JPH062817U - Wideband amplifier - Google Patents

Abstract

(57) [Abstract] [Purpose] With regard to widening the band of vertical axis amplifiers for oscilloscopes, etc., in particular, it is intended to improve the DC drift characteristics and to widen the band by compensating for changes in the operating point due to temperature changes. is there. [Configuration] FET that amplifies the DC region with an operational amplifier
In addition to avoiding the temperature drift of the above, as for the deterioration of high frequency characteristics, an AC bypass circuit is provided at the frequency where the frequency characteristics start to fall, and the frequency is increased by adding it to the subsequent stage by passing the source follower and grounded base circuit. The band was improved.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a wideband amplifier suitable for use as a wideband amplifier, particularly for a vertical axis input amplifier of an oscilloscope. The present invention relates to widening the bandwidth of an oscilloscope vertical axis amplifier or the like. In view of the above points, the present invention aims at improving the DC drift characteristic and compensating the fluctuation of the operating point due to the temperature change to widen the band.

[0002]

[Prior art]

 A circuit in which the temperature drift is improved by using the dual FET will be described with reference to FIG. In the figure, 1 is a terminal to which an input signal is applied, and the signal applied to this terminal 1 is output from an output terminal 3 through a field effect transistor (hereinafter referred to as FET) 2 which constitutes a source follower. Here, the FET 4 is an FET that compensates the bias fluctuation due to the temperature change of the FET 2. If the thermal characteristics of FET2 and FET4 do not match, drift due to temperature change cannot be completely corrected. Regarding the high frequency characteristics, it is difficult to obtain a constant gain up to a high frequency range because of the gate-drain capacitance and the gate-source capacitance of the FET2. In addition, since the impedance of the FET4 to the negative power source is capacitive, it also becomes a loss factor in the high range. Therefore, a conventional technique with improved DC stability will be described with reference to FIG. In the figure, the signal applied to the input terminal 5 is divided into a low frequency component and a high frequency component by the capacitor 6. In this case, the low frequency amplification is performed by the operational amplifier 8. Here, since the gain of the operational amplifier 8 is extremely large, for example, 100 dB, much better direct current stability than that of the dual FET circuit shown in FIG. 2 is obtained by feedback and by not requiring a wide band. You can The high frequency amplification is performed by the source follower by the FET 7.

[0003]

The above-mentioned conventional technique has a drawback that the high frequency characteristic is deteriorated by the feedback capacitance and the capacitive impedance of the bias stabilizing circuit. In order to solve these drawbacks, the present invention avoids the capacitive impedance of the bias stabilization circuit by compensating the high frequency characteristics and reducing the DC drift, in addition to the stabilization of the drift by the bias fluctuation of the conventional technology. And the purpose is to obtain a broadband amplifier.

[0004]

[Means for Solving the Problems]

 FIG. 1 is a diagram showing the present invention. In order to achieve the above-mentioned object, the present invention not only prevents the temperature drift of the FET by amplifying the DC region with the operational amplifier 8 but also reduces the high frequency characteristic. The frequency band is improved by providing an AC bypass circuit 9 for the frequency at which the frequency characteristic starts to fall, passing through the source follower, and adding to the subsequent stage by the grounded base circuit 10.

[0005]

[Action]

 The operation of the present invention will be described. Since the gain of the DC amplifier composed of the operational amplifier 8 is extremely large in the DC region, the output accurately follows the input. Therefore, the drift due to the temperature change can be determined by the performance of the operational amplifier 8. Further, since the grounded base circuit 10 has a high impedance for both the emitter and the collector, it is possible to efficiently add the DC region from the operational amplifier 8 and the high region from the AC bypass circuit 9.

[0006]

【Example】

 An embodiment of the broadband amplifier of the present invention will be described with reference to FIG. In the figure, 5 is an input terminal to which an input signal is applied, 6 is a DC cut capacitor, 7 is a FET, and this FET 7 constitutes a source follower circuit which is AC-coupled to the input terminal 5 via the capacitor 6. Reference numeral 8 is a DC amplifier which is DC-coupled to the input terminal 5, and 9 is a series circuit of a variable capacitance element and a resistor, which constitutes an AC bypass circuit. Reference numeral 10 is a grounded base amplifier including a transistor 11, and 12 is an output transistor to which the signal of the grounded base amplifier 10 is applied to the base, which constitutes an emitter follower. A negative feedback path 13 connects the emitter of the output transistor 12 and the inverting input terminal (-) of the DC amplifier 8. This operation will be described below. The input signal applied to the input terminal 5 is applied to the operational amplifier 8 in the DC region and to the FET 7 and the AC bypass circuit 9 in the AC region by the DC cut capacitor 6. The AC bypass circuit can adjust the amount of addition to the output by varying the capacity with a series of variable capacitance element and resistor. On the other hand, the DC region amplified by the operational amplifier 8 is added to the base ground amplifier 10 together with the AC bypass component. The output of the base-grounded amplifier 10 is added with the correction components in the DC region and the high region, and is added at the input side of the output transistor 12. Here, since the emitter and collector of the base-grounded amplifier 10 have high impedance, they can be added. The features of the present invention are that the temperature drift performance is determined by the performance of the operational amplifier 8 and that the FET 7 is released from the restriction of the DC characteristics, so that a product with a large gm can be selected, which is advantageous for high frequency characteristics. In addition, the output drop in the high range due to the capacitance between the gate and drain and between the gate and source can be compensated for by adding to the output from the source follower, and the high frequency characteristics due to gm change with temperature can be changed. That is, it can be corrected by the capacitive element 9.

[0007]

[Effect of device]

 By determining the DC characteristics by the performance of the operational amplifier, it is possible to select a high gm FET that is disadvantageous in terms of DC as the FET, which is advantageous in terms of high frequency and can realize a wide band. Regarding the high-frequency characteristics, the attenuation due to the capacitance between the gate and the source, which was a problem in the conventional technology, can be avoided by adding the high-frequency component to the output side. In addition, the high-frequency characteristics attributed to the gm change due to temperature Can be compensated by the variable capacitance element used in the bypass circuit.

[Brief description of drawings]

FIG. 1 is a circuit diagram of the present invention.

FIG. 2 is a prior art circuit diagram.

FIG. 3 is a prior art circuit diagram. 7 FET 8 Operational amplifier 9 AC bypass circuit 10 Base ground circuit

Claims (1)

[Scope of utility model registration request] 1. A DC cut means connected in series between an input terminal and a source follower FET, and the source follower FE.
T, an operational amplifier DC-coupled to the input terminal, a means for adding the output of the source follower FET and the output of the operational amplifier, and a bypass of a high-frequency signal connecting the gate electrode of the source follower FET and the input of the adding means. Wideband amplifier consisting of a circuit.