input terminal and an output terminal of an operational amplifier and turning on the TR when an output voltage reaches a prescribed level with respect to a reference voltage fed to the base. CONSTITUTION:A collector of a TR Q5 is connected to an inverting input (-) at an operational amplifier A, the emitter is connected to an output terminal T0 and an input terminal T1 of a reference voltage Vref is connected to the base. The TR Q5 is turned on when the output voltage Vout(=IinXRf) of the operational amplifier A reaches Vref-Vbe and limited at a prescribed amplitude at a voltage of Vref-Vbe, where Iin is the input current and Rf is the feedback resistor. Denoting Vbe as 0.7V and Vref as 0.2V, then Vref-Vbe=-0.5V. Thus, the output voltage Vout is limited to a prescribed amplitude at -0.5V.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a limiter circuit that limits the amplitude of the output voltage to a predetermined level to obtain high-speed response in a current-voltage conversion circuit using an operational amplifier. It is something.

[Technical Background of the Invention and Problems Therewith] An operational amplifier used as a current-voltage conversion circuit has a certain limit in response speed of an output voltage, and a slew rate is used as a characteristic representing this response speed. The slew rate is a response speed expressed by how many volts the output voltage changes per unit time of 1 μs.

The response characteristics become better as the slew rate value increases.
The current conversion circuit is capable of processing even higher frequency signals.

The slew rate has an amplitude dependence on the output voltage, and decreases when the amplitude of the output voltage increases by a large margin and the operational amplifier operates in the saturation region. Therefore, if the amplitude of the output voltage is limited to a predetermined value, the slew rate becomes large, and the response speed can be increased.

FIG. 9 shows a conventional limiter circuit that limits the amplitude of such an output voltage to a constant value. The non-inverting input terminal Φ of an operational amplifier is installed in the symbol in the same figure.
A feedback resistor Rf is connected between the inverting input terminal θ and the output terminal TO. A diode D is connected in parallel to the feedback resistor R[ such that the direction from the inverting input terminal e to the output terminal T1 is the forward direction. The forward voltage Vf of diode D is approximately 0.7V.

FIG. 10 shows the input/output characteristics of the limiter circuit. When the input current [in is below a certain current value, an output voltage vouj of -1in-R" proportional to the input current is applied to the output terminal T1. Then, when the input J current exceeds a certain value, the output voltage Volt becomes -Vf (=-0,
7V) or lower, the diode D hi conducts,
The amplitude of the output voltage is limited to a constant value at a voltage value of -V[.

By the way, when you want to set the slew rate to a desired value, or when you want to set the frequency characteristic to a desired frequency band and then set the amplitude range of the output voltage, which is proportional to the input current, as wide as possible, the output voltage There are cases where it is desired to set the amplitude limiting voltage of (if) to an arbitrary value.

However, in the limiter circuit described above, the amplitude limiting voltage is defined to a constant value by the forward voltage Vf of the diode, so it is not possible to set the amplitude limiting voltage to an arbitrary value.

[Objective of the Invention 1] The present invention was made based on the above circumstances, and an object thereof is to provide a limiter circuit which has a relatively simple configuration and is capable of limiting the output voltage to an arbitrary amplitude range. do.

[Summary of the Invention] In order to achieve the above object, the present invention connects a transistor between a current input terminal and an output terminal of an operational amplifier, and connects a transistor between a reference voltage applied to the base control terminal and an output obtained from the output terminal. by comparing the output voltage with the reference voltage and turning on the transistor when the output voltage reaches a predetermined voltage level with respect to the reference voltage;
This allows the output voltage of the operational amplifier to be set to any value.

[Effects of the Invention] According to the present invention, a feedback resistor is connected between a current input terminal and an output terminal, and a feedback resistor is connected between an operational amplifier that obtains an output voltage according to an input current, and the input terminal and output terminal. The circuit is relatively simple because it includes a transistor that is connected and that compares the reference voltage applied to the base control terminal with the output voltage and turns on when the output voltage reaches a predetermined voltage level. Despite the configuration, the output voltage of the operational amplifier can be limited to an arbitrary amplitude value according to the set value of the above-mentioned yuzu quasi-voltage. Therefore, the frequency characteristics of current/voltage conversion by the operational amplifier can be set to a desired high frequency.

[Embodiment 1 of the Invention A first embodiment of the invention will be described below based on FIG. 1.

A3 Figure 1, and Invasion Figures 3, 4, and 6
In the figure, components that are the same as or equivalent to the equipment and circuit elements shown in FIG.

First, to explain the configuration, in this embodiment, an operating circuit consisting of a pair of npn transistors, a first transistor QtJ3 and a second transistor Q2, is disposed downstream of the operational amplifier A.

The first transistor Q1 has its collector connected to the inverting input terminal e of the operational amplifier A, and its base (base control terminal) bl connected to the base! It! Voltage V ref input terminal 1
is connected. On the other hand, the second transistor Q2 is
A +■ voltage source is connected to its collector, and the base b2 is the output terminal T of the operational amplifier.

It is connected to the. The emitters of the intervening transistors Q1 and Q2 are commonly connected and connected to a -V voltage source.

Next, the operation will be explained with reference to the limiter characteristic diagram shown in FIG.

In the i\i region where the input current 1 inch is small, the input current 1 inch
flows to the feedback resistor Rr side, and the output voltage vout appearing at the output terminal TO becomes Vout--Iin-Rf-(1), and this output voltage vout is higher than the reference voltage Vref. The actuating circuit therefore consists of the second transistor Q
2 is in the on state, and the first transistor is in the off state.

As the input current [in increases, the value of the output voltage vout expressed by the above equation (1) decreases, and the reference voltage V re
When the relationship with f becomes V Out = V ref, the second transistor Q2 is cut off and the first transistor Q1 is turned on. Therefore, the input current 1in is expressed as the sum of the current I' to the feedback resistor Rf and the collector current Ifl of the first transistor Q1, which becomes 1in-[f+iρ-(2), and the output voltage yout is Vout- -If -Rr - (3).

At this time, the emitter current of the first transistor Q1 is
Then, the collector current [ρ is derived from the theory of the pH junction between the base and emitter and is expressed as follows.

r J2− re / (T+exp (Δv /v
t) ) ・= (4) Here △V = vout
-vref=.

vt kT /Q k: Boltzmann constant, T: absolute temperature q: electron charge amount ΔvgoO, so the above formula (4) can be written as IJ21e/(2+Δv/vt) = He/2) (1-(ΔV / 2vt)
)...(4)- to find yout in the above equation (3), vo
ut =-Rf (T in -1!1)--Rf
([in −(Ie / 2>・(1−(Δv /
2vt) ) ) = -1' [frin-(18/ 2> ・[1-(voui-vref /2vt) ] )
vout --Rf (r in(Ie/
2)-(L+-(vrcf/2vt))
) / (++ (Rf le /4vt) )...
(5) In equation (5) above, emitter current re is input current J
If you set it sufficiently large compared to in and set J34, Rf ・
Since the term lin can be ignored, the output voltage Vqut is: vout= (Rf I c vref/4
vt ) / (RIe /4vt )vr
ef
...(6).

The above equation (6) shows that the output voltage V out is the reference voltage Vre
It is shown that when it reaches the vicinity of f, the output voltage Vref is limited to a constant value by the value of the reference voltage V rcf without depending on the input current. Therefore, as shown in FIG. 2, if the reference voltage V ref is arbitrarily set to a desired value, the output voltage yout is limited to a constant +a with a corresponding amplitude of one cycle.

Next, FIG. 3 shows a second embodiment of the present invention.

This embodiment uses one transistor Q5, whereas the first embodiment uses a pair of transistors. The transistor Q5 has a collector connected to the inverting input terminal θ of the operational amplifier Δ, an emitter connected to the output terminal TO, and a base b3 connected to the reference voltage V.
The ref input terminal T1 is connected.

To explain the operation, if the voltage between the base and emitter of transistor Q5 is Vbe, the output voltage of A amplifier △■
When out = r in - Rf reaches a voltage of vref - Vbe, transistor Q5 turns on,
The output voltage yout has a constant amplitude (limited to 0) at the voltage Vre[-vbe. For example, if Vbe of the transistor Q5 is 0.7V, and the voltage level ffEVre[ is 0.2
V1. : If set, Vre "-vbe=0.2
-0.7=-0.5V. Therefore, operational amplifier A
The output voltage vOtI is limited to a constant amplitude at -0°5V.

Next, FIG. 4 shows a third embodiment of the present invention. In this embodiment, an input current of 1 inch limits the output voltage yout to a constant amplitude value in either the positive or negative direction. This is what I did. Therefore, in this practical example, in addition to the first operating circuit 1 similar to the first embodiment, a pair of pn transistors, a third transistor Q3 and a fourth transistor Q4, are used.
A second actuating circuit 2 consisting of a pl-transistor is provided.

That is, each transistor Q1, Q in the first operating circuit 1
2 and each l-transistor Q3 in the second actuating circuit 2
, Q4 are of complementary type. Therefore, the second reference voltage Vref2 applied to the second actuating circuit 2 and the collector voltage of the fourth transistor Q4 have only the polarity opposite to each voltage applied to the first actuating circuit 1. Shino is used respectively.

To explain the action using the limiter characteristic diagram in FIG. 5, for the input current (in in the positive direction (solid line direction) in FIG.
has a limiting effect. The operation at this time is almost the same as that of the first embodiment.

While the output voltage vout of the output terminal To is below Ov and the output voltage Vout is limited by the first operating circuit 1 as described above, the second operating circuit 2
- The transistor Q3 remains in the OFF state and no output limiting effect occurs.

On the other hand, for input current of 1 inch in the negative direction (in the direction of the chain line in Fig. 4), the second actuating circuit 2
side acts, and as shown in Figure 5, the positive output 'jJ voltage,
A limiting action on +yout is performed. In this case, almost the same effect as in the first embodiment occurs, except that only the polarities of voltage, current, etc. are reversed, and the output voltage v
out -I in-Rf is the second reference voltage +-Vre
When reaching around f2, the output voltage vout is limited to a constant value of the reference voltage +vref2, without depending on the input current 1 inch.

[Concrete example〕 FIG. 6 shows a specific example of the first embodiment.

Reference numeral 3 in FIG. 6 indicates an equivalent circuit of a photosensor using a PIN photodiode.
An input current ■in of -[, 20μApp, etc. is input to the inverting input terminal e of the amplifier A.

On the other hand, in the limiter circuit, the reference voltage V ref is -0
, 4V, and the emitter current re flowing through the common emitter section is 8V, which is several times the input current 1in.
It is specified as 0μA.

FIG. 7 shows the limiter characteristics of the limiter circuit, and FIG. 8 shows the frequency characteristics. As shown in FIG. 7, the output voltage vout is limited to approximately -0.4V or less, in other words, the input current Iin is limited to approximately 40 μA or more. The frequency a characteristic extends to a frequency band of approximately 10 MH2. Therefore, a high frequency signal current from a high-speed responsive sensor such as a PIN photodiode is appropriately converted into voltage.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a first embodiment of a limiter circuit according to the present invention, Fig. 2 is a characteristic diagram showing limiter characteristics of the same embodiment, and Fig. 3 is a circuit diagram showing a second embodiment of the invention. figure,
FIG. 4 is a circuit diagram showing a third embodiment of the invention, FIG. 5 is a characteristic diagram showing limiter characteristics of the third embodiment, FIG. 6 is a circuit diagram showing a specific example of the invention, and FIG. 8 is a characteristic diagram showing the frequency characteristics of the above specific example. FIG. 9 is a circuit diagram showing a conventional limiter circuit. FIG. 10 is a limiter characteristic of the conventional example above. It is a characteristic diagram without showing. 1.2... Operating circuit, A... Operational amplifier,
Ql, Q2, Q3, Q4, Q5...transistor, R
f: Feedback resistance, TO: Output terminal, T1: Reference voltage input terminal. Agent Patent Attorney l'I11 Ken Chika Yudo Patent Attorney Norifu Ogo-V Figure 4! Figure 7 One round moat (Hz) Figure 8

Claims (1)

[Scope of Claims] An operational amplifier in which a feedback resistor is connected between one of two input terminals and an output terminal, and obtains an output voltage according to an input current to the one input terminal; The output voltage is compared with a reference voltage of a predetermined value connected between the one input terminal and the output terminal of the operational amplifier and applied to the base control terminal, and the output voltage reaches a predetermined voltage level. A limiter circuit comprising: a transistor that turns on when the circuit is turned on;