JPH0239908B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field to Which the Invention Pertains] The present invention provides a transmitting preamplifier (hereinafter referred to as "preamplifier") and a line output amplifier that sends its output signal to a line. This invention relates to a transmitter amplifier circuit for a telephone.

[Description of prior art]

FIG. 1 is a circuit diagram of a conventional transmitting amplifier. In FIG. 1, preamplifier _A1 operates on a power supply voltage Vcc smoothed by constant current circuit _CC1 and capacitor _C1 connected between _L1 and _L2 . However, this power supply voltage Vcc is generally a very low and limited voltage, so in order to obtain the maximum output amplification, the preamplifier A ₁ is operated at a bias voltage convenient for the amplifier. configured to do so.

On the other hand, the line output amplifier _A2 is set to an appropriate bias voltage determined by constraints such as the direct current resistance of the telephone and the output dynamic range. Therefore, the DC potential of the output terminal T _{1 of the preamplifier A 1 and} the DC potential of the input terminal T ₂ of the line output amplifier A ₂ do not match. For this reason, preamplifier A ₁ and line output amplifier A ₂ were not directly connected, but were connected via capacitor C ₂ . This means that
Not only did it require a capacitor _C2 with a large capacity, but it also had the disadvantage of increasing the number of pins when integrated circuits were integrated.

[Purpose of the invention]

The present invention improves the above drawbacks and provides a circuit that can directly connect the output terminal of a preamplifier and the input/output terminal of a line output amplifier without using a capacitor between the preamplifier and the line output amplifier. The purpose is to

[Features of the invention]

The present invention includes a first transistor having a base connected to an output of a transmitting preamplifier, and a first transistor having a base connected to an AC ground terminal that determines a DC voltage of an output of the transmitting preamplifier. a second transistor having the same conductivity as the transistor; and a constant current circuit interposed between each emitter of the first and second transistors and one terminal of the power supply circuit of the transmitting preamplifier. a third transistor whose collector and base are connected to the collector of the second transistor and whose conductivity is opposite to that of the first transistor; and a third transistor whose collector is connected to the input of the line output amplifier. a fourth transistor having a base connected to the base and having the same conductivity as the third transistor; the collector of the first transistor and each emitter of the third and fourth transistors are connected to the other of the power supply circuit; It is characterized by being connected to the terminal of

[Explanation based on examples]

Embodiments of the present invention will be described with reference to the drawings.

FIG. 2 is a circuit diagram of a transmitter amplifier according to an embodiment of the present invention. In Figure 2, transistors Q ₁ and Q ₂ are
It is of PNP type, and transistors Q ₃ and Q ₄ are of NPN type. The emitters of the transistors Q ₁ and Q ₂ are respectively connected to the power supply Vcc via the constant current circuit CC2,
A constant current is supplied. Power supply Vcc preamplifier
This is the power supply voltage created by smoothing the voltage between terminal L ₁ and terminal L ₂ by the function of constant current circuit CC ₁ and capacitor C ₁ that also supply power to A ₁ . transistor
The base of Q ₁ is connected to the output terminal T ₁ of the preamplifier A ₁ , and the base of the other transistor Q ₂ is
Connected to AC ground terminal E of preamplifier _A1 .
The collector of transistor Q ₁ is connected to terminal L ₂ ,
The collector of transistor Q ₂ is the collector of transistor Q ₃
connected to the collector and base of the Further, the collector of the transistor Q ₂ is connected to the base of the transistor Q ₄ , and the emitters of the transistors Q ₃ and Q ₄ are both connected to the terminal L ₂ , so that the transistors Q ₃ and Q ₄ form a current mirror circuit. Furthermore, the collector of the transistor Q ₄ is connected to the input terminal T ₂ of the line amplifier 2.

The operation of the transmitting amplifier circuit configured in this way will be explained.

In FIG. 2, the bases of transistors Q ₁ and Q ₂ have almost the same potential in terms of direct current, so when there is no signal, a current equal to 1/2 of the current of constant current circuit 4 flows through transistors Q ₁ and Q ₂ . flows to the collector. The current mirror circuit of transistors Q ₃ and Q ₄ causes the same current as the collector current of transistor Q ₂ to flow through the collector of transistor Q ₄ .
The signal voltage applied from microphone M is amplified by preamplifier A ₁ and transmitted to transistors Q ₁ and Q ₂
added between the bases of.

At this time, the signal voltage applied between the bases of transistors Q ₁ and Q ₂ , that is, the preamplifier A ₁
The signal voltage between the output terminal T ₁ and the ground terminal E of V ₁
If the equivalent resistances of the emitters of transistors Q ₁ and Q ₂ are R _e1 and R _e2 , then the change in the emitter current flowing to the emitter of transistor Q ₂ when this signal voltage V ₁ is applied is the constant current circuit CC ₂ is commonly connected to transistors Q ₁ and Q ₂ , so
Mostly from terminal T ₁ to the emitter of transistor Q ₁ ,
Pass through the emitter of transistor Q ₂ to ground terminal E
It is thought that it will flow into Therefore, since the amount of change in the emitter current of this transistor Q ₂ corresponds to the emitter alternating current i _e2 , i _e2 =V ₁ /(R _e1 +R _e2 ). Let the current flowing through constant current circuit CC ₂ be Icc,
When the change in the emitter current of transistor Q ₂ is small and i _e2 ≪Icc/2, R _e1 = R _e2 = 2V _T /Icc V _T = kT/q k: Boltzmann constant, T: absolute temperature, q: electron The electric charge is, and the alternating current i _e2 is i _e2 =V ₁ /(4V _T /Icc), and the alternating current i _e2 and the signal voltage V ₁ are proportional.
When the signal voltage V ₁ is large, the emitter resistance
Due to the nonlinearity of R _e1 and R _e2 , the proportional relationship between the AC current i _e2 and the signal voltage V ₁ does not strictly hold, but as shown in Figure 3, the emitter of the transistor Q ₁ and the constant current circuit Between CC ₂ and transistor
By inserting resistors R _e1 and R _e2 of equal value between the emitter of Q ₂ and the constant current circuit CC ₂ ,
The proportional relationship between the alternating current i _e2 and the signal voltage V ₁ is improved.

Further, the alternating current i _e2 flowing to the emitter of the transistor Q ₂ is converted into an alternating current i _c4 flowing to the collector of the transistor Q ₄ by the current mirror circuit of the transistors Q ₃ and Q ₄ .

Thus, the input terminal T ₂ of the line output amplifier A ₂
An alternating current i _C4 proportional to the signal voltage V ₁ is drawn from the point, and across the resistor R ₁ of the line output amplifier A ₂ ,
An alternating voltage V _A is generated that is proportional to the signal voltage V ₁ .
If the value of the capacitor C ₃ is made sufficiently large, the AC voltage V _A becomes V _A =i _c4 /(1/R ₁ +1/R ₂ ).

As described above, since the coupling between the preamplifier A ₁ and the line output amplifier A ₂ is performed via the alternating current i _c4 flowing to the collector of the transistor Q ₄ , the DC potential of the output terminal T ₁ of the preamplifier A ₁ is and line output amplifier
There is an advantage that A ₂ can be directly connected to the DC potential of input terminal T ₂ even if the requirements for the input terminal T 2 are different.

Furthermore, as shown in FIG. 4, the transmitter output amplifier according to the present invention can
When the DC potential of the input terminal T ₂ of the line output amplifier A ₂ changes, or when the line output amplifier A ₂ is shared by multiple signal systems, such as a transmitting signal system and a dual-frequency dial signal system. can also be applied.

Furthermore, in the embodiment of the present invention, the transistors Q ₁ ,
Q ₂ is a PNP transistor, transistors Q ₃ , Q ₄
is composed of NPN transistors,
PNP transistor to NPN transistor,
The NPN transistor may be replaced with a PNP transistor, and the operation and effect are exactly the same.

〔Effect of the invention〕

As explained above, the present invention makes it possible to apply a voltage proportional to the output voltage of the preamplifier to the input terminal of the line output amplifier using a transistor, thereby eliminating the need for a capacitor. Even if the requirements for the direct current potential of the output terminal of the amplifier and the input terminal of the line output amplifier are different, there is an excellent effect that they can be directly connected. This eliminates the need for large coupling capacitors, so
This is advantageous for circuit integration, reducing the number of pins on the integrated circuit and further miniaturizing the device.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a conventional transmitter amplifier. FIG. 2 is a circuit diagram of a transmitting amplifier according to an embodiment of the present invention. Figure 3 is the improved circuit diagram. FIG. 4 is a circuit diagram of a transmitting amplifier according to another embodiment of the present invention. A ₁ ... Transmission preamplifier, A ₂ ... Line output amplifier,
C ₁ to C ₃ … Capacitor, CC ₁ , CC ₂ … Constant current circuit,
E...AC ground terminal, M...microphone, Q ₁ ...
Transistor, Q ₂ ... Transistor (same conductivity as Q ₁ ), Q ₃ , Q ₄ ... Transistor (opposite conductivity to Q ₁ ), R ₁ , R ₂ ... Resistor, T ₁ ... Transmission preamplifier output terminal , T ₂ ...Line output amplifier input terminal.

Claims (1)

[Scope of Claims] 1. A transmitting amplifier circuit for a telephone comprising a transmitting preamplifier, and a line output amplifier that sends the output of the amplifier to a line, which is based on the output of the transmitting preamplifier. and a second transistor _Q having the same conductivity as the first transistor and whose base is connected to an AC ground terminal that determines the DC voltage of the output of the transmitting preamplifier. ₂ , a constant current circuit CC ₂ inserted between each emitter of the first and second transistors and one terminal of the power supply circuit of the transmitting preamplifier, and a constant current circuit CC 2 of the second transistor. a third transistor Q3 having a collector and a base connected to each other and having a conductivity opposite to that of the first transistor; a third transistor _Q3 having a collector connected to the input of the line output amplifier and having a base connected to the base of the third transistor; A fourth transistor _Q4 having the same conductivity as the third transistor, wherein the collector of the first transistor and each emitter of the third and fourth transistors are connected to the other terminal of the power supply circuit. A telephone transmitter amplifier circuit characterized by: