JPH0212745Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a wideband amplifier circuit, and particularly to a circuit optimal for use in an oscilloscope.

(Prior art) In general, first-stage amplifiers for oscilloscopes are required to have excellent high-frequency characteristics, but the amplification factor does not change even if the impedance of the signal to be amplified changes greatly (for example, from 50Ω to 1MΩ). This is an important condition.

Therefore, conventionally, as shown in Figure 1, the input section
The input impedance at IN is determined by the bias resistance 20 of the first stage source follower 21 and this input capacitance 2.
In addition, the FET 21 and transistors 22 and 23 are of single-chip type with uniform pair characteristics in order to reduce DC drift.

However, in the conventional devices mentioned above, the active element is a single chip type, and moreover,
Since an element that has both DC and high frequency characteristics is required, not only does it significantly increase costs, it is difficult to completely prevent DC drift, and the signal source impedance significantly increases. There were drawbacks such as the inability to respond to changes in some cases.

(Purpose of the invention) The purpose of the invention is to eliminate the drawbacks of the conventional ones described above, simplify the circuit, use inexpensive elements, and thereby significantly reduce costs. An object of the present invention is to provide a wideband amplifier circuit that can reduce DC drift and is free from the risk of amplification characteristics changing even if the signal source impedance changes significantly.

(Structure of the invention) The wideband amplifier circuit of the invention includes a FET that is connected as a source follower and has an input signal connected to its gate terminal, and an emitter follower type active element that receives an output signal from the FET as an input. a voltage dividing resistor that divides the voltage of the input signal; an operational amplifier that receives the voltage-divided signal as its + side input; and a first capacitor inserted between the + side input and the emitter of the active element. , a wideband amplifier which inputs the signal from the emitter via a second capacitor to its +side input, and the output from the operational amplifier is input to its +side input terminal; a feedback circuit inserted between the negative input terminal of the operational amplifier, and constants of the voltage dividing resistor, the first capacitor, and the feedback circuit are set so as to correct the input capacitance of the operational amplifier. This makes it possible to obtain stable high-frequency characteristics and to ensure that the amplification characteristics do not change even if the signal source impedance changes.

(Embodiment) In the present invention, an input signal is separated into an AC region and a DC region and amplified, and an embodiment thereof will be described below based on FIG. 2.

In the figure, reference numeral 1 denotes an FET, which has a source follower connection, and an input signal (IN) is connected to its gate terminal. 2 is an emitter follower type active element that receives the output signal from FET 1 as input, R ₁ and R ₂ are voltage dividing resistors for dividing the input signal (IN), and Q ₁ is an operational amplifier, which A signal voltage-divided by the resistor is input to its + side input terminal.

C ₁ is a first capacitor inserted between the + side input terminal of the operational amplifier Q ₁ and the emitter of the active element 2, and Q ₂ is a wideband amplifier in which the signal from the active element 2 is It becomes its + side input via the second capacitor _C2 , and the output from the operational amplifier _Q1 is input to its + side input terminal (+).

3 is a feedback circuit inserted between the output terminal of the broadband amplifier Q ₂ and the negative input terminal of the operational amplifier Q ₁ , in which a resistor R ₃ and a capacitor C ₃ are connected in parallel. It is composed of the above −
The side input end is grounded via resistor _R4 .
Both the FET 1 and the active element 2 have good high frequency characteristics, and these mainly constitute an AC amplification section. On the other hand, operational amplifier _Q1 is provided mainly for DC amplification. The input section (+, -) of the above operational amplifier _Q1 has a capacitance, respectively.
Although C ₄ and C ₅ exist, the constants of each of the passive elements are set to correct this.

Generally, the oscilloscope signal source impedance applied to the input terminal is determined by the impedance of the probe (not shown), and the number of resistance components.
Capacitance component number PF to several 10 PF in parallel with 10 Ω to several 10 MΩ
It is a parallel circuit. In order to ignore the influence of this signal source impedance, it is sufficient that the input impedance is sufficiently large, but if it is not, correction is necessary.

Similarly, if R ₁ ≫ R ₂ , there is no need to correct the input capacitance C ₄ of the operational amplifier that mainly amplifies the DC component, but since the input of Q ₁ is attenuated with respect to the input signal of the entire circuit, its The gain must be increased by that much, and a highly accurate operational amplifier is required to counter DC drift. Therefore, in order to satisfy R ₁ ≫ R ₂ , correction for C ₄ is required, and Q ₁
The input time constants R ₂ , C ₄ and the source time constants R ₁ , A _v of Q ₁
It becomes necessary to match C ₁ .

Therefore, in this invention, _the +
The product of the side input capacitance C ₄ and the voltage dividing resistor R ₂ is the voltage dividing resistor R ₁
and gain A _v of FET1 and active element 2 and feedback capacitance
The value is set to be equal to or slightly lower than the product of C ₁ , and the time constant of the negative input capacitor C ₅ of operational amplifier Q ₁ and the gain setting resistor R ₄ is It is set to be equivalent to the product of gain setting resistor _R3 and feedback capacitor _C3 .

Expressed mathematically, R ₂ · C ₄ · 1/A _v = R ₁ · C ₁ (where A _v is the product of the gains of the source follower and emitter follower (0.8 to 0.9)), and R ₄ · C ₅ = _R3・_C3 .

Note that accuracy can be further improved by using a trimmer capacitor as the above-mentioned capacitor.

The output from the active element 2 is output via the DC cut capacitor _C2 , where it is mixed with the output from the operational amplifier _Q1 and amplified by the broadband amplifier _Q2 . .

The output gain of wideband amplifier Q ₂ is set to the most appropriate level by changing the resistance values of resistors R ₃ and R ₄ . Note that since the wideband amplifier _Q2 is included in the feedback loop of the operational amplifier _Q1 , its DC drift is completely canceled out and does not appear on the output side.

As described above, in the present invention, good characteristics can be obtained without using expensive elements unlike conventional ones.

(Effects of the invention) According to the wideband amplifier circuit according to the invention, it is possible to use inexpensive active elements and the circuit configuration is simple, so it is possible to achieve a significant cost reduction, and it is also possible to calculate the amount of DC drift. It has excellent features such as being able to reduce the drift amount to approximately the same level as the amplifier drift amount, and furthermore, the amplification characteristics do not change even if the signal source impedance changes significantly.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing a conventional wideband amplifier circuit.
FIG. 2 is a circuit diagram showing an embodiment of the broadband amplifier circuit according to the present invention. 1: FET, 2: Active element, 3: Feedback circuit, R ₁ ,
R ₂ : Voltage dividing resistor, Q ₁ : Operational amplifier, Q ₂ : Wideband amplifier.

Claims (1)

[Scope of utility model registration request] The FET has a source follower connection, and the input signal is connected to its gate terminal.
An emitter follower type active element that receives the output signal from the FET as an input, a voltage dividing resistor that divides the voltage of the input signal, an operational amplifier that receives the voltage-divided signal as its + side input, and the above + side input and the above active element. a first capacitor inserted between the emitter of the element;
A wideband amplifier which receives the signal from the emitter via a second capacitor and inputs the output from the operational amplifier to its +side input terminal; and a feedback circuit interposed between the negative input terminal of the amplifier, and constants of the voltage dividing resistor, the first capacitor, and the feedback circuit are set so as to correct the input capacitance of the operational amplifier. A wideband amplifier circuit characterized by: