Structure of feedback network of low noise amplifier

Abstract

The invention relates to the technical field of low-noise amplifiers, in particular to a structure of a feedback network of a low-noise amplifier. Comprising a VHF wave band; the amplifier of the VHF wave band realizes optimal noise matching through lossless matching; when the amplifier of the VHF wave band carries out lossless matching, due to the fact that the amplifier of the VHF wave band has the requirement of exceeding five octaves, matching from the optimal noise impedance to the nominal impedance needs to be achieved in the whole frequency range; when matching is carried out, parameter optimization is mainly carried out according to needs, and the specific optimization matching method is as follows: a feedback network is added on an amplifier of a VHF frequency band, so that a post-stage signal of the amplifier is fed back forwards to be input into an impedance matching network, and a Q value is equivalently reduced. Through a feedback network structure, under the condition that the noise level is not improved, good input and output matching design can be achieved through negative feedback.

Description

Technical Field

[0001] The present invention relates to the technical field of low noise amplifiers, in particular to the structure of a feedback network of a low noise amplifier. Background Art

[0002] VHF band microwave low noise amplifiers have an important range of applications. In particular, the 50-350MHz band includes a large number of important commercial applications such as shortwave communications, FM broadcasting, aviation communications, digital communications, satellite communications, etc., including military applications such as passive radar and scientific applications such as radio astronomy. In the past, predecessors mainly targeted communication applications in this band, and generally optimized narrower bands. And less consideration is given to low-noise design; some general low-noise amplifier chips cover this band (Minicircuits, Qorvo), but the 50MHz low-frequency end, which has higher value in shortwave communications and radio astronomy, often has larger standing waves;

[0003] At present, in response to the actual needs of the VHF band, all participating countries have carried out some low-noise amplifier design work. These designs are often based on chip-level design. Chip-level design has a high degree of design freedom and can achieve simultaneous matching of impedance and noise by controlling semiconductor materials, geometric dimensions of junction areas, etc. However, it is completely unrealistic to carry out chip-level design for small-scale application goals. This work is expensive and has a long cycle. Small-scale special applications cannot support such a model. If the design can use commercial devices, there will be advantages such as stable device supply and low prices;

[0004] However, since a large number of commercial applications are currently focused on mobile communications, commercial devices are often optimized for higher frequency bands (above 1GHz); therefore, directly applying commercial devices in the VHF band often leads to amplifier instability, excessive noise after impedance matching, and narrow matching bandwidth. A design misunderstanding of traditional low-noise amplifiers is that chip characteristics completely determine the final indicators of the amplifier. In fact, the amplifier circuit topology has a great impact on the final indicators of the amplifier, and the performance can be effectively improved by reasonably optimizing the circuit structure. In most cases, chip-level design is not required. Summary of the invention

[0005] In view of the above-mentioned deficiencies of the prior art, the present invention proposes a feedback network structure of a low noise amplifier to solve the problems that direct application of commercial devices in the VHF band often leads to amplifier instability, excessive noise after impedance matching, and narrow matching bandwidth. The present invention uses a feedback network structure to achieve a good input-output matching design by using negative feedback without increasing the noise level; the details are as follows:

[0006] The structure of the feedback network of a low noise amplifier includes an amplifier in the VHF band; the amplifier in the VHF band achieves optimal noise matching through lossless matching;

[0007] When the VHF band amplifier is losslessly matched, since the VHF band amplifier has a requirement of more than 5 octaves, it is necessary to achieve matching from the optimal noise impedance to the nominal impedance in the entire frequency range;

[0008] When matching, the parameters are optimized mainly as needed, and the specific optimization matching method is:

[0009] Step 1: Add a feedback network to the VHF band amplifier;

[0010] Step 2: After adding a feedback network to the VHF band amplifier, the amplifier post-stage signal is fed back to the input impedance matching network;

[0011] Step 3: When the amplifier's post-stage signal is fed back to the input impedance matching network, the Q value is equivalently reduced.

[0012] Preferably, after the VHF band amplifier is introduced into the feedback network, the impedance at the outlet end can be partially reflected to the preceding stage, so that the impedance is further reduced without sacrificing the noise characteristics of the circuit.

[0013] Preferably, due to the requirement of DC bias operating point, the feedback network must meet DC isolation conditions;

[0014] Among them, the input capacitor Cin plays a role in isolating DC and ensuring that the bias operating point is not affected.

[0015] Preferably, Cin itself can adjust parameters to achieve a certain degree of feedback.

[0016] Preferably, according to the frequency response relationship, the feedback network includes a variety of basic structures:

[0017] Case 1 is that only Cin is introduced in the feedback network;

[0018] Case 2 is that the introduced feedback network includes capacitor Cin and any series impedance network Zs;

[0019] Case 3 and Case 4 are two schemes of introducing a parallel microwave network (Zp) in the introduced feedback network.

[0020] Preferably, the two solutions of Case 3 and Case 4 are essentially completely equivalent; in actual work, one of the designs can be selected based on the ease of use and manufacturability of component parameters.

[0021] Preferably, the impedance matching scheme of the input stage is very free and can be configured using any ladder network.

[0022] The beneficial effects of the present invention are as follows:

[0023] 1. The structure of the feedback network of the low noise amplifier described in the present invention uses a negative feedback method in the low noise amplifier. Under the condition of not increasing the noise level, the negative feedback can achieve a good input-output matching design. The use of negative feedback in the RF low noise amplifier is an innovation for the previous design concept;

[0024] 2. The structure of the feedback network of the low-noise amplifier described in the present invention, through the feedback network circuit of the low-noise circuit and the method of determining its topology and parameters, using these methods, we have successfully achieved a breakthrough in impedance matching and gain flatness over a 5-octave frequency range while ensuring a low noise level. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be further described below in conjunction with the accompanying drawings.

[0026] Figure 1 It is the location diagram of the impedance matching pre-stage network and feedback network;

[0027] Figure 2 These are the four basic structures of the RF low noise amplifier feedback network;

[0028] Figure 3 This is the diagram of the main instruments for measuring S parameters;

[0029] Figure 4 This is the test diagram of S11 and S21 of the designed LNA;

[0030] Figure 5 It is a test chart of the noise measurement characteristics of the designed LNA;

[0031] Figure 6 This is a diagram of the measured parameters of the SKA low-noise amplifier in the prior art. DETAILED DESCRIPTION

[0032] The technical solution of the present application is described in detail below through the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present application and the specific features in the embodiments are detailed descriptions of the technical solution of the present application, rather than limitations on the technical solution of the present application. In the absence of conflict, the embodiments of the present application and the technical features in the embodiments can be combined with each other.

[0033] The key technology for realizing the present invention is to use a negative feedback method in a low noise amplifier. Under the condition of not increasing the noise level, a good input-output matching design can be realized by using negative feedback.

[0034] Embodiment 1

[0035] like Figure 1 As shown, the structure of the feedback network of the low noise amplifier of the present invention includes a VHF band amplifier; the VHF band amplifier achieves optimal noise matching through lossless matching;

[0036] When the VHF band amplifier is losslessly matched, since the VHF band amplifier has a requirement of more than 5 octaves, it is necessary to achieve matching from the optimal noise impedance to the nominal impedance in the entire frequency range;

[0037] When matching, the parameters are optimized mainly as needed, and the specific optimization matching method is:

[0038] Step 1: Add a feedback network to the VHF band amplifier;

[0039] Step 2: After adding a feedback network to the VHF band amplifier, the amplifier post-stage signal is fed back to the input impedance matching network;

[0040] Step 3: When the amplifier's post-stage signal is fed back to the input impedance matching network, the Q value is equivalently reduced;

[0041] It should be noted that, as seen in the noise model, the impedance at which the amplifier obtains the best noise is not necessarily the required nominal impedance; for one frequency point, the best noise match can be achieved through lossless matching; however, the amplifier in the VHF band has a requirement of more than 5 octaves, so it is necessary to achieve matching from the best noise impedance to the nominal impedance over the entire frequency range; however, due to the limitations of the Fano-Bode condition, it is impossible to achieve complete matching for one frequency band, so the parameters can only be optimized as needed;

[0042] At present, most commercial pHEMT devices have high impedance and capacitive input impedance at 50-250MHz. After using large-scale Monte Carlo simulation, the required impedance matching cannot be directly achieved. Either the final bandwidth of the amplifier is too narrow, or the noise factor cannot meet the requirements, or the system cannot be stable. For example, in the existing related literature design, low noise is finally achieved at the expense of standing wave ratio. Such a design cannot meet the strict requirements of occasions.

[0043] In order to solve the matching problem, the common practice in the VHF band is to use inductive elements at the input stage to achieve low impedance to high impedance matching; Figure 1 As shown, the Q value of this matching structure is completely determined by the input impedance (50 ohms) and the amplifier components ( Figure 1 The bandwidth characteristics of the final amplifier are completely determined by the amplifier components;

[0044] To this end, the present invention uses a VHF band amplifier for lossless matching to achieve optimal noise matching; parameter optimization is performed as needed, and the proposed optimization method is: adding a feedback network to the VHF band amplifier so that the amplifier rear stage signal is fed back to the input impedance matching network, which equivalently reduces the Q value;

[0045] in, Figure 1 L1 and L2 in the figure are common matching structures, and inductors L1 and L2 play the role of impedance matching; wherein, the circuit structure access method cited in the figure is marked in the dotted box; the input port is input, the output port is output, PHEMT is the transistor component of the amplifier, and FEEDBACK is the feedback network; the irrelevant DC bias part in the figure is the prior art and is not shown.

[0046] Embodiment 2

[0047] After the feedback network is introduced into the amplifier of the VHF band, the impedance at the outlet end can be partially reflected to the preceding stage, so that the impedance is further reduced without sacrificing the noise characteristics of the circuit;

[0048] Due to the requirement of DC bias operating point, the feedback network must meet DC isolation conditions;

[0049] Among them, the input capacitor Cin plays a role in isolating DC, ensuring that the bias operating point is not affected;

[0050] Cin itself can adjust parameters to achieve a certain degree of feedback;

[0051] like Figure 2 As shown in the figure, according to the frequency response relationship, there are four basic feedback networks:

[0052] Case 1 is that only Cin is introduced in the feedback network;

[0053] Case 2 is that the introduced feedback network includes capacitor Cin and any series impedance network Zs;

[0054] Case 3 and Case 4 are two schemes of introducing parallel microwave network (Zp) in the introduced feedback network;

[0055] The two solutions in Case 3 and Case 4 are essentially completely equivalent; in actual work, one of the designs can be selected based on the ease of use and manufacturability of component parameters;

[0056] Due to the requirement of DC bias operating point, the feedback network must meet the DC isolation condition; therefore, the input capacitor Cin plays a role in isolating DC and ensuring that the bias operating point is not affected; Cin itself can adjust parameters to achieve a certain degree of feedback. According to the frequency response relationship, case 1 only has Cin, so it has a greater impact on high-frequency signal feedback; case 2 includes capacitor Cin and any series impedance network Zs; when Cin is large enough, Zs determines the feedback performance of the system, so the structure of Zs can be selected through methods such as Q matching to achieve feedback matching requirements; cases 3 and 4 are two solutions for introducing parallel microwave networks (Zp); the two solutions are essentially completely equivalent; in actual work, one of the designs can be selected based on the ease of use and producibility of component parameters;

[0057] The impedance matching scheme of the input stage is very free and can be configured using any ladder network; the present invention includes four basic structures, and other extensions to the structure also fall within the scope of feedback network application.

[0058] In order to verify the technical effect of the present invention, the present invention designs a complete set of VHF band low noise amplifiers and conducts characteristic tests;

[0059] 1. Measurement of S parameters;

[0060] The main instruments used for measurement are (such as Figure 3 (shown): Keysight 5071B network analyzer, E3534A DC power supply, and Fieldfox 9916 microwave analyzer;

[0061] During the circuit debugging stage, the present invention uses a Keysight 5071B network analyzer to measure the circuit S parameters, an E3534A DC power supply is used for DC power supply, and a Fieldfox 9916 microwave analyzer is used in conjunction with a 246B noise source for noise coefficient measurement;

[0062] The present invention uses a SOLT (short, open, direct connection) S parameter calibration scheme, using a 75-ohm mechanical calibration piece; because the present invention adopts a 75-ohm design, an N-port 75-50 resistive gasket is also used when measuring S parameters; the network analyzer is calibrated to a 75-ohm nominal impedance with a measurement port;

[0063] like Figure 4 As shown, Figure 4 The S11 and S21 of the designed LNA are shown. S22 is not shown because the measured value of S22 is lower than -30dB, which far exceeds the requirements of the scenario application.

[0064] For comparison with other designs, the S11 corresponding to VSWR1.5 is -14db; it can be seen that within the frequency band required by the present invention, the gain change is less than 1db, and the VSWR is better than 1.8; for more than 90% of the bandwidth, the VSWR is better than 1.5, and the gain change is less than 0.5dB.

[0065] 2. Measurement of noise parameters

[0066] The present invention uses fieldfox 9916 microwave analyzer to complete the measurement in an electromagnetic shielding room. The measurement results are as follows: Figure 5 As shown, Figure 5 To complete the noise measurement characteristics of LNA;

[0067] according to Figure 5 It can be seen that in the entire frequency band, the noise is better than 55K (noise factor 0.8), and in more than 90% of the frequency band, the noise temperature is better than 40K (noise factor 0.6).

[0068] In order to further verify the technical effect of the present invention, the low noise amplifier designed by the present invention is compared with the amplifier designed at chip level in the prior art in the same frequency band; Figure 6 As shown, Figure 6 The measured parameters of the SKA low noise amplifier in the prior art; wherein the upper figure is the noise coefficient, and the lower figure is S11, S21, S22;

[0069] according to Figure 6 It can be seen that even with the chip-level design, the SKA low noise amplifier in the prior art is still higher than the design of the present invention in terms of noise coefficient; especially in the low frequency band of 100MHz, the SKA low noise amplifier in the prior art is nearly 10 times higher than the amplifier in the present invention; at the same time, the amplifier of the present invention achieves a standing wave ratio better than 1.5 at 50-250Mhz, which is far better than the 1.9 (s11=-10dB) level of the SKA low noise amplifier in the prior art; this fully demonstrates the excellent performance of the existing design scheme in the circuit topology structure.

[0070] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. The structure of the feedback network of the low noise amplifier is characterized by: The invention comprises an amplifier in the VHF band; the amplifier in the VHF band realizes optimal noise matching through lossless matching; When the VHF band amplifier is losslessly matched, since the VHF band amplifier has a requirement of more than 5 octaves, it is necessary to achieve matching from the optimal noise impedance to the nominal impedance in the entire frequency range; When matching, the parameters are optimized mainly as needed, and the specific optimization matching method is: Step 1: Add a feedback network to the VHF band amplifier; Step 2: After adding a feedback network to the VHF band amplifier, the amplifier post-stage signal is fed back to the input impedance matching network; Step 3: When the amplifier's post-stage signal is fed back to the input impedance matching network, the Q value is equivalently reduced.

2. The structure of the feedback network of the low noise amplifier according to claim 1, characterized in that: After the feedback network is introduced into the amplifier of the VHF band, the impedance at the outlet end can be partially reflected to the preceding stage, so that the impedance is further reduced without sacrificing the noise characteristics of the circuit.

3. The structure of the feedback network of the low noise amplifier according to claim 1, characterized in that: Due to the requirement of DC bias operating point, the feedback network must meet DC isolation conditions; Among them, the input capacitor Cin plays a role in isolating DC and ensuring that the bias operating point is not affected.

4. The structure of the feedback network of the low noise amplifier according to claim 1, characterized in that: Cin itself can adjust parameters to achieve a certain degree of feedback.

5. The structure of the feedback network of the low noise amplifier according to claim 4, characterized in that: According to the frequency response relationship, there are many basic feedback networks: Case 1 is that only Cin is introduced in the feedback network; Case 2 is that the introduced feedback network includes capacitor Cin and any series impedance network Zs; Case 3 and Case 4 are two schemes of introducing a parallel microwave network (Zp) in the introduced feedback network.

6. The structure of the feedback network of the low noise amplifier according to claim 5, characterized in that: The two solutions in Case 3 and Case 4 are essentially completely equivalent; in actual work, one of the designs can be selected based on the ease of use and manufacturability of component parameters.

7. The structure of the feedback network of the low noise amplifier according to claim 6, characterized in that: The impedance matching scheme of the input stage is very free and can be configured with any ladder network.

Images