JPH0241927Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an impedance conversion circuit.

Generally, the input circuit of an oscilloscope uses a circuit that outputs a signal input with high impedance with low impedance, and conventionally, impedance conversion circuits as shown in Figures 1 and 2 have been used. Ta. In the case shown in Fig. 1, transistors Q ₅ and Q ₆ are connected to the output terminal V ₀ .
Since an operating voltage is generated by V _BE , a differential circuit must be used in the next stage circuit that should receive this output. Therefore, the circuit configuration becomes complicated,
There were drawbacks such as increased costs. Furthermore, the one shown in Fig. 2 is a so-called composite amplifier, but in this type, the resistance values of resistors _R1 and _R2 require precision, so adjustment is troublesome, and the output terminal V When the load R _L at ₀ changes, an attenuator is formed by the load R _L and the output impedance of the output transistor Q ₇ , so the resistors R ₁ and R ₂ must be adjusted every time the load changes. .
If this adjustment is neglected, only the high-frequency gain will be affected by load fluctuations, resulting in distortions such as overshoot and tail in the square wave characteristics. Therefore, it is necessary to constantly adjust the It was also something that was lacking.

An object of the present invention is to provide an impedance conversion circuit that eliminates the above-mentioned drawbacks of the conventional circuit and provides high reliability with a simple circuit configuration.

Hereinafter, the configuration of the present invention will be explained based on FIGS. 3 to 5, which are examples thereof.

In the figure, 1 is a first active element that can input an input voltage at high impedance, and in the embodiment, an FET serving as a source follower is used. Reference numeral 2 denotes a transistor as a second active element which receives the output from the FET 1 and is capable of outputting at low impedance, and is used as an emitter follower. 3
4 is a FET as a third active element which is a current source of the FET 1, and 4 is a transistor as a control element for controlling the amount of current of the FET 3.

In the first embodiment shown in FIG. 3, the base-emitter voltage of transistor 4 is equal to the base voltage V _B2 of transistor 2 and the gate voltage Vi of FET 1, and between V _B2 and V ₀ (output voltage). It is designed to be equal to the voltage of That is, FET
The source current I _S1 of transistor 1 is the sum of the source current I _S3 of FET 3 and the base current I _B2 of transistor 2 (I _S1 = I _S3 +
I _S2 ), and the difference between the voltage difference V _F between point 5 (arrow line) and the emitter 6 of transistor 4 and the voltage difference V _G between the base voltage V _B2 of transistor 2 and the gate voltage Vi of FET 1. (V _F −V _G ) is R _S1 (resistance) + 1/g _n (gm is mutual conductance).

So, base 7 of transistor 4 and point 5
The voltage difference {(R _S1 +1/g _n )·I _B2 } can be canceled by setting the resistance R _B between the two to R _S1 +1/g _n .

The biggest factor that makes the operating point unstable is V _GS ,
It is known that temperature drift in V _BE , I _D , I _B etc. is caused by temperature drift, but it is important to set resistance R _S1 = resistance R _S2 = R _S and to set resistance R _B = R _S +1/g _n . Due to the drift of FET 1 and FET 3 and transistor 2
The drift between the transistor 4 and the transistor 4 can be canceled out. Also, since the voltage of emitter 6 is the gate voltage of FET 3 and also the emitter voltage of transistor 4, correspondingly, FET 1
The gate voltage Vi of transistor 2 and the emitter voltage V ₀ of transistor 2 are equal to each other.

As mentioned above, since there is no element drift or harmful voltage difference, the operating points of the input and output can be made equal; for example, when the input is
If it is OV, the output can also be OV. Moreover, since the configuration is a combination of a source follower and an emitter follower, there is no adverse effect caused by load fluctuations as explained in FIG. 2. Note that FIG. 4 shows transistors 2 and 4, respectively.
A second embodiment in which the transistor is replaced with a PNP type transistor,
FIG. 5 shows a third embodiment in which the emitter-collector of each of the transistors 2 and 4 is connected to an independent power supply in the first and second embodiments. Although N-channel FETs are shown in the drawings, P-channel FETs may also be used, and it is also possible to use general transistors; FETs 1 and 3, transistors 2,
By using a material with uniform characteristics as 4, the drift can be further reduced (to about 10 mV/° C.).

According to the impedance conversion circuit according to the present invention, the first active element is capable of inputting an input voltage at high impedance, and the second active element is configured to receive the output from the first active element and output it at low impedance. , a third active element serving as a current source for the first active element, and a control element for controlling the amount of current of the third active element. It is possible to match the operating points with the two, and high accuracy can be obtained.

In addition, it has excellent features such as being able to greatly improve the temperature characteristics against drift, as well as stabilizing the operation even with changes in load resistance, thereby improving the reliability of the circuit. .

[Brief explanation of the drawing]

1 and 2 are circuit diagrams showing conventional impedance conversion circuits, FIGS. 3 to 5 show embodiments of the impedance conversion circuit according to the present invention, and FIG. 3 shows the first embodiment. The circuit diagrams shown in FIGS. 4 and 5 are circuit diagrams showing the second and third embodiments, respectively. 1... FET as the first active element, 2...
A transistor as a second active element, 3...FET as a third active element, 4... A transistor as a control element.

Claims (1)

[Scope of utility model registration request] The first type that can input the input voltage with high impedance
a second active element that receives the output from the first active element and is capable of outputting at low impedance; and a third active element that is a current source for the first active element. , and a control element for controlling the amount of current of the third active element.