JP2018064225A - Current-voltage conversion circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a current-voltage conversion circuit that achieves high added value by using a vacuum tube, while reducing the number of parts as much as possible without sacrificing electrical characteristics, and can be practically used without adjustment.SOLUTION: By realizing a low input impedance by a base grounded circuit, adverse effects on a current signal source can be prevented. The circuit current and the input terminal voltage are regulated by a constant current source and a bias circuit 6 so that circuit operation becomes proper. In order to achieve load matching, common mode noise removal, and DC component removal effect, a transformer 7 is provided in a load circuit.SELECTED DRAWING: Figure 6

Description

The present invention relates to a circuit that converts a current signal into a voltage signal.

In recent years, electron tubes have been used in high-priced acoustic electronic devices, and using an electron tube as an amplifying element leads to high added value of products.
A digital-analog conversion device is important in reproducing a digital sound source. However, if a vacuum tube is used for the signal circuit, it will lead to further added value, so that it is economically useful. On the other hand, since it is not technically easy, it has not been commercialized much.
The electron tube here refers to an amplifying electron tube (triode, pentode, so-called vacuum tube) that has a thermionic cathode and can be controlled by a grid.

Current voltage conversion by resistance or using a voltage output type digital-analog conversion device and amplifying by inputting to the grid of the vacuum tube has been put to practical use, but the non-linearity of the vacuum tube remains as harmonic distortion as it is Therefore, the original performance of the digital-analog conversion device is impaired.
This can be solved to some extent by performing negative feedback, but the GB product as an amplifier circuit is not large and the effect is limited. Forcing it increases the complexity of the circuit, creating maintenance and cost problems. (Non-Patent Document 1)
In addition, voltage output type digital-analog converters are limited in variety and disadvantageous from the viewpoint of noise compared to current output type, so using this is appropriate for high-priced electronic equipment. is not.

There are vacuum amplifiers at the first and last stages of the current-voltage conversion circuit using operational amplifiers, but the instability of the operating point of the first-stage vacuum tube appears as direct current drift in the output as it is, and the product yield rate is reduced. In terms of failure, it is not intrinsically safe.
Therefore, in practice, some compensation circuit such as negative feedback (so-called DC servo circuit) in the DC region is required to correct this DC drift. This means an increase in the number of parts, which causes maintenance and cost problems. (Non Patent Literature 2)

Soundfort “High-Resolution USB DAC Equipped Vacuum Hybrid Amplifier QS-9” Introduction Page 2016 http://www.soundfort.jp/products/qs-9/ September 2016

Akihiko Kaneda “Current Transmission Audio DC Amplifier” Seibundo Shinkosha October 2013 pp162

"PCM1702" data sheet Burr Brown (currently Texas Instruments) 1993 (Texas Instruments document number SBAS026)

“DSD1794A” Data Sheet Texas Instruments 2004 (Texas Instruments Document SLES116A)

A conventional current-voltage conversion circuit using an electron tube with a simple circuit configuration generates voltage at the input current injection point and is not appropriate for the current signal source. Depending on how the constant is selected, the current signal source is normal. It hindered the movement.

A conventional current-voltage conversion circuit using an electron tube with a simple circuit configuration has a high harmonic distortion and is not suitable for high fidelity reproduction.

A conventional current-voltage conversion circuit using an electron tube in which an operational amplifier type circuit is configured has a large number of elements, and therefore is not appropriate in terms of yield, failure, and cost.

Due to the nature of the electron tube, the characteristics change during use time and the electrical characteristics deteriorate. Therefore, in the conventional circuit, adjustment and confirmation are required not only at the time of manufacture but also at the time of use.

The present invention suppresses unnecessary voltage (hereinafter referred to as residual voltage) generated at the current input terminal by lowering the input impedance, reduces harmonic distortion, minimizes the number of components, and makes the electron tube unstable. In this case, no adjustment is realized.

Among the problems to be solved by the present invention, in order to suppress the residual voltage, the input impedance is lowered by using a conventional grounded cathode type amplifier circuit as a grounded grid type. The input impedance of the grid ground circuit is

(Equation 1)

Zin = 1 / gm


Therefore, if an electron tube with a gm (mutual conductance) of 10 mS is used, it becomes 100Ω, and if an electron tube with a 40 mS is used, it becomes 25Ω. Accordingly, when current-voltage conversion can be performed with a resistance of 500 to 1 kΩ as in a general digital-analog conversion device (Non-patent Document 3), the input impedance viewed from the current signal source alone is 20% to 2.5%. %, Which is more advantageous than the combined use of voltage-current conversion by resistance and a grounded cathode amplifier circuit.

Even when the input impedance of the grid grounding circuit is used, when a digital-analog converter (Non-patent Document 4) having a low voltage operation and a large current output is used as a signal source, the residual voltage is further suppressed. This is not sufficient because the input impedance of the conversion circuit needs to be 100Ω or less. Therefore, in order to increase the equivalent gm, the problem can be solved by configuring a cascode circuit using a bipolar transistor or a field effect transistor. In the case of a bipolar transistor, its gm is

(Equation 2)

gm≈40 × Ic

Therefore, it is possible to obtain an arbitrary gm simply by increasing the circuit current. If Ic = 20 mA, 0.8S is obtained, so Zin is 1.25Ω. Therefore, the present invention can also be applied to recent digital-analog conversion devices (Non-patent Document 4).
Furthermore, since the apparent gm of the circuit is defined by the gm of the transistor, a special high gm electron tube is not required.

Among the problems to be solved by the present invention, with respect to harmonic distortion, the plate side current of the grid grounding circuit (base grounding circuit) used as the current source input is the same as the input current when the current amplification factor of the circuit is sufficiently high. (Current gain is 1), so in principle it is a distortion-free circuit. That is,

(Equation 3)

ik = iin

(Equation 4)

ip = iin

(Equation 5)

eo = ip × RL


Therefore, essentially low distortion is expected.

Among the problems to be solved by the present invention, regarding the characteristic fluctuation / deterioration of the electron tube, a constant current source is provided on the cathode side in order to define the operating current. As a result, the circuit current is automatically determined, so that adjustment and confirmation of the operating point are not required.

Explanatory drawing which showed the implementation method (Example 1) Explanatory drawing which showed the implementation method (Example 2) Explanatory drawing which showed the implementation method (Example 3) Explanatory drawing which showed the implementation method (Example 4) Explanatory drawing which showed the implementation method (Example 5) Explanatory drawing which showed the implementation method (Example 6)

FIG. 1 shows the application of the present invention to the output of a current output type digital-analog converter. The operating current at the time of no signal of the electron tube is set so that the cut-off or the like does not occur even when the digital-analog converter generates a current at the maximum signal.
When the PCM 1702 is used for the digital-analog converter, the output current is ± 1.2 mA. Therefore, a minimum signal operating current of 600 μA is required. In reality, it is necessary to flow current so that the input impedance can be made sufficiently low due to the balance with gm.
When 12BH7A is used for the electron tube, it is defined as 3.1 mS when the plate current is 11.5 mA. Therefore, if the circuit current is determined as 11.5 mA, the input impedance becomes 32.3Ω, and the residual voltage becomes 38 mVp-p. .
A part of the signal current is shunted to the internal resistance of the signal source and the resistance on the cathode side, but most of it reaches the plate side and becomes a signal voltage. For example, if the load resistance is 5 kΩ, a signal voltage of ± 6 V can be obtained.
The resistance and negative power source on the cathode side are for regulating the current of the vacuum tube and for setting the voltage of the input terminal to around 0V. The capacitor in parallel with the load resistor is for suppressing unnecessary high frequency components in the high frequency range. A time constant may be set so that components generated by oversampling are suppressed.

FIG. 2 shows the application of the present invention to the output of a current output type digital-analog converter. In place of FIG. 1, a cascode circuit of a bipolar transistor and an electron tube is used. The basic principle of setting constants is the same.
When DSD1794 is used for the digital-analog converter, its output current is 7.8 mAp-p, and -6.2 mA when there is no signal. Therefore, a circuit current of 10.1 mA or more is required. Since this digital-analog converter is a differential output, two sets of cascode circuits are provided.
If the circuit current is set to 15 mA considering a certain margin, the gm of the bipolar transistor is 0.6 S and the input impedance is 1.67Ω. Therefore, even if 7.8 mAp-p is received, only a residual voltage of about 13 mVp-p is generated.
In this case, any electron tube may be used as long as it can withstand a current of 15 mA (positive peak 18.9 mA). However, when using a low gm electron tube, the electron tube may be cut off unless the voltage between the collector of the transistor and the cathode of the electron tube is kept high.
A bias circuit is provided to define the operating point of the base ground circuit using bipolar transistors and the cascode circuit using electron tubes.
The negative power supply and the resistor on the base side define the circuit current and set the voltage at the input terminal around 0V. The capacitor in parallel with the load resistor is for suppressing unnecessary high frequency components in the high frequency range. A time constant may be set so that components generated by oversampling are suppressed.

3 and 4 show the case where the present invention is applied to the output of a current output type digital-analog converter. 1 and 2 are provided with a current source in order to regulate the circuit current. As a result, the current is automatically regulated, so that it is stable against fluctuations and deterioration of the characteristics of the electron tube. Further, the signal current that cannot reach the plate side is reduced by simply using the resistance.

FIG. 5 shows the application of the present invention to the output of a current output type digital-analog converter. In FIG. 4, a bias circuit is provided to define the voltage at the input terminal. As a result, the voltage at the input terminal is automatically regulated, so that the digital-analog converter can be prevented from entering an inappropriate operating state.
Here, when the characteristics of the transistors 3a and 3b of the cascode circuit and the transistor 6a of the bias circuit are the same and the emitter current density is the same, the base voltage of each transistor is the same, and the emitter of the transistor 6b is connected to the ground. Therefore, the voltage at the input terminal is automatically regulated around 0V. In reality, monolithic type matching transistors are used for 3a, ab, and 6a in order to align the characteristics. For example, MAT04 (manufactured by Analog Devices) may be used.
The transistor 6b is for compensating the base current.

FIG. 6 shows the application of the present invention to the output of a current output type digital-analog converter. In FIG. 5, a transformer is provided as a matching circuit for output matching. As a result, the output impedance can be set to an arbitrary value, and an effect of removing unwanted DC components and common-mode noise is provided.
The transformer 7 must intentionally increase the leakage inductance in order to limit the high-frequency response and have a high common-mode noise removal capability at a high frequency. The capacitor 7a is for cutting off the direct current component, and is applied if necessary to prevent saturation of the transformer.

DESCRIPTION OF SYMBOLS 1 Current signal source 2 Base ground amplifier circuit 3 Cascode amplifier circuit 3a Transistor 3b Transistor 4 Load resistance 5 Current source 6 Bias circuit 6a Transistor 6b Transistor 7 Transformer 7a Capacitor 8 Output terminal 9 Power supply

Claims (5)

A current-voltage conversion circuit that injects signal current from the cathode side of the electron tube.
A current-voltage conversion circuit comprising a semiconductor cascode circuit in the circuit of claim 1.
A current-voltage conversion circuit in which a constant current circuit is provided in the circuits according to claim 1 and 2.
A current-voltage conversion circuit comprising a bias circuit for regulating a voltage at an input terminal in the circuit according to claim 1 or 2.
4. The current-voltage conversion circuit according to claim 1, wherein a transformer is provided for common-mode noise removal and load matching.

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