JPH0262104A - Nmr signal amplifier - Google Patents

Abstract

PURPOSE:To form the amplifier of a low input output noise ratio coefficient with a low impedance input circuit by connecting plural amplifying circuits such as an electric field effect transistor in parallel and increasing the amplification factor of an apparant amplifier. CONSTITUTION:In a parallel circuit to connect the gate and drain of electric field effect transistors FET1 and 2, resistances 8 and 9 and a resistance for a gate bias and a load resistance 3 are made common, source resistances 4 and 5 are made individual and the variance of FET1 and 2 is prevented. Capacitors 6 and 7 are used as the high frequency bypass capacitor of the resistances 4 and 5. Thus, the drain current to flow at the resistance 3 can be made two times as much as the individual FET. Here, an input output noise ratio coefficient NF is reduced by the influence of the input gate capacity of the FET and with the capacity as a part of an input resonance circuit, the influence can be removed. Thus, a low NF amplifier can be formed by the low impedance input circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention provides a high frequency low noise amplifier, particularly an N
The present invention relates to a preamplifier for MR signals.

[Conventional technology]

Generally, it is advantageous to set the output impedance of an NMR signal detection probe in an MRI apparatus to a low impedance (Zo=50Ω) from the viewpoint of suppressing noise voltage and handling of the signal line. For this reason, when using high input impedance field effect transistors (hereinafter abbreviated as FETs), etc., conventionally Electronics Letters, Vol. 27, 1
November 975, pages 596 to 597 (Electr
onicsLetters, 596-697.27t)
+, 11.1975), an impedance conversion circuit using a transformer circuit was used.

[Problem to be solved by the invention]

FIG. 4 shows an example of a conventional amplifier circuit. In the figure, a signal from a probe 44 that detects an NMR signal is propagated via a coaxial line (usually Zo = 50Ω) 45 to an amplifying element 41 such as a high input impedance FET from an impedance conversion circuit 42, where the signal is amplified and sent to a load 43. It is output from A problem with such a circuit is signal propagation loss in the impedance conversion circuit 42. It is generally difficult to reduce the loss in the above-mentioned circuit that handles minute signals, which is also a factor that deteriorates NF (Noise Figure: input/output noise ratio evaluation index).

[Means to solve the problem]

The present invention is based on a circuit configuration in which a plurality of amplifying elements such as FETs are used in parallel without using a conventional impedance conversion circuit.

[Effect]

NF is the mutual conductance gm of the element (input voltage V
, to the output current i, g @ = l 4 / V t
) and the signal source impedance Rs. The present invention achieves the above object by using a plurality of amplifying elements in parallel to increase the apparent g of the amplifier so that NF does not deteriorate even when Rs is in a low impedance state.

〔Example〕

In order to facilitate understanding of the present invention, the explanation will be given with reference to FIGS. 5 and 6, which are background circuits. FIG. 5 shows a circuit in which an FET 51 is directly connected to a signal source circuit 55 through a capacitor 56, and the impedance conversion circuit 42 shown in FIG. 4 is not provided. The signal source 55 consists of a signal source impedance (Rs) 11 and a voltage source (vs) 12. Resistors 53 and 54 are FET51
This is a gate bias circuit. The output voltage vo is the load (R
) 52 as an amplified signal. Fifth
The NF relationship in the circuit shown in the figure will be explained using the equivalent circuit shown in FIG. In FIG. 6, reference numeral 61 indicates an internal equivalent path of the FET 51 of FIG. In such a circuit, the noise figure NF is now hl·Vl.

As is clear from equation (1), glI influences at low frequencies, and input capacitance 0162 influences at high frequencies.

Embodiments of the present invention will be described below with reference to FIGS. 1, 2, and 3. FIG. 1 shows a parallel circuit in which the gates and drains of F E 'I' 1 and 2 are connected. Resistors 8 and 9 are the gate bias resistor and load resistor 3 are shared, and source resistors 4 and 5 are separate and the drain current i of FETI and 2 is
111. This prevents variations in L a2. Further, capacitors 6 and 7 are used as high frequency bypass capacitors for resistors 4 and 5. In this way, the drain current i flowing through the resistor 3
a= i aI+ i ax , ga of the apparent circuit
(g+s=g, t+g bite 2; gel and g-2 are FETI
, glI of 2) can be doubled. The equivalent circuit of FIG. 1 is exactly the same as that of FIG.

Figure 2(a) shows the signal source impedance Rs (horizontal axis 21)
FIG. 3 is a characteristic diagram of the NF (vertical axis 22). However, 1 frequency f
is fixed. Characteristic 23 in the figure corresponds to the case where the FET in FIG. 5 is in one stage, and characteristic 24 corresponds to the case in which the FET in FIG. 1 is in two stages in parallel. Characteristic 24 is obtained by moving 23 almost parallel to the direction in which the signal source impedance R521 is smaller. In addition, the left side of the minimum NF value for both 23° and 24 characteristics is expressed by equation (1) ()
The equation on the left side of the second term in the figure shows that the NF becomes higher (worse) due to the influence of the third term. Next, FIG. 2(b) shows the characteristics of frequency f (horizontal axis 25) versus NF (vertical axis 26) (however, Rs is constant). A characteristic diagram 27 shows characteristics that tend to be common to the circuits shown in FIGS. 1 and 4.

As the frequency f increases, due to the influence of the second term of the inner cylinder in equation (1), N
F deteriorates (increases). In this case, the NF characteristics will improve if the influence of the input gate capacitance Ct of the FET is removed.

FIG. 3 shows a second embodiment of the present invention in which C1 is used as part of an input resonant circuit to ensure NF at high frequencies.

In the figure, a resonant circuit 35 includes an inductance 31, capacitors 32 and 33, and an input gate capacitance 34. In this way, NF is guaranteed at the desired high frequency selection position fo, as shown by the characteristic line 28 in FIG. 2(b).

Note that although the circuit of the present invention has been explained using a general FET, other JFET, GaAsF
It goes without saying that a similar circuit can be easily constructed using active elements such as E T and bipolar.

〔Effect of the invention〕

According to the present invention described in detail above, a low impedance conversion circuit is not required for input to a low impedance signal source, and NF deterioration when using the conversion circuit is eliminated. A low NF amplifier can be easily constructed using an impedance input circuit.

[Brief explanation of the drawing]

FIG. 1 is a basic circuit diagram of an embodiment of the present invention, FIG. 2 is an NF characteristic diagram for supplementary explanation of FIGS. 1 and 3 of the present invention, and FIG. 3 is a diagram showing an applied example of the present invention. Figure 4 is the conventional circuit diagram,
FIG. 5 is a supplementary explanatory circuit diagram related to the present invention, and FIG. 6 is an equivalent circuit diagram of FIG. 5. 1.2...FET, 3...Output load resistance, 4,5...
...Source resistance, 6,7.13...High frequency bypass capacitor, 8,9...Gate bias resistor, 10.
... High frequency signal source, 31 ... Resonance coil, 32.3
3... Variable capacitor for resonance, 34... Equivalent gate capacitance of FET, 35... Resonant circuit. 1.1! Knee - Figure 1 cause Figure 2 tVf)

Claims (1)

[Scope of Claims] 1. An NMR signal amplifier characterized in that it has a circuit configuration in which a plurality of amplifier circuit elements such as field effect transistors are used and input terminals and output terminals are commonly connected. 2. An NMR according to claim 1, characterized in that a circuit configuration is implemented in which a resistor and a capacitor are added in parallel between the ground potential side terminals of a plurality of amplifying elements and the ground.
signal amplifier. 3. The NMR signal amplifier according to claim 2, characterized in that the circuit has a circuit configuration in which a coil or a capacitor is added in parallel between the input signal source side and the input terminal of the amplifying element.