JPH0669126B2 - Wideband DC Amplifier - Google Patents

Description

Detailed Description of the Invention Field of Industrial Application The present invention relates to a wide-band DC amplifier with high input impedance for an oscilloscope, and provides a wide-band DC amplifier with little drift and capable of changing the output level setting and adjusting the high-frequency characteristics with a simple configuration.

The input stage of a conventional oscilloscope typically requires a DC amplifier with an impedance of 1 MΩ and a wide bandwidth, and is particularly required to have excellent high frequency characteristics with little drift over long periods of time.

The input signal to an oscilloscope is usually an unbalanced signal, which is converted from unbalanced to balanced at the amplifier input stage to provide the required amplification and deflect the vertical axis of the cathode ray tube to obtain the desired display.

Problems to be Solved by the Invention However, in conventional DC amplifiers in the input stage, the characteristics of FETs and transistors change depending on the temperature, so each element used in the input stage must have consistent characteristics in order to maintain temperature balance. However, this has the disadvantage that it is difficult to obtain elements that are excellent up to high frequencies, and they are expensive.
Therefore, the circuit must be configured to reduce the influence of such active elements.

For example, Japanese Utility Model Application No. 54-47572 (Japanese Utility Model Application Laid-Open No. 55-148208)
As shown in the microfilm, the comparison amplifier and FE
An unbalanced input circuit was configured using a T and a transistor. However, in the previous example, even though the comparison amplifier had a high input impedance, there was an offset current, and a weak input current was an essential condition, so there was a problem of "zero level shift" occurring, where the reference position of "0V" displayed on the oscilloscope screen moved when the input terminal was grounded and when it was forcibly set to GND (ground) using the oscilloscope's control knob.

Furthermore, because oscilloscopes require good frequency characteristics over a wide range, the difference between the gain of the low-frequency components of the comparison amplifier that forms the negative feedback and the gain of the high-frequency components obtained in the FET and transistor circuit configuration requires multiple adjustments in the circuit after input impedance conversion.However, in conventional input impedance conversion circuits, when a general-purpose operational amplifier is used as the comparison amplifier for drift correction, the stray capacitance of the input terminal is approximately 3 to 7 pF, which increases the capacitance of the oscilloscope's input terminal, making it impossible to perform drift correction over a wide band.

Furthermore, even in a high input impedance wide band amplifier having a means for dividing the output signal voltage, as shown in Japanese Patent Laid-Open Publication No. 57-33807, the gain setting of the high frequency component and the output level cannot be adjusted separately, which makes the adjustment process complicated.

The present invention solves the above-mentioned problems of the conventional art, and aims to provide an excellent wide-band DC amplifier which allows the output DC level and the gains of the low-frequency and high-frequency components to be set independently and easily by adjusting two circuit elements separately, and which has a wide band and little drift.

To achieve the above object, the present invention provides a wideband DC amplifier comprising a first source-follower FET AC-coupled to an input terminal, a comparison amplifier having one input directly coupled to the input terminal, and a negative feedback circuit which adds the output of the first source-follower FET and the output of the comparison amplifier and applies the resulting signal to the base of an emitter follower to obtain an output signal, and which couples this output signal to the other input of the comparison amplifier, the wideband DC amplifier comprising a means for voltage-dividing the input and output signals, and a negative feedback circuit which couples each of the voltage-divided signals to a pair of second source-follower FETs.
and means for varying the voltage applied to the means for voltage-dividing the output signal, thereby adjusting the output DC level without changing the voltage-dividing ratio of the output signal.

Therefore, according to the present invention, the output DC level and the gains of the low-frequency and high-frequency components can be set by simple independent adjustments without impairing the input balance of the comparison amplifier, the DC level of the amplifier output can be easily changed, drift can be reduced, if the input signal is zero in DC terms, the DC level of the output signal can also be made zero, the gains of the low-frequency and high-frequency signals can be made equal, and further, since a voltage divider is used on the input signal side of the comparison amplifier, the input stray capacitance of the comparison amplifier added to the input side of the oscilloscope is reduced in accordance with the voltage division ratio, and good frequency characteristics can be obtained.

An embodiment of the present invention will be described below with reference to FIG. 1. Reference numeral 1 denotes an input terminal, to which an input signal to an oscilloscope is normally applied. Reference numerals 3 and 4 denote resistors, which provide a voltage dividing means for the input signal and an input impedance. Reference numeral 6 denotes an FET, and reference numerals 7 and 8 denote resistors, which provide the required bias and act as a source follower. Reference numeral 5 denotes a capacitor, which transmits a high-frequency signal to FET 6. Reference numeral 9 denotes a transistor, and reference numeral 10 denotes a resistor, which supplies current to FET 6 and amplifies the low-frequency signal at the same time. Reference numeral 16 denotes a transistor, and reference numeral 17 denotes a resistor, which constitutes an emitter follower, receiving the signal from the previous stage and providing an output. Reference numeral 24 denotes an output terminal, which supplies a signal to the vertical amplifier of the oscilloscope. Reference numerals 18, 20, and 21 denote resistors, and reference numeral 19 denotes a variable resistor, which varies the voltage dividing and the ratio of the output signal, thereby matching the low-frequency gain to the high-frequency gain. Reference numeral 22 denotes a resistor,
23 is a variable resistor, which changes the DC level of the output by changing the voltage. 12 and 13 are FETs.
The resistors 14 and 15 are each configured as a source follower, and receive the input and output divided voltage signals to perform impedance conversion. The reference numeral 11 is a comparison amplifier consisting of an operational amplifier or the like, and the difference signal between the input signal and the output signal is amplified and output to the transistor 9.
This reduces the output fluctuations via the base of the .25.25 is a ground terminal.

Next, the operation of one embodiment of the present invention will be described. The DC component of the signal applied to input terminal 1 is cut by coupling capacitor 5, and the high-frequency component is transmitted by source-follower FET 6 and emitter-follower transistor 16, and obtained as an output at terminal 24. Meanwhile, the low-frequency component is voltage-divided by resistors 3 and 4 and transmitted via source-follower FET 12 to one input of comparison amplifier 11, and the output signal is coupled to the other input of comparison amplifier 11 via voltage dividers 18, 19, 20, and 21 and source-follower FET 13, and the difference signal is amplified and transmitted via transistor 9 to resistor 8 and transistor 16.

As described above, this amplifier amplifies low-frequency and high-frequency components separately, providing a stable, wideband DC amplifier. Generally, when no signal is applied, the output voltage slowly fluctuates from the initial setting due to the passage of time and temperature changes, which is called drift. In this invention, the amplifier is divided into low-frequency and high-frequency components, and the characteristics of each are utilized, suppressing and reducing drift on the low-frequency side through the negative feedback action of the circuit.
Since FETs 12 and 13 are source followers and have very high input impedance, the voltage dividing point of voltage dividing circuits 3 and 4 is at ground potential, and therefore the source of FET 12 is at a constant value, taking into account the bias. FET 13 operates in a similar manner, so the input of comparison amplifier 11 is in a balanced state. If the voltage at output terminal 24 becomes positive due to various fluctuation factors, the comparison amplifier 11 will go in the positive direction via output side resistors 18, 19, 20, and 21 and FET 13. Since the other input is constant, the output of comparison amplifier 11 will be positive, and the phase will be inverted by transistor 9.
It operates so that the output returns to the set value via FET 6 and transistor 16. It is clear that the opposite of the above occurs when the output becomes negative. This means that if the input voltage is constant, the output will also be constant, and at the same time, since the output faithfully follows the input signal, it is possible to provide an extremely stable impedance converter with little drift as a low-frequency amplifier, including DC. Furthermore, when it is desired to adjust the DC level on the output side, the variable resistor 19 or 23 is adjusted as desired, and the input voltage is compared by the comparison amplifier 11.

FIG. 2 shows frequency characteristics, with a indicating low frequency characteristics and b indicating high frequency characteristics, and the characteristics shown in c are obtained as a result of the above-mentioned operations.

As is clear from the above examples, the present invention is a general-purpose high-frequency FE.
By using T and a pair of FETs and a comparison amplifier, the DC drift is reduced and at the same time the input balance of the comparison amplifier is balanced by adjusting the variable resistor, so that the input balance of the comparison amplifier is not affected.
The gain setting of the low frequency component and the high frequency component and the output DC level can be set by simple independent adjustment, and the DC level of the amplifier output can be easily changed, drift can be reduced, and if the input signal is DC zero, the DC level of the output signal can also be set to zero.

Furthermore, by providing a voltage divider at each input of the comparison amplifier,
This also has the effect of reducing the influence of the input stray capacitance of the comparison amplifier and achieving good frequency characteristics by matching the gains of the high and low frequency components.

[Brief explanation of the drawings]

1 is a circuit diagram of a wide-band DC amplifier according to an embodiment of the present invention, and FIG. 2 is a frequency characteristic diagram thereof. 1...input terminal, 6, 12, 13...FET, 9, 16...transistor, 11...comparison amplifier, 19, 23...variable resistor, 24...output terminal.

──────────────────────────────────────────────────── Continued from the front page (72) Inventor: Yukimichi Okamura 4-3-1 Tsunashima Higashi, Kohoku-ku, Yokohama, Kanagawa Prefecture Address: Matsushita Communication Industrial Co., Ltd. (56) References: JP 54-80782 (JP, A) JP 57-33807 (JP, A) Utility Model Application Publication No. 148208 (JP, U)

Claims (1)

[Claims] [Claim 1] A wideband DC amplifier comprising a first source-follower FET AC-coupled to an input terminal, a comparison amplifier having one input directly coupled to the input terminal, and a negative feedback circuit which adds the output of the first source-follower FET and the output of the comparison amplifier and applies the resulting signal to the base of an emitter follower to obtain an output signal which is then coupled to the other input of the comparison amplifier, characterized in that the wideband DC amplifier further comprises means for voltage-dividing the input and output signals, means for coupling each of the voltage-divided signals to the input of the comparison amplifier via a second source-follower FET paired with the input signal, and means for varying the voltage applied to the means for voltage-dividing the output signal, thereby adjusting the output DC level without changing the voltage division ratio of the output signal.