JP2965567B2 - Low frequency amplifier circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a low-frequency amplifier circuit used for audio amplification of a telephone device.

(Prior Art) A conventional low frequency amplifier for audio signals of a telephone device has a configuration as shown in FIG. The input signal from the input terminal 15 is input to the
The signal is input to the negative input terminal of the operational amplifier 13 constituting the amplification circuit of the stage. A feedback resistor 11 is connected between the negative input terminal and the output terminal of the operational amplifier 13. The positive input terminal is connected to a reference power supply.

The output signal from the first-stage amplifier circuit is input to the negative input terminal of the operational amplifier 7 constituting the second-stage amplifier circuit via the resistor 10, the coupling capacitors 9, 8 and the input resistor 6. The positive input terminal is connected to a reference power supply, similarly to the amplifier 13.

A feedback resistor 5 and a feedback capacitor 19 are connected in parallel between the negative input terminal and the output terminal of the operational amplifier. The output from the operational amplifier 7 is output to the output terminal 1 via the coupling capacitor 4. Zener diodes 2 and 3 are connected in reverse series to output terminal 1 and are connected to ground.

A collector of the transistor 16 is connected to a connection point of the coupling capacitors 8 and 9 to form a mute circuit.
The emitter terminal is grounded.

The gate of the transistor 16 is connected via a current limiting resistor 17
CPU 18 is connected.

Next, the operation of the circuit of FIG. 2 will be described. This circuit is a low-frequency amplifier circuit in which two stages of negative feedback inverting input amplifier circuits are connected.

The transistor 16 is a mute circuit. When a high-level control signal is output from the CPU 18, the transistor 16 is turned on, the input of the second-stage amplifier including the operational amplifier 7 is shunted, and the mute state is set. When a low-level control signal is output from the CPU 18, the transistor 16 is turned off and has no effect on the amplifier circuit.
The zener diodes 2 and 3 connected in reverse series constitute a limiter circuit, and limit the amplitude of the output signal.

Here, the Zener voltage of Zener diodes 2 and 3 is Vz
Assuming that the forward ON voltage of the diode is 0.7 V, when the output signal exceeds + (Vz + 0.7) V, the Zener diodes 2 and 3 are turned on, and the output signal is shunted to the ground. Therefore, the output signal of the amplifier circuit does not exceed (Vz + 0.7) V. Similarly, the output signal does not fall below-(Vz + 0.7) V.

Therefore, the output amplitude of this circuit is 2 (Vz + 0.7) V _pp
Is limited to

(Problems to be Solved by the Invention) As described above, the circuit shown in FIG. 2 uses a Zener diode to constitute a limiter circuit. Therefore, in order to limit the amplitude of the output signal in both the positive direction and the negative direction, there is a problem that two expensive zener diodes are necessarily required.

Further, when a semiconductor varistor is used instead of a Zener diode, only one device is required, but there is a problem that the device is more expensive and the shape of the element becomes relatively large. Further, when the Zener diode is a semiconductor varistor, there is a problem that the limit value of the output amplitude is limited by the forward ON voltage of the diode or the Zener voltage.

Further, there is also a problem that a wide-range distortion component when the output signal is clipped by the limit circuit appears as it is in the output.

The present invention has been made to solve such a problem, and it is an object of the present invention to provide a low-frequency amplifier circuit which can realize a limiter circuit with one diode by combining with an existing mute transistor. Aim.

[Configuration of the invention]

(Means for Solving the Problems) According to the present invention, a first amplifier circuit for amplifying an input signal from an input terminal and outputting the amplified signal to a connection point, and an output signal of the first amplifier circuit at the connection point A second amplifier circuit for amplifying and outputting to the output terminal; a transistor having a collector connected to the connection point and an emitter connected to ground; a diode having an anode connected to the connection point and a cathode connected to ground. And a control circuit that supplies a control signal to the base of the transistor and controls the transistor. The mute / limiter circuit is configured such that when the control signal is at a high level, the transistor is turned on. When the control signal is at a low level, the transistor is turned off. When the connection point exceeds a predetermined voltage, the diode is turned on and shunts to ground.When the connection point falls below a predetermined voltage, the collector and emitter of the transistor are turned on and shunted to ground, thereby limiting the limiter. Provided is a low-frequency amplifier circuit that operates.

(Operation) The present invention utilizes the fact that the transistor constituting the mute circuit has a 0.7 V limiting action on the negative side of the signal, and this section limits the 0.7 V limit on the positive side of the signal. It has an effect.

If a diode is connected in parallel with the transistor for this purpose, this diode has a 0.7 V limiting action on the positive side.

The signal amplitude is limited to 2 × 0.7 = 1.4 V _pp by the mute transistor and the diode. That is, a limiter circuit of 1.4 V _pp is formed.

The output limiter level can be arbitrarily determined by appropriately setting the amplification degree of the amplifier circuit located behind the limiter circuit. In addition, by providing the amplifier circuit with a frequency characteristic for lowering the wide band frequency, a high frequency component generated by the limiter operation can be reduced, and output distortion can be reduced.

Embodiment An embodiment of 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 present invention. The same components as those of the conventional circuit shown in FIG. 2 are denoted by the same reference numerals, and detailed description thereof will be omitted.

The difference from the conventional circuit shown in FIG. 2 is that the Zener diodes 2 and 3 connected to the output terminal 1 are omitted, and a diode 20 is sequentially provided between the collector and the emitter of the transistor 16 constituting the mute circuit. It is a point connected in the direction.

In the circuit shown in FIG. 1, since the transistor 16 is an NPN transistor, the diode 20 is connected in a forward direction between the collector and the emitter. Is connected to the power supply terminal, the diode 20 is also connected between the power supply terminal and the collector terminal.

Next, the operation of the circuit of FIG. 1 will be described. The low-frequency signal input from input terminal 15 is coupled to coupling capacitor 14,
The signal is amplified by a negative feedback inverting input amplifier circuit comprising a feedback resistor 11, an operational amplifier 13, and is output to the next stage.

When the CPU 18 is outputting a high-level control signal, the base current of the transistor 16 flows through the resistor 17 and the transistor 16 is turned on. For this reason, the output of the first stage amplifier is shunted to ground via the resistor 10 and the capacitor 9, and is not transmitted to the second stage.
Does not show a signal.

On the other hand, when the CPU 18 outputs a low-level control signal, the base current does not flow through the transistor 16, so that the transistor 16 is turned off. Therefore, the output of the first stage amplifier circuit is composed of the input resistor 6, the feedback capacitor 1
The signal is amplified by a second-stage non-feedback inverting input amplifier circuit including a feedback resistor 5 and an operational amplifier 7, and output to an output terminal 1 via a coupling capacitor 4.

The second-stage amplifier circuit operates as a low-pass filter having a cutoff frequency fH = 1 / 2πR ₅ C ₁₉ because of the feedback capacitor 19.

Note here R ₅ is the resistance value of the feedback resistor 5, C ₁₉ represents a capacitance value of the feedback capacitor 19, respectively.

Next, the limiter circuit will be described. When the mute circuit is operating, there is no output, and there is no need to operate the limiter circuit. Therefore, a state where the mute circuit is not operating, that is, a case where the CPU 18 is outputting a low-level control signal will be described.

The connection point of the coupling capacitors 8 and 9 is now point A. If the signal amplitude at point A exceeds + 0.7V, a diode
20 turns on and the signal is shunted to ground. Therefore, the amplitude is limited to 0.7 V at the + side. If the signal amplitude at point A falls below -0.7V,
Since the base potential of 16 is the ground potential, the base potential viewed from the collector potential (potential at point A) increases by 0.7 V or more, so current flows from the base to the collector,
The area between the collector and the emitter is turned on, and the point A is shunted to the ground. Therefore, the amplitude at the point A does not become lower than -0.7V.

In this case, the emitter terminal of the transistor 16 operates as a collector, and the collector terminal operates as an emitter. This is a phenomenon that occurs because the transistor 16 has an NPN junction structure.

Therefore, at point A, the diode 20 and the transistor 16
As a result, the amplitude is limited by the limiter voltage of 0.7 × 2 = 1.4V.

The output limiter voltage can be arbitrarily set by appropriately adjusting the amplification degree of the second-stage amplifier circuit. The low-pass filter function of the second-stage amplifier reduces the wide-range distortion component generated when the signal is clipped by the limiter circuit, so that the output distortion rate is improved as compared with the conventional circuit shown in FIG.

〔The invention's effect〕

As described above in detail based on the embodiment, in the present invention, a diode is connected between the collector and the emitter of the mute transistor to provide one-sided amplitude limiting effect, thereby achieving the one-sided amplitude limiting effect of the mute transistor. Together, a limiter circuit can be configured. Therefore, there is an advantage that a low-frequency amplifier circuit having a built-in limiter circuit can be configured at a low cost only by adding one diode.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention, and FIG.
FIG. 9 is a circuit diagram illustrating a conventional low-frequency amplifier circuit. 7 ... operational amplifier, 8, 9 ... coupling capacitor, 16 ... transistor, 20 ... diode.

Claims (1)

(57) [Claims] A first amplifier circuit that amplifies an input signal from an input terminal and outputs the amplified signal to a connection point; and a second amplifier circuit that amplifies an output signal of the first amplifier circuit at the connection point and outputs the amplified output signal to an output terminal. A mute circuit comprising: an amplifying circuit, a transistor having a collector connected to the connection point and an emitter connected to the reference voltage terminal, and a diode having an anode connected to the connection point and a cathode connected to the reference voltage terminal. A limiter circuit, a control circuit that supplies a control signal to the base of the transistor and controls the transistor, wherein the mute / limiter circuit is configured such that the transistor is turned on when the control signal is at a high level. A mute operation is performed. On the other hand, when the control signal is at a low level, the transistor is turned off, and the connection point is set at a predetermined level. When the voltage exceeds the voltage, the diode is turned on and is set to the same level as the reference voltage terminal, and when the connection point falls below a predetermined voltage, the collector and emitter of the transistor are turned on and set to the same level as the reference voltage terminal. A low-frequency amplifier circuit that performs a limiter operation by being set.