JPH0351810Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a line input amplifier for a tape recorder with improved S/N (signal-to-noise ratio).

Conventionally, there has been a line input amplifier as shown in FIG.

In the same figure, VR is a variable resistor for input, Tr ₁
is an amplification transistor. The AC equivalent circuit when the power is cut off when the circuit in the same figure is viewed from the line input side is:
It is shown in FIG.

In FIG. 5, the input impedance Zi is minimum when the variable resistance VR is maximum (indicated by a broken line), and its value is Zi=R ₁ +R _B1 /R _B2 r _D1 +R _E /r _D2 VR. However, r _D1 and r _D2 are amplification transistors
This is the forward series resistance of the PN junction of Tr ₁ .

Of these, the second and third terms are extremely small, so the input impedance Zi when the power is turned off
is approximately determined by the resistance R ₁ .

Generally, the input impedance of a tape recorder must be 47KΩ or higher in any state, including when the power is turned off or on.

Therefore, in order to ensure the input impedance when the power is turned off, in conventional line input amplifiers,
It was set to R ₁ ≧47KΩ.

However, if the resistor R ₁ is set to 47KΩ or more in this way, it becomes a source of thermal noise during operation of the line input amplifier, which becomes an obstacle to improving the S/N ratio.

The purpose of the present invention is to overcome the above-mentioned drawbacks of the conventional line input amplifier, and by increasing the input impedance of the line input amplifier when the power is cut off, the resistance value of the resistor _R1 , which was conventionally set to 47KΩ or more, can be lowered. It is an object of the present invention to provide a line input amplifier for a tape recorder that can suppress the generation of thermal noise and improve the S/N ratio.

Hereinafter, the present invention will be specifically explained with reference to the drawings.

FIG. 2 is a circuit diagram of a line input amplifier according to an embodiment of the present invention.

What is important in the figure is that a new transistor Tr 2 is provided between the variable resistor VR and the amplification transistor Tr ₁ , the collector of Tr ₂ is connected to the variable resistor VR, and _{the emitter of transistor Tr 2 is} connected to the transistor Tr ₁ . (base of) and the base of transistor Tr ₂ is connected to the power supply via resistor R ₂ with a large resistance value.

In the figure, FIG. 6 shows an AC equivalent circuit when the power is cut off as seen from the line input side.

In Fig. 6, when the power is cut off, the new transistor Tr ₂ separates the circuit from the subsequent stage, so even when the variable resistor VR is at its maximum (dotted line position), the input impedance is Zi=R ₁ +VR/r _D3 R ₂ becomes. However, r _D3 is the forward series resistance of the PN junction of the transistor Tr ₂ .

Therefore, even if the resistance R ₁ is not large,
High input impedance can be maintained by resistor _R2 with a large resistance value.

Furthermore, when the line input amplifier is in operation, the transistor Tr ₂ simply operates as a switching transistor and transmits the input signal to the subsequent amplification transistor Tr ₁ .

Note that the new transistor Tr ₂ also supplies bias current to the amplification transistor Tr ₁ , and also serves to suppress increases in additional components and costs.

FIG. 3 shows a modification of the present invention, in which a new section consisting of a transistor Tr ₂ and a resistor R ₂ is provided separately from the input amplifier A and at the previous stage thereof.
Even in this case, the same effects as in the above embodiment can be achieved. Further, in this case, since the input amplifier A at the subsequent stage is completely isolated from the input end when the power is cut off, there is an advantage that the circuit constants of the amplifier can be set quite freely.

In the present invention, if the resistance R ₂ cannot be made large enough, the transistor Tr ₂ and the resistance R ₂ can be modified as shown in Figure 4 to obtain high impedance by _R It can be effective.

As explained above, according to the present invention, the subsequent circuit is cut off by the switching transistor Tr ₂ when the power is cut off, and the resistance value of the resistor R ₂ is large, so the value of the input impedance can be easily increased. can do. Therefore, conventionally
Resistor _R1 , which has been set to 47KΩ or more, can be made to a lower value, thereby reducing thermal noise and improving the S/N of the line input amplifier.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional amplifier, Figure 2 is a circuit diagram of a line input amplifier according to an embodiment of the present invention, Figures 3 and 4 are circuit diagrams of modified examples, and Figure 5 is a circuit diagram of a line input amplifier according to an embodiment of the present invention. The equivalent circuit diagram in Figure 1, and the equivalent circuit diagram in Figure 6.
This is an equivalent circuit diagram of the figure. VR...variable resistor, Tr ₁ ...amplification transistor, Tr ₂ ...new transistor.

Claims (1)

[Scope of utility model registration request] In a line input amplifier for a tape recorder having a variable resistance VR for input, a resistance _R1 is connected between the variable resistance and the line signal input terminal, and a transistor Tr is connected between the variable resistance and the amplification transistor _Tr1 . Connect ₂ and this transistor
By connecting the collector of Tr ₂ to the variable resistor, connecting the emitter to the amplifying transistor, and connecting the base to the power supply via a resistor R ₂ with a large resistance value, the variable resistor and amplification are connected when the power is cut off. A line input amplifier for a tape recorder, characterized in that the line input amplifier for a tape recorder is configured to have separate transistors for high input impedance.