JPH05136639A - Multiple stage amplifier circuit for high frequency - Google Patents

Abstract

PURPOSE:To provide a multiple stage amplifier circuit for high frequency not generating the spurious oscillation by connecting plural transistor amplifier parts, forming an amplifier stage and providing a band pass filter between amplifier stages. CONSTITUTION:Plural transistor amplifier parts 1 are connected so that respective parts can form the continuous amplifier state. A band pass filter 2 including a strip line with the frequency which is provided between the amplifier parts 1 and made into an object as a center frequency is provided. The filter 2 is arranged by using the half-wavelength side surface coupling filter of the strip line, sandwiching an element B of the half length of a wavelength on the substrate of the frequency which is the object and making approximate elements A and C in parallel by lambda/4 only. Plural elements B of the lambda/2 length are dislocated by lambda/4 and arranged in a multiple stage. Thus, with easy circuit constitution, a multiple stage amplifier circuit not generating the spurious oscillation can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high frequency amplifier circuit for amplifying a microwave band signal by using a transistor,
In particular, the present invention relates to a high-frequency multistage amplifier circuit in which a plurality of transistor amplifier circuits are connected in two or more stages in order to obtain a high gain.

[0002]

2. Description of the Related Art In a high frequency amplifier circuit using a transistor for amplifying a signal in the so-called microwave band of 1 GHz or more, it is difficult to obtain a sufficient gain even if a transistor for the microwave band is used. As shown in FIG. 2, the transistor amplifier circuits are connected in two or more stages to obtain a desired gain. In FIG. 11, the first transistor Tr ₁ and the second transistor Tr ₂ form the first-stage and second-stage amplifier circuits, respectively. Each amplifier circuit is formed of a capacitor and an inductance element in addition to the transistor. C ₁ , C ₂ , C ₃ and C
₄ plays a role of a filter for removing the DC component.

When performing amplification over a plurality of stages as shown in FIG. 11, it is necessary to provide an impedance matching circuit 3 for matching the impedance between the stages. This is the output impedance of the first transistor Tr ₁ and the second transistor at the signal frequency of interest.
It matches the input impedance of Tr ₂ , and if the impedances do not match, loss increases and sufficient gain cannot be obtained.

The transistor used for amplifying a signal in the microwave band is made so as to obtain a high gain even at such a high frequency. However, as shown in FIG. 12, the gain obtained as the frequency increases. Decreases. The maximum gain that can be obtained is called the maximum effective gain, and generally decreases at a rate of -6 dB / oct above a certain frequency. The signal frequency f _r to be the order object when selected in a region where the gain is decreasing, towards a lower frequency than the signal frequency f _r the gain is increased.

In the circuit of FIG. 11, since the impedance matching is performed with respect to the signal frequency f _r , the gain becomes small for other frequencies. However, in most bipolar transistors, both output impedance and input impedance are not much different from 50Ω. Gain without much decrease even for frequencies lower than Therefore signal frequency f _r, the gain characteristic as shown in FIG. 13 is obtained as a whole circuit.

[0006]

Since the circuit of FIG. 11 has a high gain in a low frequency band as shown in FIG. 13,
Or apply feedback for such stabilization of the output level, the unwanted feedback is generated from the relationship of the wiring pattern, it is liable to cause a oscillating at a frequency lower than the signal frequency f _r.

In order to prevent such oscillation, measures are taken such as sufficiently removing the AC component when applying feedback and sufficiently separating the input and output positions in the pattern layout. However, the measures taken in the pattern layout mean that the degree of freedom in design becomes smaller accordingly. Further, even if measures such as providing a bypass capacitor are taken, it is difficult to completely prevent unnecessary feedback, and the cost increase for the measures is also a problem.

The present invention has been made in view of the above problems, and an object of the present invention is to realize a high-frequency multistage amplifier circuit that does not cause unnecessary oscillation with a simple circuit configuration.

[0009]

In the high-frequency multistage amplifier circuit of the present invention, in order to solve the above-mentioned problems, a stripline filter having a half wavelength of a target signal is a bandpass filter whose center is the signal frequency. Utilizing the characteristics, the band pass filter is provided between each amplification stage.
FIG. 1 is a diagram showing a basic configuration of a high-frequency multistage amplifier circuit of the present invention. In the drawings, parts having the same function are denoted by the same reference numerals.

That is, the high-frequency multistage amplifying circuit of the present invention is provided with a plurality of transistor amplifying units 1 connected so as to form continuous amplifying stages, and a target signal provided between the transistor amplifying units 1. A high-frequency multistage amplifier circuit including a bandpass filter 2 including a stripline having a frequency as a center frequency.

[0011]

Function: Target signal frequency in multi-stage amplifier circuit
f_rThis low frequency is the reason why oscillation is likely to occur at lower frequencies.
This is because the gain is high at high frequencies. Therefore, each tran
Signal frequency f between the transistor amplifier 1_rIs the center frequency
If a band pass filter that_rAt lower frequencies
The gain of is reduced. Figure 2 shows such a bandpass filter
It is the frequency characteristic of the circuit when 2 is provided. Frequency in Figure 13
F compared to the characteristics _rLow gain at lower frequencies
It becomes difficult to shake.

In the microwave band, the strip line having a half wavelength has a characteristic that low frequencies are sufficiently attenuated, and such a band pass filter can be easily realized.
Further, since the band-pass filter removes the DC component, the capacitor provided between the stages of the DC-cutting capacitors as shown in FIG. 11 becomes unnecessary.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The transistor amplification section 1 and the impedance matching circuit of the circuit of FIG. As described above, in the present invention, the bandpass filter 2 is realized by using the strip line. A half-wave end coupling filter, a half-wavelength side coupling filter, and an interdigital filter are known as the types of strip-line bandpass filters, and the half-wavelength surface coupling type is generally used. FIG. 3 shows an example of the shape of this half-wavelength surface coupling filter. As shown in the figure, A and C are arranged in close proximity to each other with an element B having a half length of the wavelength λ on the substrate of the target frequency sandwiched in parallel with each other by λ / 4 minutes,
A plurality of elements B having a length of λ / 2 may be shifted in a plurality of λ / 4 and arranged in multiple stages.

The stripline bandpass filter including the half-wavelength side-coupling filter shown in FIG.
The degree of coupling between striplines, that is, the degree of coupling from A to B and from B to C is very small, and the effective Q of element B is
Since the value is high, the pass bandwidth is narrow. Therefore, if such a bandpass filter is used, the gain of the circuit is reduced for frequencies other than the signal frequency f _{r, and} there is no fear of oscillation.

However, the bandpass filter as shown in FIG. 3 has a problem that the bandpass is too narrow and the bandpass filter is used in a certain bandwidth. There is also a problem that the area of the pattern must be widened depending on the signal frequency. Therefore, it is advisable to use an example in which a stripline and a capacitor that is a lumped constant element are combined as a bandpass filter.

As shown in FIG. 4, such a bandpass filter is constructed by combining two lumped constant type capacitors having the same capacitance before and after a stripline slightly shorter than a half wavelength of a signal of interest. A method of designing such a bandpass filter will be described below. FIG. 5 (a)
It is known that the input impedance Z _in of the strip line having the length l _r and the impedance Z _o as shown in FIG.

[0017]

[Equation 3] Here, if l _r is a half wavelength λ / 2, Z = ∞, and the input impedance Z _{in in} that case is as follows.

[0018]

[Equation 4] Considering here when l _r changes before and after λ / 2, l
_{When r} approaches λ / 2 from the one smaller than λ / 2,
The change in impedance tends to become ∞ in terms of capacitance. Also, when l _r is larger than λ / 2 and approaches λ / 2, it is in the direction of ∞ in terms of inductive properties. Such a characteristic is the same as the change in the impedance characteristic of the parallel resonant circuit, and the half-wave strip line can be represented as an equivalent circuit of the parallel resonant circuit as shown in FIG. 5B. From the impedance characteristic in the vicinity of the resonance circuit of FIG. 5B, the capacitance C and the inductance L in the figure can be expressed by the following equation with the impedance Z _{O of the} strip line and the signal frequency f ₀ .

[0019]

[Equation 5]

[0020]

[Equation 6] Now, as shown in FIG. 5C, there are lumped constant type capacitors before and after the strip line, and when the capacitance is C _S / 2, an equivalent circuit is represented as shown in FIG. 5D. In the circuit of Fig. 5 (c), the length of the strip line is set to half the wavelength when the frequency is 2GHz, and the analysis result of the frequency characteristic by the calculation when the capacitance of the capacitor is changed is shown. 6 to 9. These figures show the case where the capacitance of capacitors is 0.01pF, 0.2pF, 0.5pF, 1pF in order.

As is clear from the results shown in FIGS. 6 to 9, when the capacitance of the capacitor is very small, the resonance frequency f
_It can be seen that ₀ coincides with the strip line alone, and the resonance frequency f ₀ decreases and the filter width increases as the capacitance of the capacitor increases. Therefore, in designing the bandpass filter, first, the desired frequency f ₀ and the width of the filter are determined. From the width of this filter, the capacitance C _S / 2 of the capacitor is determined with reference to the characteristics shown in FIGS. Next, the length l _r of the strip line is determined.

Assuming that the resonance frequency of the strip line having the length l _r alone is f ₁ , the equivalent capacitance C in that case can be obtained from the following equation (4).

[0023]

[Equation 7] Here, as shown in FIG. 5D, a capacitor having a capacitance C _S / 2 is added before and after the strip line, and it is sufficient that the resonance frequency becomes f ₀ . F ₀ at this time
The relationship between and f ₁ is obtained by the following equation.

[0024]

[Equation 8] Since a quadratic equation of C ₁ is obtained from the equation (7), solving it gives C _{1 as} the following equation.

[0025]

[Equation 9] Therefore, desired characteristics can be obtained by selecting the equivalent capacitance C ₁ of the strip line alone for f ₀ and C _S as shown in equation (8). The relationship between C ₁ and f ₁ is obtained from the equation (6). Since the stripline length l _r is the half wavelength of the signal of f ₁ , it can be obtained from the following equation.

[0026]

[Equation 10] However, in the above, V _c is the speed of light, and S is the shortening rate of the wavelength of the substrate forming the stripline with respect to the wavelength in vacuum. In the circuit shown in FIG. 6, the signal frequency is set to 2
Figure 10 shows the circuit and characteristics of the strip line length obtained as above when the frequency is GHz and the capacitor capacitance is 0.5 pF.

Further, the length l ₀ of the half-wavelength stripline at the desired frequency is obtained, the equivalent capacitance C ₀ in this case is obtained in the same manner as the equation (6), and from this and C ₁ obtained by the equation (8). You may obtain | require the shortening rate like the following formula.

[0028]

[Equation 11] As described above, a desired bandpass filter can be easily obtained by combining the stripline and the capacitor.

[0029]

According to the present invention, in a high-frequency multistage amplifier circuit in which transistor amplifier circuits are connected in multiple stages, oscillation is performed at a low frequency even if the signal frequency is selected as a high frequency at which the transistor gain is lower than the maximum gain. It is possible to realize an amplifier circuit without.

[Brief description of drawings]

FIG. 1 is a diagram showing a basic configuration of a high-frequency multistage amplifier circuit of the present invention.

FIG. 2 is a diagram showing a basic form of frequency characteristics of a high-frequency multistage amplifier circuit of the present invention.

FIG. 3 shows an example in which a bandpass filter provided between amplifier stages is realized by a stripline.

FIG. 4 is a diagram showing an example in which a bandpass filter is realized by combining a stripline and a lumped-constant capacitor connected before and after.

FIG. 5 is a diagram illustrating a resonance circuit and an equivalent circuit using a strip line.

FIG. 6 is a diagram showing a circuit and characteristics of a filter in which a stripline and a capacitor are combined.

7 is a diagram showing characteristics when the capacitance of the capacitor is set to 0.2 pF in the circuit of FIG.

FIG. 8 is a diagram showing characteristics when the capacitance of the capacitor is set to 0.5 pF in the circuit of FIG.

9 is a diagram showing the characteristics when the capacitance of the capacitor is set to 1.0 pF in the circuit of FIG.

10 is a diagram showing a circuit and characteristics when the signal frequency is 2 GHz and the capacitor capacitance is 0.5 pF in the circuit of FIG. 6.

FIG. 11 is a diagram showing an example of a high-frequency multistage amplifier circuit using conventional transistors.

FIG. 12 is a diagram showing an example of frequency characteristics of a high frequency transistor.

13 is a diagram showing frequency characteristics of the circuit of FIG. 5 when f _r is selected as shown in FIG. 6.

[Explanation of symbols]

 1 ... Transistor amplifier 2 ... Bandpass filter 3 ... Impedance matching circuit

Claims (3)

[Claims] 1. A plurality of transistor amplifiers (1) connected to each other so as to form a continuous amplifier stage, and a frequency of a target signal provided between the transistor amplifiers 1 as a center frequency. High-frequency multi-stage amplifier circuit including a bandpass filter (2) including a stripline. 2. The band-pass filter (2) is composed of a strip line and a lumped-constant type capacitance element provided before and after the strip line, and a target signal frequency is f _O , When the impedance of the line is Z _O , the capacitance of the capacitive element is C _S / 2, the speed of light is V _C, and the wavelength shortening rate with respect to the wavelength in vacuum due to the substrate material on which the strip line is formed is S,
The length l _r of the stripline is The multistage amplifier circuit for high frequencies according to claim 1, wherein the length is slightly shorter than the half wavelength represented by. 3. A bandpass filter for a signal of frequency f ₀ , comprising a stripline and lumped-constant type capacitance elements provided before and after the stripline, wherein the impedance of the stripline is Z _O Where the capacitance of the capacitive element is C _S / 2, the speed of light is V _C, and the wavelength shortening rate for the wavelength in vacuum due to the substrate material on which the strip line is formed is S, the length of the strip line l _r is given by A bandpass filter having a length slightly shorter than the half wavelength represented by.