JP2772418B2 - High input impedance broadband amplifier for oscilloscope - Google Patents

Description

The present invention relates to a high input impedance broadband amplifier for an oscilloscope.

[Conventional technology]

Amplifiers used at the input stage of measuring instruments such as oscilloscopes need to operate accurately without saturation over a wide voltage range from millivolts to hundreds of volts to handle a wide range of measurement signals. . Also, the input impedance of the amplifier keeps a constant value for gain switching,
And it needs to be expensive. This is to minimize the effect on the circuit under test.

Conventionally, a circuit as shown in FIG. 4 is used as a circuit that satisfies such requirements.

In FIG. 4, a measurement signal supplied to an input terminal 31 includes an input coupling switch 32, an input attenuator 33, and an overvoltage protection circuit 34.
Is supplied to the broadband buffer amplifier 35 via the. The output voltage of the buffer amplifier 35 is output from the output terminal 36. Here, the input coupling switch 32 switches between DC input and AC input, and has a switch S31 therefor. The input attenuator 33 attenuates the measurement signal to an appropriate level. Here, for simplicity of explanation, the input signal is 1 /
減 衰 1 attenuation transmitted by 1 and 1/10 transmission of input signal ÷ 1
There are two steps of 0 attenuation. The switch S32 is a selection switch for that purpose, and the contacts S32-a and S32-b switch the attenuation rate in conjunction with each other. The overvoltage protection circuit 34
This is a protection circuit for preventing an overvoltage from being applied to the buffer amplifier 35. The buffer amplifier 35 is a broadband amplifier having a small offset voltage between input and output and a high input impedance.

[Problems to be solved by the invention]

However, the above-described conventional technology has the following problems.

(1) Switches of input coupling switch 32 and input attenuator 33
It is difficult to use a semiconductor switch for S31 and S32. That is, since a voltage of several hundred volts may be applied to the switches S31 and S32, and the stray capacity between these switches and the ground must be as small as possible, it is difficult to employ a semiconductor switch. Becomes For this reason, it is difficult to perform remote control using a semiconductor switch.

(2) If a relay is adopted for the switches S31 and S32, power consumption increases. In addition, it is difficult to reduce the size of the device and to save labor in manufacturing the device, and the cost is high.

(3) In the buffer amplifier 35, a DC offset voltage is generated between input and output. This offset voltage drifts with time and temperature.

For this reason, a measurement error occurs, and it is difficult to increase the sensitivity.
When high sensitivity is required, an improvement measure as described in JP-B-63-39122 is required. That is, the AC component and the DC component of the input signal are amplified by separate amplifiers, and the output of the AC component amplifier is fed back to the DC component amplifier to reduce the offset voltage.
Improvement measures are needed.

The present invention has been made under such a background,
An object of the present invention is to provide a high input impedance broadband amplifier for an oscilloscope that does not use a mechanical switch such as a relay or a rotary switch.

It is another object of the present invention to provide a high input impedance broadband amplifier for an oscilloscope with low drift, low power consumption and low cost.

[Means for solving the problem]

To achieve the above object, a high input impedance broadband amplifier for an oscilloscope according to the present invention includes a first measurement signal input terminal for inputting a signal to be displayed on an oscilloscope, and the first measurement signal input terminal. A first input resistor having one end connected to the terminal, an inverting input terminal connected to the other end of the first input resistor, a non-inverting input terminal grounded, and an output terminal; 1 composite amplifier, a first feedback resistor provided between an output terminal and an inverting input terminal of the first composite amplifier, and one of the first feedback resistors is connected to the first composite amplifier. First switching means selectively connected between an output terminal and an inverting input terminal;
And a capacitor connected in parallel to each of the first feedback resistors, wherein the time constant of each parallel connection is equal to the first constant.
And a second phase correction capacitor that has a time constant substantially equal to an input resistance of the first composite amplifier and a parallel capacitor of the first composite amplifier. The first composite amplifier is connected to an inverting input terminal of the first composite amplifier. And a differential amplifier having an inverting input terminal connected to an output terminal of the follower circuit, and an amplifier for a high frequency region, and a non-inverting input terminal of the first composite amplifier. A non-inverting input of the operational amplifier is connected to a non-inverting input of the first composite amplifier, and the inverting input of the operational amplifier is connected to the The operational amplifier is connected to the inverting input terminal of the composite amplifier, and the output terminal of the operational amplifier includes an operational amplifier for a low frequency region connected to the non-inverting input terminal of the differential amplifier.

(Operation)

According to the invention, the measurement signal is supplied to the inverting input of the composite amplifier through a high impedance input circuit. Since the inverting input terminal is normally at zero potential, no high voltage is applied to the feedback circuit. Further, since the gain changeover switch is in the feedback circuit, some stray capacity is allowed. Therefore, a semiconductor switch such as an FET switch can be used as the gain switch. A semiconductor switch can also be used for the input coupling switching circuit for the same reason.

The composite amplifier is configured by combining a broadband amplifier having a normal DC offset and a temperature drift characteristic with an amplifier for a low frequency region including an inexpensive and low-drift general-purpose operational amplifier. The temperature drift is corrected.
As a result, a high input impedance broadband amplifier with low drift, low power consumption, and low DC offset can be provided.

Furthermore, since semiconductor switches can be used for gain switching and input coupling switching, these switches can be operated by remote control.

〔Example〕

 Hereinafter, the present invention will be described in detail with reference to the drawings.

First Embodiment FIGS. 1 and 2 are circuit diagrams showing a configuration of a first embodiment of the present invention.

In FIG. 1, 1 is an input circuit, and 2 is a composite amplifier. The input circuit 1 comprises a parallel circuit of an input resistor R1 and a capacitor C1, connects an input terminal 3 to an inverting input terminal of a composite amplifier 2, and substantially defines an input impedance of the device. On the other hand, the non-inverting input terminal of the composite amplifier 2 is grounded. Therefore, the composite amplifier 2 operates as an inverting amplifier.

The output terminal 4 of the composite amplifier 2 is connected to the inverting input terminal via a plurality of feedback circuits. That is, the first feedback circuit 5 comprising a parallel circuit of the feedback resistor R5 and the capacitor C5.
And a second feedback circuit 6 composed of a parallel circuit of a feedback resistor R6 and a capacitor C6 are connected to the inverting input terminal of the composite amplifier 2. One of these feedback circuits 5, 6 is connected to a gain changeover switch S2. Is selectively connected to the output terminal 4. The time constant (R5 · C5) of the feedback circuit 5 and the time constant (R6 · C6) of the feedback circuit 6 are set substantially equal to the time constant (R1 · C1) of the input circuit 1. Furthermore, the values of the feedback resistors R5 and R6 are
R5 = R1 and R6 = R1 / 10 are set. Therefore, when the feedback circuit 5 is connected, the gain becomes 1, and when the feedback circuit 6 is connected, the gain becomes 1/10.

The output terminal 4 of the composite amplifier 2 is further connected to an inverting input terminal of the composite amplifier 2 via a low-pass filter 8. The low-pass filter 8 includes a resistor R8 and a capacitor C8 functioning as an integrating circuit, and an operational amplifier 9, a resistor R9, and a capacitor C9 functioning as a smoothing circuit. The output of the operational amplifier 9 (the output of the low-pass filter 8) is connected to the coupling switch S1. The switch S1 switches between AC coupling and DC coupling. In the case of AC coupling, the inverting input terminal of the composite amplifier 2 is connected to the output terminal of the operational amplifier 9 through the resistor R10. Switch to ground through R10. Therefore, the resistor R10 functions as a feedback resistor when AC coupling is selected. That is, the DC voltage component of the output of the composite amplifier 2 is supplied to the inverting input terminal via the low-pass filter 8 so that the DC voltage component becomes zero.

FIG. 2 is a circuit diagram showing a specific configuration example of the composite amplifier 2. The composite amplifier 2 includes a source follower FET.Q11 functioning as an input stage 2a, transistors Q12 and Q13 forming a differential amplifier 2b, an emitter follower transistor Q15 functioning as an output stage, and an amplifier 2 for a low frequency region.
The operational amplifier 23a functioning as 3 is mainly configured.

The inverting input terminal 21 of the composite amplifier 2 is connected to the inverting input terminal of the differential amplifier 2b, that is, the base of the transistor Q12, via the source follower FET.Q11 as the input stage 2a.
On the other hand, the non-inverting input terminal 22 of the composite amplifier 2 is connected to the non-inverting input terminal of the differential amplifier 2b, that is, the base of the transistor Q13, via the low frequency region amplifier 23 and the voltage dividing resistors R12 and R13. ing. A resistor R11 and a capacitor C11 functioning as a low-pass filter are connected to the amplifier 23. Therefore, the amplifier 23 amplifies only the DC component and the low frequency component.

The output of the differential amplifier 2b is supplied to an emitter follower transistor Q15 via a common base transistor Q14 for improving high frequency characteristics. The emitter voltage of the emitter follower transistor Q15 is output through the output terminal 25.

In the drawing, CR11 is an overvoltage protection circuit for protecting the circuit from an overvoltage that enters when the feedback circuit is switched or when the output of the composite amplifier 2 is saturated. Further, a resistor R15 is an emi resistance for determining the gain of the differential amplifier 2b, a resistor R16 is a resistor for flowing a bias current to the transistors Q12 and Q13, a resistor R17 is a resistor for supplying a bias current to the transistors Q12 and Q14, and a resistor R18 is Is another resistor that determines the gain of the differential amplifier 2b. In addition, 24 are amplifiers
23 is a variable resistor for adjusting the DC offset voltage of the input.

According to such a configuration, the operation as already described in the section of “operation” can be obtained. That is, since the voltage at the inverting input terminal of the composite amplifier 2 is almost zero, application of a high voltage to the feedback circuits 5 and 6 can be avoided. Further, since the feedback circuits 5 and 6 have high impedance in the feedback circuits, some stray capacitance can be tolerated. Therefore, the semiconductor switch S2 can be included in the feedback circuit.

Further, at the time of AC coupling, through the low-pass filter 8,
By negatively feeding back the output of the composite amplifier 2 to the inverting input terminal, the DC offset voltage of the composite amplifier 2 is reduced,
Temperature drift can be suppressed.

Second Embodiment FIG. 3 is a circuit diagram showing a configuration of a second embodiment of the present invention. The main differences between the second embodiment and the first embodiment are as follows.

(1) An emitter follower is used in the input stage of the composite amplifier 42 instead of the source follower.

(2) Three feedback circuits 45, 46 and 47 are provided so that three gain switches 1/1, 1/10 and 1/100 can be selected.

(3) The gain switch and the input coupling switch can be remotely controlled by the analog switch control circuit 50.

Hereinafter, further description will be made focusing on these points. Input terminal
43 is connected to the input stage 42a of the composite amplifier 42 via the input circuit 41 and the damping resistor R42. The damping resistor R42 is used to reduce linking and overshoot and to stabilize circuit operation.
It is set to a slightly smaller value than. The input stage 42a of the composite amplifier 42 includes a coupling capacitor C43, an emitter follower transistor Q41, an input termination resistor R43, and a load resistor R44, and current-amplifies an input signal and supplies it to the differential amplifier 2b. Differential amplifier 2b, transistor Q1
4, and the emitter follower transistor Q15
This is the same as the first embodiment shown in the figure.

The output of the emitter follower transistor Q15 is output from the inverted output terminal 44 of the composite amplifier 42, and is also supplied to feedback circuits 45, 46, 47 and a low-pass filter 48.

The feedback circuit 45 includes a feedback resistor R45, a capacitor C45 connected in parallel with the feedback resistor R45, and an analog switch S45 connected in series with the parallel circuit. Similarly,
The feedback circuit 46 includes a feedback resistor R46, a capacitor C46, and an analog switch S46. The feedback circuit 47 includes a feedback resistor R47, a capacitor C47, and an analog switch S47. Here, the ratio between the feedback resistors R45, R46, R47 and the input resistor R41 is set to 1/1, 1/10, 1/100. Therefore, the gain of the composite amplifier 42 becomes one of 1/1, 1/10, and 1/100 depending on the analog switch that is alternatively turned on. Note that the time constants R45 and C45, R46 and C46, and R47 and C47 of the feedback circuits 45, 46, and 47 are adjusted so that the time constants R41 and C41 of the input circuit 41 are substantially equal.

The low-pass filter 48 is configured around an operational amplifier 49, and the output is supplied to the feedback resistor R50 and the analog switch S49 via the analog switch S48. The analog switches S48 and S49 are exclusively turned on to switch the input coupling. That is, when the analog switch S48 is on and S49 is off, the output of the low-pass filter 48, that is, the DC component of the output of the composite amplifier 42 is negatively fed back to the inverting input terminal of the composite amplifier 42 via the feedback resistor R50. And AC coupling. Conversely, when the analog switch S48 is off and S49 is on, the inverting input of the composite amplifier 42 is
It is grounded via a feedback resistor R50 and an analog switch S49, and is DC-coupled.

The switching of these analog switches is controlled by an analog switch control circuit 50. That is, the control line 51 selects the feedback circuit 45 having a gain of 1/1,
The control line 52 includes a feedback circuit 46 having a gain of 1/10
53 selects a feedback circuit 47 having a gain of 1/100.
Further, the control lines 54 and 55 operate exclusively from each other,
Switches between AC coupling and DC coupling.

According to the second embodiment, the same operation and effect as those of the first embodiment can be obtained. Further, gain switching and input coupling switching can be performed by remote control.

Third Embodiment FIG. 5 shows a cascaded gain-switching inverting amplifier having a small step of gain 5, 2.5, and 1 to an inverting gain attenuating amplifier 42 of 1/1, 1/10, 1/100 of FIG. Connect the volume VR between both circuits
151 is arranged to provide a gain fine adjustment circuit. According to this circuit, a non-inverted output can be obtained from the output terminal 154. The advantages of this embodiment are as follows.

(1) Non-inverted output.

(2) Fine gain adjustment is possible. It is most suitable for an oscilloscope because it has a small step gain switching of 1-2-5 steps.

(3) Since the second stage feedback circuit has a low circuit impedance, no high-frequency compensation capacitor is required.

Fourth Embodiment When the constant of the feedback circuit, that is, the gain ratio is large to some extent,
When the gain of the composite amplifier is constant, a stable feedback circuit over a wide band may not be obtained. FIG. 6 solves this problem by interlocking with the changeover switch S5 of the feedback circuit and also changing over the changeover switch S20 of the composite amplifier 2 so that the gain is set to the optimum value. According to this embodiment, stable amplification over a wide band can be obtained.

Fifth Embodiment FIG. 7 shows an input circuit provided with a voltage dividing / dividing circuit 70, switching the input current to 1/1, 1/100 and combining it with 1/1, 1/10 of the feedback circuit to obtain 1/1. , 1/10, 1/100, and 1/1000 gains are realized.

As described with reference to FIG. 6, the resistance of the input circuit is set to 1/10 so that the ratio of the feedback circuit does not need to be increased more than necessary.
Set to 0, discard 99/100 of the input current as ground current, and
Is supplied to the inverting input section to obtain 1/100 attenuation.

Sixth Embodiment FIG. 8 shows a circuit in which the circuits of FIG. 3 are provided in parallel to form a differential type. In configuring the AC / DC switching circuit,
The outputs of both composite amplifiers 42 and 42 'are connected by a resistor 81, an analog switch 82 and a resistor 81', a common voltage is detected, and this is fed back to each inverting input via low-pass filters 48 and 48 '. Set the common voltage to zero.

Therefore, even when a bias voltage of, for example, several hundred volts is applied to both input terminals, amplification can be performed with high sensitivity without saturating the differential signal.

Applications include differential input oscilloscopes and floating probes.

〔The invention's effect〕

As described above, according to the present invention, the measurement signal
The input is fed to the inverting input of the composite amplifier through a high impedance input circuit, which is usually
Since the potential is zero, no high voltage is applied to the feedback circuit. Further, since the gain changeover switch is in the feedback circuit, some stray capacity is allowed. Therefore, a semiconductor switch such as an FET switch can be used as the gain switch. A semiconductor switch can also be used for the input coupling switching circuit for the same reason.

Further, the composite amplifier is configured by combining a broadband amplifier having a normal DC offset and temperature drift characteristics with an amplifier for a low frequency region including a low-cost and low-drift general-purpose operational amplifier. The offset and the temperature drift are corrected. As a result, a high input impedance broadband amplifier with low drift, low power consumption, and low DC offset can be provided at low cost.

Furthermore, since semiconductor switches can be used for the gain changeover switch and the input coupling switch, these changeover switches can be operated by remote control.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of a high input impedance broadband amplifier according to a first embodiment of the present invention, FIG. 2 is a circuit diagram showing a configuration of a composite amplifier 2 of the first embodiment, and FIG. FIG. 4 is a circuit diagram showing a configuration of a high input impedance broadband amplifier according to a second embodiment of the present invention, FIG. 4 is a circuit diagram showing a configuration of a conventional high input impedance broadband amplifier, and FIGS. Third embodiment to sixth embodiment
FIG. 2 is a circuit diagram illustrating a configuration of an example. 1,41 input circuit, 2,42 composite amplifier, 2a, 42a input stage of composite amplifier, 2b, 42b differential amplifier of composite amplifier, 3,43 input terminal, 4,44 …… Output terminals, 5,6,45,46,47 ……………………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………… Numerical products, etc. Switch, S1, S48, S49 …… Coupling changeover switch, 50 …… Analog switch control.

Continuation of the front page (56) References JP-A-2-199907 (JP, A) JP-A-63-139403 (JP, A) JP-A-62-123814 (JP, A) JP-A-63-296403 (JP, A) , A) JP-A-59-132366 (JP, A) JP-A-57-161660 (JP, A) JP-A-1-303806 (JP, A) Shigeaki Suzuki "How to use an analog switch" (Showa 55 -5-5) CQ Publishing Company one two Three

Claims (9)

(57) [Claims] A first input terminal for inputting a signal to be displayed on an oscilloscope; a first input resistor having one end connected to the first input terminal for measurement; A first composite amplifier having an inverting input terminal connected to the other end of the input resistor, a non-inverting input terminal grounded, and an output terminal; and an output terminal and an inverting input terminal of the first composite amplifier. A first feedback resistor provided between the first composite amplifier and an output terminal of the first composite amplifier. Switching means, a first phase correction capacitor connected in parallel to the first input resistor, and a capacitor connected in parallel to each of the first feedback resistors, wherein the time constant of each parallel connection is The time constant of the first input resistor and its parallel capacitor is approximately equal to A second phase correction capacitor, wherein the first composite amplifier comprises: a follower circuit connected to an inverting input terminal of the first composite amplifier; and an output terminal of the follower circuit. An amplifier for a high-frequency region having a differential amplifier having an inverting input terminal connected to the operational amplifier connected to a non-inverting input terminal of the first composite amplifier. A non-inverting input terminal is connected to a non-inverting input terminal of the first composite amplifier; an inverting input terminal of the operational amplifier is connected to an inverting input terminal of the first composite amplifier via a low-pass circuit; A high input impedance broadband amplifier for an oscilloscope, wherein an output terminal of the operational amplifier comprises an operational amplifier for a low frequency region connected to a non-inverting input terminal of the differential amplifier. 2. The high input impedance broadband amplifier for an oscilloscope according to claim 1, wherein a value of said first feedback resistor is smaller than a value of said first input resistor. 3. A low-pass filter having an input terminal connected to an output terminal of the first composite amplifier, and an output terminal and a ground terminal of the low-pass filter are selectively connected to an inverting input terminal of the first composite amplifier. 2. The high input impedance broadband amplifier for an oscilloscope according to claim 1, further comprising: a second switching unit connected to the oscilloscope. 4. An analog switch provided between an output terminal of the first composite amplifier and each of the first feedback resistors, and an on / off control of the analog switch. The high input impedance broadband amplifier for an oscilloscope according to claim 1, further comprising control means. 5. The high input impedance broadband amplifier for an oscilloscope according to claim 4, wherein said control means is connected to a remote control device. 6. A second composite amplifier cascaded to said first composite amplifier via a variable resistor, and a plurality of feedback circuits for feeding back the output of said second composite amplifier to its inverting input terminal. 3. An oscilloscope according to claim 1, further comprising: third switching means for selectively connecting one of said feedback circuits between an output terminal and an inverting input terminal of said second composite amplifier. Input impedance broadband amplifier for use. 7. A gain setting resistor and fourth switching means are connected in series to an emitter of a transistor constituting said differential amplifier of said first composite amplifier, and said fourth switching means and said first switching means are connected in series. 2. The high input impedance broadband amplifier for an oscilloscope according to claim 1, wherein said means is linked to said means. 8. A voltage dividing / dividing circuit provided between the first input resistor and the first composite amplifier, and a fifth switching for switching a voltage dividing / dividing ratio of the voltage dividing / dividing circuit. 2. The high input impedance broadband amplifier for an oscilloscope according to claim 1, further comprising: 9. A second measurement signal input for inputting a signal to be displayed on an oscilloscope together with a third composite amplifier connected in parallel to the first composite amplifier and the first measurement signal input terminal. A second input resistor connected between the second measurement signal input terminal and the inverting input terminal of the third composite amplifier; an output terminal and an inverting input terminal of the third composite amplifier A plurality of second feedback resistors provided between the third composite amplifier and one of the second feedback resistors are selectively connected between an output terminal and an inverting input terminal of the third composite amplifier. Switching means, a third phase correction capacitor connected in parallel to the second input resistor, and a fourth phase correction capacitor connected in parallel to each of the second feedback resistors. The time constant of the connection is such that the second input resistor and its parallel capacitor High input impedance wideband amplifier for oscilloscope according to claim 1, characterized by comprising a capacitor which is to be substantially equal to to the time constant.