JPH0236142Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a transmission circuit for a nuclear magnetic resonance apparatus that can perform various NMR measurements with a single transmission circuit by using FETs (field effect transistors).

In a nuclear magnetic resonance apparatus, it is necessary to change the frequency of the excitation high frequency wave depending on the nuclide to be measured. The variable range is, for example, from 10MHz to several 100MHz,
Since a single excitation coil cannot efficiently irradiate the sample with excitation high frequency waves over such a wide range, excitation coils or excitation coils are used.
The entire NMR probe is replaced depending on the nuclei to be measured. Conventionally, power amplifiers employing bipolar transistors have been used to amplify the high frequency waves sent to the excitation coil, but they cannot efficiently amplify the high frequency excitation waves over a wide frequency band. A power amplifier is provided for each band, and a switch is used to select the amplifier to be used.

Therefore, the configuration is complicated and it is disadvantageous in terms of price. Therefore, for example, it is possible to use a transistor with good high-frequency characteristics, but since the impedance of the excitation coil that serves as the load changes greatly when exchanging samples or measuring variable temperature, there is a risk of destruction of the transistor. There is. To prevent this, it is possible to use transistors with sufficient power or protection circuits with high-speed response, but this would lead to a significant increase in costs.

The present invention was developed in view of the above-mentioned conventional problems, and is a core technology that can perform NMR measurements over a wide band with a single transmitter circuit (power amplifier) by using FETs (field effect transistors). The present invention aims to provide a transmitting circuit for a magnetic resonance apparatus.

The present invention provides a transmission circuit for a nuclear magnetic resonance apparatus that amplifies a high-frequency signal generated by an oscillator and sends the amplified signal to an irradiation coil placed near a sample. a transformer connected between a field effect transistor and the irradiation coil; and an LC matching circuit connected between the transformer and the field effect transistor, the LC matching circuit connecting the transformer and the field effect transistor. It is characterized by the provision of an LC matching circuit comprising an L to be connected and a C to be inserted between the connection point of the transformer and L and the ground. Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 shows the configuration of an embodiment of the present invention. In the figure, 1 is a high frequency oscillator, and the high frequency excitation generated from the oscillator 1 is sent to the final stage power amplification MOSFET 3 via an intermediate amplifier 2. . Applicable FET
The source S of the transistor 3 is directly grounded to enable large amplitude operation, and a feedback circuit 4 for flattening the gain is connected between the drain D and the gate G, if necessary. The impedance of the excitation coil 6 (including tuning capacitors, etc.) in the NMR probe 5, which serves as a load, is normally close to 50Ω;
A broadband transformer 7 and an LC impedance matching circuit 8 are inserted between the NMR probe 5 and the FET 3 in order to make the load the optimum load for the MOSFET over a wide band. This LC matching circuit 8 includes an L connecting between the transformer 7 and the FET 3, and an L that connects the transformer 7 and the FET 3.
and C inserted between the connection point of L and ground. 9 is a coupling capacitor for DC cut, 10 is an inductor for high frequency cut,
Vg is the gate bias voltage and Vd is the drain voltage.

In the above configuration, the MOSFET has a negative temperature coefficient, and the output current decreases as the temperature increases during high output. Therefore, even if the load condition changes significantly, the operating condition shifts to a safe one, so high reliability can be obtained without the need for a special protection circuit or the like.

Further, as mentioned above, the source of MOSFET 3 is directly grounded, and FET 3 is capable of large amplitude operation. A feature of the present invention is that this large-amplitude, high-power operation can be realized over a wide band.

That is, the MOSFET load condition is given by the following equation.

Rl=(Vd−Vsat) ² /2P where Rl is the high power load resistance and Vsat is
The drain saturation voltage of the MOSFET, P is the output power.

In the above equation, Vsat increases as the frequency increases, and the output power P decreases. Therefore, in order to maintain the output power P at a large value up to high frequencies, it is necessary to reduce Rl according to the frequency. To achieve this, a transformer 7 and an LC matching circuit 8 are used. The transformer 7 converts the impedance of the excitation coil from 50Ω to a lower impedance, for example 12.5Ω.

FIGS. 2a and 2b show the individual frequency characteristics of the amplifier circuit composed of the FET 3 and the feedback circuit 4 and the LC matching circuit 8. It can be seen from FIG. 2 that the cutoff frequency ₂ of the LC matching circuit 8 is set higher than the cutoff frequency ₁ of the amplifier circuit. When setting ₂ high in this way, the value of L is chosen to be sufficiently small, and the value of C is chosen to be relatively large compared to it.

When such settings are made, in the frequency range lower than frequency ₁ , the impedance of the LC matching circuit is affected by the large impedance of C.
It is sufficiently larger than 12.5Ω and can be virtually ignored. Therefore, in such a frequency range, 12.5Ω becomes the drain load resistance Rl of MOSFET 3.

On the other hand, in the region between frequencies ₁ and ₂ , the impedance of C becomes smaller, and accordingly the impedance of the matching circuit also becomes smaller, reaching a value that cannot be ignored compared to 12.5Ω. This region corresponds to the bulge of the characteristic curve in FIG. 2b. Therefore, in this region, the drain load resistance Rl is
12.5Ω, and as a result, the output power P is
Even if the frequency increases and Vsat becomes larger, it does not decrease based on the above equation and remains high as shown by the broken line in FIG. 2a.

In this way, a single power amplifier can operate at high power over a wide frequency band, which not only simplifies the configuration and is advantageous in terms of cost, but also eliminates the saturation behavior of the MOSFET itself. Combined with the speed of recovery time from , it becomes possible to quickly rise and fall the excitation pulse modulated high frequency, shorten the carrier high frequency switching time, and perform highly accurate NMR measurements.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of an embodiment of the present invention, and FIG. 2 is a diagram for explaining its operation. 1: High frequency oscillator, 3: MOSFET, 5:
NMR probe, 6: excitation coil, 7: transformer, 8: LC matching circuit.

Claims (1)

[Scope of utility model registration request] In a transmission circuit for a nuclear magnetic resonance apparatus for amplifying a high frequency signal generated by an oscillator and sending it to an irradiation coil placed near a sample, a field effect transistor for amplifying the high frequency signal; a transformer connected between the irradiation coil; and an LC matching circuit connected between the transformer and the field effect transistor; 1. A transmitting circuit characterized in that an LC matching circuit is provided, which is comprised of a C that is inserted between a connection point between a transformer and an L, and a ground.