A broadband power amplifier device for FM transmitters

Abstract

This utility model relates to the field of radio frequency power amplifier technology. It provides a broadband power amplifier device for an FM transmitter, comprising an exciter, a driver stage power amplifier, a final stage power amplifier, a filter, an attenuator, and a power meter connected in sequence. An input matching network is provided between the driver stage power amplifier and the final stage power amplifier, and an output matching network is provided at the output terminal of the final stage power amplifier. Both the input matching network and the output matching network adopt a microstrip line structure based on a printed circuit board. This solves the problem that existing coaxial cable matching networks generate severe heat under high power, limiting the output power and stability of the power amplifier module.

Description

Technical Field

[0001] This utility model relates to the field of radio frequency power amplifier technology, and more specifically, to a broadband power amplifier device for an FM transmitter. Background Technology

[0002] All-solid-state FM transmitters, with their excellent anti-interference capabilities, signal stability, and wide bandwidth characteristics, have become core equipment in modern broadcast transmission systems. Among them, the output capability and stability of the high-power broadband power amplifier module are key factors determining the overall performance of the transmitter.

[0003] Currently, mainstream FM transmitter power amplifier modules generally employ coaxial cable matching for their input and output matching networks in pursuit of high efficiency and high power output. However, this matching method has significant drawbacks under high-power operating conditions: the transmission of radio frequency energy in the cable generates a large amount of heat, causing the output matching cable temperature to rise sharply. Excessively high cable temperatures not only become a bottleneck limiting the continuous output power of the power amplifier module but also severely affect the operational stability and long-term reliability of the power amplifier transistors, becoming a key issue restricting the stable high-power output of the power amplifier module across the entire frequency band. Utility Model Content

[0004] The purpose of this invention is to provide a broadband power amplifier device for FM transmitters, which aims to solve the problem that existing coaxial cable matching networks generate severe heat at high power, limiting the output power and stability of the power amplifier module.

[0005] This utility model is achieved through the following technical solution:

[0006] A broadband power amplifier device for an FM transmitter includes an exciter, a driver stage power amplifier, a final stage power amplifier, a filter, an attenuator, and a power meter connected in sequence; an input matching network is provided between the driver stage power amplifier and the final stage power amplifier, and an output matching network is provided at the output end of the final stage power amplifier; both the input matching network and the output matching network adopt a microstrip line structure based on a printed circuit board.

[0007] Optionally, the input matching network and the output matching network adopt a two-layer microstrip structure, including top layer traces and bottom layer traces of the printed circuit board.

[0008] Optionally, the input matching network splits the 25Ω impedance signal into two 12.5Ω impedance signals via a bottom microstrip line and inputs them to the final stage power amplifier tube.

[0009] Optionally, the output matching network synthesizes a 25Ω impedance signal through two 12.5Ω microstrip lines and outputs it through the bottom microstrip line.

[0010] Optionally, a groove is formed on the substrate of the final stage power amplifier module in the area corresponding to the bottom microstrip line.

[0011] Optionally, the groove is filled with insulating and thermally conductive silicone.

[0012] Optionally, the system also includes a thermal imager connected to the final stage power amplifier; wherein the thermal imager is used to monitor the operating temperature of the final stage power amplifier in real time.

[0013] Optionally, the RF signal input terminal of the final stage power amplifier is equipped with a gate voltage filter circuit and a power supply resistor.

[0014] Optionally, the input end of the input matching network is provided with a first capacitor matching unit; wherein, the first capacitor matching unit is used to optimize the impedance matching of the radio frequency signal input end.

[0015] Optionally, the input terminal of the output matching network is provided with a second capacitor matching unit; wherein, the second capacitor matching unit is used to optimize the impedance matching of the RF signal output terminal.

[0016] The technical solution of this utility model has at least the following advantages and beneficial effects:

[0017] By replacing the traditional coaxial cable matching network with a microstrip line structure based on a printed circuit board (PCB), the large amount of heat generated during the transmission of radio frequency energy through the coaxial cable is eliminated. The microstrip line has a larger surface area and a better heat dissipation path (conducted through the PCB to the heat sink), which can effectively reduce the temperature of the matching network during operation. This solves the problem of output power being limited by cable overheating at high power, enabling the power amplifier module to stably output higher power across the entire frequency band.

[0018] After solving the key bottleneck of matching network heat generation, the power amplifier module is no longer limited by this, and its design potential for broadband, high efficiency and high power output can be fully realized; it can achieve more stable and sustained high power output across the entire frequency band, improving the overall performance of the transmitter.

[0019] The microstrip line structure is directly integrated on the PCB, eliminating the need for bulky coaxial cables and their connectors (such as N-type connectors) that require precision installation. This not only simplifies the structural design of the power amplifier module and reduces potential connection failure points (such as poor soldering or poor contact), but also improves the mechanical stability and environmental adaptability of the entire matching network, further enhancing the overall reliability of the system.

[0020] PCB microstrip line structures facilitate standardized design and mass production, offering good consistency and relatively simple assembly. Compared to traditional methods requiring manual soldering and precision installation of coaxial cables, this reduces production costs and assembly complexity. During maintenance, the integrated PCB structure also makes inspection and replacement easier. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the overall structure of the FM transmitter broadband power amplifier device according to an embodiment of the present invention;

[0022] Figure 2 A schematic diagram of the substrate provided in the broadband power amplifier device of the FM transmitter according to an embodiment of the present utility model;

[0023] Figure 3 This is a schematic diagram of the printed circuit board structure of the broadband power amplifier device for the FM transmitter according to an embodiment of the present invention;

[0024] Figure 4 This is a schematic diagram of the circuit structure of the broadband power amplifier device for the frequency modulation transmitter according to an embodiment of the present invention;

[0025] Icons: 1-substrate, 101-groove, 2-printed circuit board, 201-power amplifier tube soldering area, 3-microstrip line. Detailed Implementation

[0026] The following is a detailed description of the embodiments, in conjunction with the accompanying drawings.

[0027] Reference Figure 1 A broadband power amplifier device for an FM transmitter includes an exciter, a driver-stage power amplifier, a final-stage power amplifier, a filter, an attenuator, and a power meter connected in sequence. An input matching network is provided between the driver-stage power amplifier and the final-stage power amplifier, and an output matching network is provided at the output of the final-stage power amplifier. Both the input and output matching networks adopt a microstrip line structure based on a printed circuit board. A signal source (such as audio + pilot) enters the exciter to generate a radio frequency (RF) signal conforming to the FM broadcast standard. The RF signal output by the exciter is sent to the driver-stage power amplifier for preliminary amplification. The signal output by the driver-stage power amplifier passes through the input matching network (microstrip line structure) and is then sent to the final-stage power amplifier for final power amplification. The high-power RF signal output by the final-stage power amplifier undergoes impedance transformation and filtering through the output matching network (microstrip line structure). The signal from the output matching network enters the filter (which can be a bandpass filter) to further filter out out-of-band spurious signals and harmonics, ensuring compliance with the spectrum template requirements. The filtered signal passes through the attenuator (used for power fine-tuning or protecting subsequent instruments) and is finally monitored by the power meter for output power.

[0028] The input matching network is designed on a dedicated PCB board or integrated on the PCB transition board between the driver stage amplifier and the final stage amplifier. A PCB substrate with excellent high-frequency performance, stable dielectric constant, and low loss tangent (such as Rogers RO4000 series, Taconic RF series, or high-performance FR4) can be selected. Based on the operating frequency band (e.g., 88-108MHz) and required impedance transformation, the width (affecting characteristic impedance), length (affecting phase and matching point), and possible shapes (e.g., straight line, gradient line, T-type, π-type, etc.) of the microstrip line are precisely calculated. Discrete components such as surface-mount capacitors and inductors (or microstrip line inductors) can be integrated on the PCB to form the matching network together with the microstrip line. These components are connected through metallized vias and traces on the PCB; using SMA, N-type, or other RF connectors, or directly through PCB pads / press connections, they are connected to the output of the driver stage amplifier and the input of the final stage amplifier.

[0029] Output matching networks can be designed on dedicated, robust, and heat-dissipating PCBs, directly connecting to the output ports (e.g., drain / collector) of the final stage power amplifier transistors. They typically require larger dimensions to handle high power and facilitate heat dissipation. Similar to input matching, they can utilize high thermal conductivity substrates (e.g., metal substrates-IMS, such as aluminum substrates; or composite substrates filled with high thermal conductivity ceramic particles) to efficiently conduct heat generated by the power amplifier transistors and lost within the matching network itself. The microstrip conductors (copper layers) of the output matching network need to be sufficiently wide and thick (typically starting at 2 oz copper thickness, or even 3 oz or more) to reduce current density, conductor losses, and heat generation; substrates with lower loss factors can be selected, and the microstrip line shape can be optimized to reduce discontinuities.

[0030] In some embodiments, the input and output matching networks employ a two-layer microstrip line structure, including top and bottom traces on the printed circuit board. The input and output matching networks are designed on a double-sided copper-clad printed circuit board (PCB). The top trace, as the primary signal transmission path, forms the conductor portion of the microstrip line, and its width, length, and shape (straight line, gradient line, T-branch, π-network, etc.) are designed based on impedance matching calculations. The bottom trace, as a complete or partial ground plane, is fundamental for the normal operation of the microstrip line structure (providing a reference ground and controlling impedance). The electrical connection between the top and bottom layers is achieved through metallized vias. For example:

[0031] For discrete components (such as surface mount capacitors and inductors), one pad is connected to the microstrip line on the top layer, and the other pad is connected to the bottom ground plane (bypass capacitor) through a via; branches of T-type or π-type matching networks may need to switch paths between the top and bottom layers; the source / emitter ground pins of the power amplifier tubes are directly connected to the bottom ground plane through multiple vias to minimize ground inductance.

[0032] Surface mount capacitors, inductors (or inductors formed by short microstrip lines), resistors, etc., can be flexibly arranged on the top and bottom layers and interconnected through vias and microstrip lines to form a matching network.

[0033] For high-current paths (especially at the input of the output matching network), microstrip segments can be designed in parallel on the top and bottom layers and connected by a dense array of vias. This is equivalent to increasing the cross-sectional area of ​​the conductor, significantly reducing current density and conductor losses.

[0034] In some embodiments, the input matching network splits the 25Ω impedance signal into two 12.5Ω impedance signals via a bottom-layer microstrip line and inputs them to the final stage power amplifier transistors. This precise splitting of the 25Ω signal into two 12.5Ω signals via the bottom-layer microstrip line achieves equal power distribution, allowing the final stage power amplifier transistors (typically two parallel transistors) to operate simultaneously at high efficiency. This avoids the power imbalance problem associated with traditional coaxial cable branching, thereby improving overall output power. As part of the PCB structure, the bottom-layer microstrip line typically has a conductor (copper layer) thickness of 2 oz or more, effectively reducing current density and resistance loss. Compared to coaxial cables, microstrip lines generate less heat during high-power RF transmission, avoiding the "thermal bottleneck" problem caused by a rapid increase in cable temperature.

[0035] In some embodiments, the output matching network synthesizes a 25Ω impedance signal through two 12.5Ω microstrip lines and outputs it via the bottom microstrip line. The precise synthesis of a 25Ω impedance signal from the two 12.5Ω microstrip lines achieves impedance matching between the final stage power amplifier output and subsequent circuits (such as filters), significantly reducing reflection losses during RF signal transmission. This ensures efficient transfer of high-power energy from the final stage power amplifier tubes to subsequent links, improving overall power transmission efficiency and avoiding power waste due to impedance mismatch. The microstrip lines of the output matching network (especially the bottom microstrip lines) use copper conductors with a thickness of 2oz or more, and the current path is optimized through the PCB structure. The thicker copper conductor effectively increases the conductive cross-sectional area, reducing current density and resistance loss; simultaneously, referring to... Figure 3 The microstrip line 3 is integrated on the printed circuit board 2, which is mounted on a substrate (such as a high thermal conductivity metal substrate or a low-loss high-frequency substrate). The heat generated during the synthesis process can be efficiently conducted and diffused through the PCB. Figure 3 The power amplifier tube welding area 201 is used for welding and installing power amplifier tubes. Compared with the problem of "rapid temperature rise" caused by high cable loss and poor heat dissipation when traditional coaxial cables are combined, this design significantly reduces the heat generation in high-power RF transmission, breaks through the "thermal bottleneck" of traditional structures, and supports the continuous and stable output of high power by the power amplifier module.

[0036] In some embodiments, refer to Figure 2A groove 101 is formed on the substrate 1 of the final stage power amplifier module in the area corresponding to the bottom microstrip line. This increases the heat dissipation surface area of ​​the substrate 1 in this area, improves the convective heat transfer efficiency with air, or promotes the diffusion of heat to other areas of the substrate through the heat dissipation channel formed by the groove 101, reducing the local accumulation of heat directly below the microstrip line. If the substrate 1 is a high thermal conductivity metal substrate (such as an aluminum substrate) or a composite heat dissipation substrate, the groove 101 can reduce the thermal resistance between the microstrip line and the external heat dissipation structure (such as heat sink fins or cooling modules), allowing the heat generated by the bottom microstrip line to be conducted to the external heat dissipation system more quickly through the substrate 1, avoiding local overheating.

[0037] In some embodiments, the groove 101 is filled with insulating thermally conductive silicone. Insulating thermally conductive silicone has excellent thermal conductivity, with a thermal conductivity coefficient much higher than that of air. After filling, it can effectively eliminate air gaps in the groove 101 (air is a poor thermal conductor), allowing the heat generated by the underlying microstrip line (conductor copper layer) to be quickly conducted to the substrate 1 (such as a metal substrate or composite heat dissipation substrate) through the silicone, and then diffused from the substrate to the external heat dissipation system (such as heat sink fins, cooling modules), reducing the local temperature of the microstrip line and matching network, and avoiding performance degradation or reliability problems caused by heat accumulation under high power.

[0038] In some embodiments, a thermal imager is also included, connected to the final stage power amplifier; wherein the thermal imager is used to monitor the operating temperature of the final stage power amplifier in real time. As the core component of high-power RF signal amplification, the final stage power amplifier's power transistors and matching network are prone to heat generation due to losses during high-power operation. By monitoring the temperature in real time, the thermal imager can accurately capture areas with abnormally high temperatures (such as microstrip line connections, component solder joints, and the core area of ​​the power transistors), issuing warnings before the temperature approaches a critical threshold. This prevents performance degradation, parameter drift, or even burnout of the power transistors due to localized overheating, effectively protecting critical components and reducing the risk of sudden equipment failure.

[0039] In some embodiments, refer to Figure 4In this diagram, RFin represents the RF signal input terminal of the final stage power amplifier module, used to receive the RF signal transmitted from the driver stage power amplifier through the input matching network; C1 represents the first capacitor matching unit of the input matching network, optimizing the input impedance matching (fine-tuning the impedance between the RF signal source and the input matching network, close to the target value of 25Ω), reducing reflection loss; C7 and C12 represent RF signal coupling capacitors, which couple the two RF signals after splitting to the two input terminals of the power amplifier tubes respectively, realizing signal injection; C2 and C4, together with R1, form a gate voltage filter capacitor, and C3 and C5, together with R2, form a gate voltage filter capacitor. The two sets of filter networks correspond to the gates of the power amplifier tubes of the two RF inputs, filtering out... High-frequency noise, power supply ripple, and electromagnetic interference in the gate voltage are eliminated to ensure gate voltage stability and prevent power amplifier tube gain drift and nonlinear distortion. R1 and R2 represent the gate power supply resistors of the power amplifier tubes, which are connected in series with the gate voltage filter capacitor to limit current and divide voltage, stabilize the power supply current, and prevent overcurrent damage to the power amplifier tubes caused by power supply sudden changes or abnormal loads. C17, C19, and C21, together with R3, and C18, C20, and C22, together with R4, form an RF signal harmonic filtering network. The two circuits correspond to the two RF inputs respectively, suppressing harmonics generated during power amplifier operation and improving the harmonic characteristics of the power amplifier module (reducing spurious radiation). N3 represents the final stage power amplifier tube module (integrating two parallel power amplifier tubes). The following structures are used to achieve high-power amplification of RF signals: L3 represents the power coupling inductor at the output of the power amplifier tube, used for power coupling output and improving power amplifier efficiency (optimizing energy transfer and reducing losses); C23 represents the power coupling capacitor, which couples the RF signals output from the two power amplifier tubes, preparing for subsequent combining; C31 and C32 represent impedance matching capacitors, improving impedance matching during power combining of the two RF signals and reducing reflection losses during the combining process; C35 and C36 represent the second capacitor matching unit of the output matching network, optimizing output impedance matching (fine-tuned to the 25Ω target value), reducing output reflections, and improving power transfer efficiency; C50 represents the output coupling capacitor, which couples the matched... The RF signal is coupled to RFout; +50V represents the power supply (drain / collector power supply) for the final stage power amplifier transistor; L6 represents the power supply filter inductor, which, together with subsequent capacitors, forms an LC filter network to suppress the coupling of power supply noise to the power amplifier circuit; C24, C25, C26, C27, C28, C29, C30, C33, and C34 form a power supply filter capacitor array to filter out ripple and high-frequency noise in the +50V power supply, improve power supply purity, and reduce the impact of power supply noise on power amplifier performance; GND represents the circuit reference ground, providing a grounding loop for each component. The source / emitter of the power amplifier transistor is connected to the ground plane through metallized vias, etc., minimizing grounding inductance.

[0040] The final stage power amplifier's RF signal input terminals are equipped with gate voltage filter circuits and power supply resistors. The gate voltage filter circuits consist of surface-mount capacitors, with two sets of filter capacitors corresponding to each of the two RF signal inputs. Specifically, the gate voltage filter circuit at the first RF signal input terminal includes capacitors C2 and C4, while the gate voltage filter circuit at the second RF signal input terminal includes capacitors C3 and C5. These capacitors are directly connected to the power supply path of the power amplifier transistor's gate, forming a filtering network for the gate voltage. Power supply resistors R1 and R2 are configured for each of the two RF signal inputs. R1 is connected in series with the gate voltage filter circuit (C2, C4) at the first input terminal, and R2 is connected in series with the gate voltage filter circuit (C3, C5) at the second input terminal, providing a stable operating voltage for the power amplifier transistors.

[0041] The gate voltage filter capacitors (C2, C3, C4, C5) effectively filter out high-frequency noise, power supply ripple, and external electromagnetic interference in the gate voltage, ensuring a stable and pure voltage input to the power amplifier tube's gate. This avoids interference from noise signals in the RF amplification process, reduces power amplifier tube gain drift and nonlinear distortion caused by gate voltage fluctuations, and improves the accuracy and stability of RF signal amplification. The power supply resistors (R1, R2) limit current and divide voltage, stabilizing the power amplifier tube's supply current and preventing excessive current from flowing into the power amplifier tube due to sudden changes in power supply voltage or abnormal load. This avoids overcurrent damage to the device, extends the power amplifier tube's lifespan, and enhances the long-term reliability of the power amplifier module.

[0042] In some embodiments, the input end of the input matching network is provided with a first capacitor matching unit; wherein, the first capacitor matching unit is used to optimize the impedance matching of the RF signal input end. The first capacitor matching unit of the input matching network is composed of a surface-mount capacitor C1, which is directly connected between the RF signal input end (RFin) of the final stage power amplifier module and the microstrip line of the input matching network. Specifically, one end of capacitor C1 is connected to the RF signal input port (RFin), and the other end is connected to the initial microstrip line segment of the input matching network. The impedance of the input end is finely adjusted by the capacitive reactance characteristic of the capacitor, so that the impedance between the RF signal source and the input matching network is closer to the ideal matching state (such as the target impedance of 25Ω), laying the foundation for subsequent impedance transformation and signal splitting. Good impedance matching reduces the accumulation of reflected energy at the input end and reduces the additional power loss caused by reflection (such as the heat converted from standing wave loss). Combined with the microstrip line structure of the input matching network, compared with the traditional cable matching method, it further reduces the lossy heat generation in high-power transmission, alleviates the "thermal bottleneck" problem, and works synergistically with the heat dissipation design of substrate slotting and thermally conductive silicone to reduce the overall temperature of the power amplifier module.

[0043] In some embodiments, a second capacitor matching unit is provided at the input of the output matching network; wherein, the second capacitor matching unit is used to optimize the impedance matching at the RF signal output. The second capacitor matching unit of the output matching network consists of surface-mount capacitors C35 and C36, which are connected between the signal synthesis microstrip line and the output microstrip line of the output matching network. Specifically, after the two 12.5Ω microstrip lines synthesize a signal with a 25Ω impedance, the synthesized RF signal first passes through capacitors C35 and C36, and then enters the bottom 25Ω microstrip line of the output matching network. C35 and C36 are connected in parallel between the output node of the synthesis microstrip line and the ground plane (or in series in the signal path), and through their capacitive reactance characteristics, they finely adjust the impedance at the output end, making the impedance between the output impedance of the final stage power amplifier tube and the output matching network and subsequent circuits (such as filters) closer to the ideal matching state (such as the target impedance of 25Ω), laying the foundation for the final signal to be coupled and output to RFout through C50. Good output impedance matching reduces the accumulation of reflected energy at the output end and reduces the additional heat generated by standing wave loss. Combining the thick copper microstrip structure (2oz or more copper layer) of the output matching network with the high thermal conductivity substrate design, the impedance optimization of C35 and C36 further reduces lossy heat generation in the output path. This, along with the substrate slotting and the heat dissipation design using insulating thermally conductive silicone, effectively alleviates the "thermal bottleneck" problem. See Tables 1 and 2 below:

[0044] Table 1: Temperature of Existing Broadband Power Amplifier Modules During Power Output

[0045]

[0046] Table 2: Temperature of the broadband power amplifier module during power output in the embodiments of this utility model

[0047]

[0048] Comparing Table 1 and Table 2 above, the power amplifier tube temperature is significantly reduced after optimization.

Claims

1. A broadband power amplifier device for an FM transmitter, characterized in that, It includes an exciter, a driver stage power amplifier, a final stage power amplifier, a filter, an attenuator, and a power meter connected in sequence; an input matching network is provided between the driver stage power amplifier and the final stage power amplifier, and an output matching network is provided at the output end of the final stage power amplifier; both the input matching network and the output matching network adopt a microstrip line structure based on a printed circuit board.

2. The broadband power amplifier device for an FM transmitter as described in claim 1, characterized in that, The input matching network and the output matching network adopt a two-layer microstrip structure, including top layer traces and bottom layer traces of the printed circuit board.

3. The broadband power amplifier device for an FM transmitter as described in claim 2, characterized in that, The input matching network splits the 25Ω impedance signal into two 12.5Ω impedance signals via the bottom microstrip line and inputs them to the final stage power amplifier tube.

4. The broadband power amplifier device for an FM transmitter as described in claim 2, characterized in that, The output matching network synthesizes a 25Ω impedance signal through two 12.5Ω microstrip lines and outputs it through the bottom microstrip line.

5. The broadband power amplifier device for an FM transmitter as described in claim 1, characterized in that, The substrate (1) of the final stage power amplifier module has a groove (101) in the area corresponding to the bottom microstrip line.

6. The broadband power amplifier device for an FM transmitter as described in claim 5, characterized in that, The groove (101) is filled with insulating and thermally conductive silicone.

7. The broadband power amplifier device for an FM transmitter as described in claim 1, characterized in that, It also includes a thermal imager, which is connected to the final stage power amplifier; wherein the thermal imager is used to monitor the operating temperature of the final stage power amplifier in real time.

8. The broadband power amplifier device for an FM transmitter as described in claim 1, characterized in that, The final stage power amplifier is equipped with a gate voltage filter circuit and a power supply resistor at its radio frequency signal input terminal.

9. The broadband power amplifier device for an FM transmitter as described in claim 1, characterized in that, The input matching network is provided with a first capacitor matching unit at its input end; wherein the first capacitor matching unit is used to optimize the impedance matching of the radio frequency signal input end.

10. The broadband power amplifier device for an FM transmitter as described in claim 1, characterized in that, The input terminal of the output matching network is provided with a second capacitor matching unit; wherein, the second capacitor matching unit is used to optimize the impedance matching of the RF signal output terminal.

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