JPH02180412A - Broad band amplifier - Google Patents

Abstract

PURPOSE:To expand a usable frequency band by providing a feedback circuit formed with the use of a drain grounding field effect transistor(FET) between an input and an output with 180 deg.C phase difference separately from a negative feedback circuit, and suppressing the input/output VSWR of the broad band amplifier in an allowable range. CONSTITUTION:An FET 20 controls a feedback quantity from an output terminal 9 to an input terminal 8. The feedback quantity is determined by the gains of an amplifying part 30 and the FET 20. Since the gain of the amplifying part 30 and the FET 20 are both decreased as the frequencies are increased, the feedback quantity by the FET 20 is decreased as the frequencies are increased. Accordingly, the gain of the amplifier 30 is approximately flat with respect to the frequencies. Consequently without largely deteriorating the input/ output VSWR of the amplifier, the frequency characteristic of the gain can be drastically improved. The feedback circuit by the FET 20 controlling the feedback quantity can be used between the terminals having the 180 deg.C phase difference of a microwave voltage.

Description

[Detailed Description of the Invention] Skin! TECHNICAL FIELD The present invention relates to wideband amplifiers, and more particularly to wideband amplifiers using field effect transistors.

In the circuit configuration of a conventional wideband amplifier using a conventional Koki strain field effect transistor (hereinafter also referred to as rFETJ), the FET shown in (1) in FIG. 4 is used as a common source.

(2) is the DC blocking capacitor, (3) is the FET drain bias resistance, (4) is the FET gate bias resistance, (5), (6) and (7) are feedback from the FET output terminal to the input terminal. (8) is the input terminal of the amplifier, and (9) is the output terminal of the amplifier. In a wideband amplifier, gain flatness over a wideband and low VSWR (voltage standing wave ratio) are important. Therefore, the feedback resistance (5),
(6) and (7) are used to configure a parallel negative feedback circuit to lower the input/output impedance of the FET and flatten the gain. FET equivalent circuits are often used to design their resistance values. The equivalent circuit of the FET is shown in Figure 5, where Rg, Rs, and Rd are the gate resistance, source resistance, and drain resistance, respectively, and Cgs, Cgd, and C
ds is the capacitance between the gate and the source, the capacitance between the gate and the drain, and the capacitance between the drain and the source, respectively, Ri is the input resistance, Gd is the drain conductance, V is the voltage applied to Cgs, and gm is the mutual conductance. The gate width of all FET (1) in Fig. 4 is set to 800.
μm, the gate length is 0.6 μm, and F E T (1
) is 3V and the drain current of each FET is 30+*A, the frequency characteristics of the amplifier gain and human output VSWR shown in FIG. 4 are as shown in FIG. 6.

The values used here are Rg=1.5Ω and Rs=1.5Ω.

Rd=L, OΩ, Cgs=0.7pF,
Cg d =0.07pFCds=0.1pF, Ri
= 1.5Ω+ G d engineering 4 ms+ gm-90
o+S, and the feedback resistors (5), (6), and (7) are 180Ω, 220Ω, and 350Ω, respectively, and the resistor (3
L (4) is 350Ω and 3Ω. Further, the capacitor (2) was set to 10 pF. Note that the feedback resistance value is calculated based on matching conditions at low frequencies.

In this conventional example, the gain decreases as the frequency increases, and the frequency at which the gain decreases by 3 dB is approximately 3.561 (z,
It decreases by 6dB, which is about 5. Q(Jz). On the other hand, the input/output VSWR is approximately 2 or less up to 5.0 GHz.

The amount that the day tries to do.

It is desirable that the gain characteristics of a wideband amplifier be flat with respect to frequency. However, in the conventional example described above, the gain decreases as the frequency increases.

If the usage limit of an amplifier is the frequency at which the gain decreases by 3 dB (hereinafter, this frequency will be referred to as Fc), then in the conventional example, it is approximately 3 dB.
．． 5GH2 is the limit.

In view of these points, it is an object of the present invention to provide an amplifier in which the usage limits of the amplifier are significantly improved.

In order to achieve the above-mentioned object to solve the problem, the present invention provides a wideband transistor in which one or more field effect transistors are connected in multiple stages, and a negative feedback circuit is connected between the output terminal and the input terminal of the field effect transistor. In the amplifier, 180° apart from the feedback circuit
A feedback circuit formed using a grounded drain field effect transistor is provided between input and output with a phase difference.

With this configuration, the amount of feedback decreases as the frequency increases due to the feedback circuit using the common drain FET. As a result, the overall gain of the amplifier becomes flat with respect to frequency.

Implementation - construction The present invention will be explained in detail below using examples.

An embodiment of the present invention is shown in FIG. (1) to (9) in the figure
is the same as in Figure 4. (20) is an FET that constitutes a feedback circuit from the output terminal (9) to the input terminal (8) of the wideband amplifier, and operates with the drain connected to the ground. The reason why the drain is grounded here is to apply feedback without inversion.
In addition, grounding the drain has a high input impedance and does not affect the feedback voltage extraction point. (21),
(22) and (23) are the gate, source, and drain terminals of FET (20), respectively.

Also, (24) is the source bias resistor of FET (20), (25) is the gate bias resistor, and (30) is the same circuit as the conventional example in FIG. 4, which is famous for the amplification section of the wideband amplifier.

FET equivalent circuit parameters (Rg, gm, etc.) and (2
The values of the circuit bar meters in ) to (7) are the same as in the conventional example. Here, F E T (20) is the output terminal (
9) to the input terminal (8). The amount of feedback is determined by the gain of the amplifier (30) (ie, the amplifier circuit of FIG. 4) and the gain of FET (20). Both the gain of the amplifier section (30) and the gain of FET (20) decrease as the frequency increases, so
The amount of feedback due to T(20) decreases as the frequency increases. In order to make the overall gain of the amplifier of FIG. 1 approximately equal to that of FIG. 4, the gain due to F E T (20) must be significantly reduced. Also, in order not to increase the input/output VSWR of the entire amplifier, FET
The input/output impedance of the feedback circuit based on (20) needs to be sufficiently larger than the input/output impedance of the amplifier section (30). Therefore, the structure of FET is (1) and (2)
0), the gate width of FET (20) is 7μ+i, and the resistance values of (24) and (25) are 3Ω. Calculation of the frequency characteristics of the gain and input/output VSWR of a wideband amplifier. The results are shown in Figure 2.

As can be seen from the gain characteristics in FIG. 2, according to the present invention, the gain of the amplifier is approximately flat with respect to frequency, so that the usability limit is significantly improved. In the characteristics of the conventional example shown in Fig. 4,
While the band was 3.5 GHz, it has been improved to 5.5 GHz in the embodiment of the present invention shown in FIG. Further, as can be seen from FIG. 2, the VSWR, which is important for a wideband amplifier, is approximately 2 or less in this embodiment as well. Therefore, according to this embodiment, the input/output VSW of the amplifier
The frequency characteristics of the gain can be significantly improved without sacrificing much R.

The presence of the feedback circuit using the FET (20) makes it possible to improve the frequency characteristics of the gain without significantly sacrificing the VSWR. Therefore, it is important to select the gate width of F E T (20) to control the amount of feedback. Using the embodiment of FIG. 1, the bandwidth: Fc, the in-band maximum value of the input and output VSWR (Vm t, respectively) as a function of the gate width of the FET (20).

Figure 3 shows the calculation of Vmo). In the figure, the amplifier gain (i) decreases monotonically with frequency in the 81 region l, (ii) has a maximum value in the fiI region 2, and the difference between the maximum value and the gain at low frequencies is less than 3 dB. can be,(
in) 81 region 3 has the maximum value as in region 2, but exhibits a characteristic in which the difference from the low frequency gain is 3 dB or more.

Therefore, Fc in the figure is defined as the frequency at which the gain changes by 3 dB from the low frequency. The permissible human output VSWR for a broadband amplifier is about 3 - for boat fishing, so reading the gate width of F ET (20) that satisfies this from FIG. 3, it is about 2 to 15a. FET (20)
By setting the gate width within the above range, it is possible to suppress the input/output VSWR of the amplifier within the permissible range and significantly expand the usable frequency band.

As explained above, according to the present invention, the input and output V of the amplifier
The frequency characteristics of the gain can be significantly improved without significantly deteriorating the SWR.

Note that the present invention is not limited to the embodiment shown in FIG. 1; the feedback circuit based on FET (20) that controls the amount of feedback can be used as long as the phases of the microwave voltages differ by 180°. It is possible to do so. Further, the parallel negative feedback circuit formed by the feedback resistors (5), (6), and (7) in the embodiment of FIG. 1 is not necessary for all FETs, and for example, the feedback resistor (6) can be omitted.

The number of FET stages is not fixed at 3, and the gate width can also be selected arbitrarily.For example, the gate widths of all FETs can be made equal, or the gate widths can be made wider as they go to later stages. It is. Furthermore, semiconductors such as St, GaAs, and InP can be used as materials for the FET, and the circuit configuration can be either hybrid or monolithic.

As explained above, in the present invention, a feedback circuit using a common drain FET is added, so the input/output VSWR of the wideband amplifier can be suppressed within the permissible range, and the usable frequency band can be greatly expanded. becomes possible.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a wideband amplifier embodying the present invention, FIG. 2 is a diagram showing its gain and frequency characteristics of input/output VSWR, and FIG. FIG. 2 is a diagram showing the bandwidth and input/output VSWR of FIG. 4 is a circuit diagram of a conventional example. FIG. 5 is an equivalent circuit III diagram of the FET. Figure 6 shows the gain and input/output V of the conventional example.
It is a figure showing the frequency characteristic of SWR. (1)-・FET, (2)-・DC blocking capacitor. (3) - drain bias resistance. (4)・-Gate bias resistance. (5) (6) (7) - Feedback resistor, (8) - Input terminal. (9) - Output terminal, (20) - Drain grounded F
E.T. (24)---Source bias resistance. (25) --- Gate bias resistance. (30)...-Amplification section.

Claims (1)

[Claims] (1) Connecting one or more field effect transistors in multiple stages,
In the wideband amplifier in which a negative feedback circuit is connected between the output terminal and the input terminal of the field effect transistor, the feedback circuit is formed using a common drain field effect transistor between input and output with a 180° phase difference apart from the feedback circuit. A wideband amplifier characterized by having a circuit.