JP2911232B2 - Transmission output circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

 [Object of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-cost transmission output circuit requiring no adjustment.

[0002]

2. Description of the Related Art FIG. 3 shows a conventional example. T1 and T2 are terminals connected to a telephone line. The transmission signal appears at T2. Reference numeral 1 denotes a transmission circuit for generating a transmission signal such as a DTMF dialer or FSK MODEM, which is connected to the amplifier 2 via a variable resistor VR1. The amplifier 2 amplifies the DTMF signal or the FSK signal output from the transmission circuit 1 to a desired level. In this example, the operational amplifier OP1 and the resistor R2
It is described with an inverting amplifier consisting of R3. The amplification degree of the amplifier 2 is changed by the variable resistor VR1. C1 is provided for the purpose of AC coupling a transmission signal amplified by the amplifier 2 to a telephone line by a capacitor. R1 is provided by a resistor for the purpose of determining the output impedance to the telephone line, and usually about 600Ω is selected.

[0003]

In such a conventional circuit, there is a variation in the transmission circuit 1 and a variation in the capacitor C1. Therefore, the amplifier is controlled by the variable resistor VR1 so that the output level from the terminal T2 becomes a desired level. It was necessary to adjust the amplification degree of No. 2. Further, since the variable resistor VR1 is expensive, the product itself is expensive. An object of the present invention is to provide a low-cost transmission output circuit that does not require adjustment. [Configuration of the Invention]

[0004]

According to the present invention, there is provided a transmission output circuit for outputting a transmission signal from a transmission circuit to a telephone line via an output impedance. Amplifying means connected to one input terminal for amplifying a transmission signal from the transmission circuit; an AC coupling capacitor for outputting an AC component of the transmission signal amplified by the amplification means to a telephone line; A feedback capacitor for feeding back the AC component output to the telephone line via the capacitor, a comparing unit for comparing the amplitude of the AC component fed back by the feedback capacitor with the amplitude of the reference voltage, When the amplitude of the AC component is larger than the amplitude of the reference voltage, the comparator is connected in parallel to an integrator that integrates a signal output by the comparing unit and a feedback resistor of the amplifying unit. It is composed of a variable resistor means which is variable by the output signal of the integrator.

[0005]

In such a configuration, the comparing means compares the amplitude of the AC component of the transmission signal with the amplitude of the reference voltage and outputs a signal when the amplitude of the AC component of the transmission signal is larger.
The variable resistor means is varied by the output signal via the integrator to adjust the amplification degree of the amplifying means, so that the adjustment is omitted and the cost is reduced.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram of the transmission output circuit of the present invention. T
1 and T2 are terminals connected to a telephone line. The transmission signal is T
Appears in 2. 1 is DTMF dialer or FSK MODEM
Is connected to the amplifier 2 by a transmission circuit for generating a transmission signal as described above. The amplifier 2 receives the DT output from the transmission circuit 1
The MF signal and the FSK signal are amplified to a desired level. In this example, the variable resistance element 4 includes an operational amplifier OP1, resistors R2 and R3, and is connected in parallel with the resistor R2. C
A capacitor 1 is provided for the purpose of AC coupling with a telephone line. R1 is provided by a resistor for the purpose of determining the output impedance to the telephone line, and usually about 600Ω is selected. The capacitor C2 is used to determine the amount of alternating
The comparator OP2 is biased to the ground potential by a resistor R4.

Next, the operation will be described. The comparator OP2 is connected to the reference voltage Vref on the − terminal side, and outputs an “L” level when the input voltage on the + terminal side falls below the − terminal side voltage. Vref is a reference voltage, the + side is grounded, and the − side is connected to the − terminal of the comparator OP2. Assuming that the reference voltage Vref outputs vr, -vr is applied to the-terminal of the comparator OP2. The reference voltage Vref is often configured using a tuner diode, a band gap reference, and a power supply voltage. D1 is a diode and a current flows only when the output of the comparator OP2 is at the "L" level, and the diode D1 conducts with the integrator 3. An integrator 3 comprises an operational amplifier OP3, a resistor R5, and a capacitor C3. The output of the integrator 3, which integrates the "L" level voltage of the comparator OP2, is connected to a variable resistance element 4 to control the amplification of the amplifier 1. I have. 4
Is a variable resistance element, which is represented by a Cds photocoupler in this example. The LED side is connected to the integrator 3 via the resistor R6, and the resistor side is connected in parallel with the resistor R2 of the amplifier 1. When the current on the LED side increases, the resistance on the resistor side decreases.

FIG. 2 shows a waveform diagram. From the transmission circuit 1
The waveform shown in (a) is output. Signal a is amplifier 2
The signal passes through the capacitor C1 to be AC-coupled to the telephone line to reach T2. Here, the AC component of the signal passing through the capacitor C1 becomes (b) and the desired output amplitude becomes 2
Let it be Va ((1) of waveform b). The signal b is applied to the comparator OP2 by the capacitor C2.
The capacitor C2 is provided for transmitting only the AC component to the comparator OP2. -Of the comparator OP2
The reference voltage Vref is connected to the terminal side, and the voltage vr of the reference voltage Vref is selected so that vr = Va. That is, since the + side of the reference voltage is grounded, -vr is applied to the-terminal side of the comparator OP2. Therefore, when the voltage of the signal b input to the + terminal side of the comparator OP2 falls below the -terminal side voltage -vr91 ((2) of the waveform b), that is, when the signal b exceeds the desired input voltage amplitude 2Vb, the comparator OP2 Becomes "L" ((3) of waveform b). Here, a current flows through the diode D1 and conducts with the integrator 3. Then, the integrator 3
Integrates the 'L' level voltage of the comparator OP2 and d
Output a voltage like In other words, comparator O
When the output of P2 becomes the "L" level, the integrator 3 performs the integration operation during the "L" level ((5) of the waveform d) and the "L".
When the level changes from the “H” level to the “H” level, the current at that time is continued ((6) of waveform d). When the output of the comparator OP2 goes low again, the integrator 3 performs the integration operation ((7) of waveform d). Such an operation is repeated until vr = va. The signal d is converted into a current by the resistor R6 and flows into the variable resistance element 4. When the current on the LED side increases, the resistance of the resistor connected in parallel with the resistor R2 of the amplifier 2 decreases. When the resistance of the variable resistance element 4 on the resistor side decreases, the amplification degree of the amplifier 2 decreases and the amplitude of the signal b also decreases accordingly. A desired amplitude can be obtained by repeating the above operation and controlling the amplification degree by a circuit.

[0009]

As described above, according to the present invention, the variable resistance means for adjusting the amplification degree of the amplification means uses a signal output from the comparison means when the amplitude of the AC component of the transmission signal becomes larger than the amplitude of the reference voltage. Since the transmission output circuit is variable, it is possible to obtain a low-cost transmission output circuit that can be automatically adjusted.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a transmission output circuit of the present invention.

FIG. 2 is a waveform chart showing the operation of the transmission output circuit of the present invention.

FIG. 3 is a configuration diagram of a conventional transmission output circuit.

[Explanation of symbols]

 2 ... amplifier, 3 ... integrator, 4 ... variable resistance element

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04M 1/00 H04M 1/60 H03G 3/30

Claims (1)

(57) [Claims] 1. A transmission output circuit for outputting a transmission signal from a transmission circuit to a telephone line via an output impedance, wherein an output terminal is connected to one input terminal via a feedback resistor. Amplification means for amplifying the
An AC coupling capacitor for outputting the AC component of the transmission signal amplified by the amplifying means to a telephone line; a feedback capacitor for feeding back the AC component output to the telephone line via the AC coupling capacitor; Comparing means for comparing the amplitude of the AC component fed back by the feedback capacitor with the amplitude of the reference voltage; and a signal output by the comparing means when the amplitude of the AC component is larger than the amplitude of the reference voltage. A transmission output circuit comprising: an integrator for integration; and a variable resistance means connected in parallel to a feedback resistor of the amplification means and variable by an output signal of the integrator.