Digital TV Transmitter Power Amplifier

Digital television transmitter power amplifier is an important part of. Usually, digital TV transmitters in the way COFDM modulated signals output by the IF analog signals, amplified by the conversion into the part. The modulation includes IFFT (8M) and IFFT (2M) modes, respectively, 6817 and 1705 from the carrier composition. each carrier frequency spacing between the very close, so very easy to fall-modulation signal frequency band, causing intermodulation distortion. digital television transmitters more traditional type, linearity, stability has a higher demand. on the transmitter power amplifier required to work at a higher linear state, the gain stability.

Enlarged part of the launch system is divided into motivation and the main amplifier circuit. Which motivate some of the broadband power amplifiers, terrestrial digital television transmission to ensure the normal stability, need to have good stability and reliability, its frequency band in the 470MHz ~ 860MHz, work state of AB type; required gain greater than 10dB, intermodulation suppression is less than-35dB, the noise power density greater than 130dBc/Hz. In this paper, the latest LDMOS FET devices, and balanced amplifier structure? smoked design digital television transmitters in drive-level power amplifier, optimized and debug, to meet the system requirements.

1 Power Amplifier

1.1 Selection of power amplifier chips to enlarge

In this paper, the device MRF373 Motorola LDMOS FET power amplifier as the amplification chip. The chip linear gain and output capability relative to BJT devices have more to upgrade, so that the transmitter greatly improved reliability and maintainability. With the traditional sub- UHF amplifier tube than bipolar, LDMOS FET has the following significant advantages:

* At high standing wave ratio (VSWR = 10:1) case work;

* High gain (typical 13dB);

* Saturation curve smoothing is beneficial to both analog and digital RF signal amplification;

• You can drive power to bear too great, especially for DVB-T COFDM modulation in multi-carrier signal;

* Bias circuit is simple, without complex temperature compensation is active with low-impedance bias circuit.

LDMOS manufacturing process combines the BPT and GaAs technology. And standard MOS technology is different in the device package on, LDMOS without using BeO beryllium oxide isolation layer, but directly Yingjie in the substrate, the thermal performance, improved The temperature of the device, greatly extend the life of the device. LDMOS tube because the negative temperature effect, the leakage current flow in the heat all the time automatically, and not as bipolar tube is temperature effect on the formation of the collector current local hot spots , and thus easily damaged pipe. 所以 LDMOS tube greatly enhance the load mismatch and over Ji Li's Chengshounengli. Tongyangyouyu LDMOS control of automated Jun Liu 作用, the input - Shuchutexing curve at the 1dB compression point (large signal Yunyong the saturated zone ) under the more moderate bent, so the dynamic range was increased, benefit analog and digital TV RF signal amplification. LDMOS small signal amplification in approximately linear time, little or no distortion, very much simplifies the calibration circuit. MOS devices of DC gate current is almost zero, the bias circuit is simple, without complex temperature compensation is active with low-impedance bias circuit.

1.2 Selection and comparison circuit

Small-signal S parameters can be used for A amplifier design, that is, the signal amplification required basic limitations of linear region in the transistor. However, related to big power amplifier, due to amplifier operating in nonlinear regions, it is usually small-signal approximation invalid. this time to obtain large-signal transistor S parameters or impedance, in order to get a reasonable design effect.

In general, the distortion coefficient A minimum working conditions, good linearity. But in the high-power applications, due to low efficiency Category work status (50%) does not apply. With Class B push-pull amplifier circuit form, can be similar to the linear amplifier and the A index.

Push-pull circuit form two independent and free from any internal connections constitutes a single-tube amplifier, through the two balun for power distribution and synthesis of the vector. As Barron itself has changed the characteristics of resistance, thus greatly reducing drag ratio with varying to the impedance matching difficult, and Barron for the even harmonics have a very good extent. However, as Barron both sides of the interval is too small, a larger two-way interaction, the application balun amplifier was used, and the circuit's input and output VSWR are relatively poor.

Digital TV Transmitter Power Amplifier

In this paper, the form of a balanced amplifier structure shown in Figure 1. The working principle and circuit balun structure similar to, but because the application of 3dB bridge, making two-way RF signal isolation between the good, beneficial to the two ports match. relative to the single-tube amplifier structure, its advantages, such as Table 1.

Digital TV Transmitter Power Amplifier

1.3 Matching Network Design

As MRF373 not provided within the match, so to build the amplifier matching network. Digital TV reflex amplifier system working in the 470MHz ~ 860MHz, need to be achieved within the broadband impedance matching. Wideband amplifier matching circuit design the basic idea is : De input and output and the amplifier stage are 采用 reactance between the multi-stage impedance matching networks transform. The network is only matched Zuo Yong Qi, not Ewai loss of power, to the maximum of Zhuanshuxishu Bao Zheng, from Junheng role on Device Performance, Bing meet Ji Tong bandwidth requirements needed.

IV curves using the device or by the output power, voltage and other parameters can determine the load RL. To make the maximum output power, with RL that the device's internal drain load as the target output matching circuit. If a network to a complex impedance has the best match, the network's output impedance equals the complex conjugate load impedance value. Now the load impedance is purely real RL, so the best output matching circuit to the device reflected the drain load impedance is the complex conjugate value of RL , namely:

RL = (VDD-VDS (sat)) 2/2P

Where VDD is the operating voltage, VDS (sat) is the inflection point voltage, P is the output power.

Based on the type can be worked out, MRF373 of RL is about 6Ω.

This separation of the amplification circuit components and component mix of distributed parameter method. As the inductance capacitance higher than the heat loss, so in such circuits normally avoid using inductors, and use high-impedance transmission line to replace. Mixed type matching network usually includes several paragraphs to link the transmission line and the interval between the parallel capacitor configuration. The amplifier is part of the input matching impedance with a 4 even transform, the output matching impedance transform using five hybrid circuit with the form. input and output matching network topology in Figure 2, Figure 3 shows.

Digital TV Transmitter Power Amplifier

Digital TV Transmitter Power Amplifier

2 circuit optimization and simulation results

As digital television transmission system requirements to enlarge the circuit must be working online Xing Fangdazhuangtai, Ke Yi Yong 小 信號(hào) S 參數(shù) analysis. Jiezhuqijian Changshangtigong the small-signal S parameters Wen Jian, you can use ADS whole circuit is simulated small-signal S parameters, are small signal gain, port match, isolation and stability factor K. Table 2 for the MRF373 in (Vce = 26V, Ic = 500mA) under the S parameter.

Digital TV Transmitter Power Amplifier

For circuit simulation using ADS and can not meet the design requirement and should on this basis the circuit optimization. 當(dāng) only small-signal S parameter as a model to design power amplifier, the circuit optimization step is generally as follows: First, as Yi RL (relative maximum output power of the load resistance) match for the target, optimize and determine the output matching circuit component values; and then optimize the input matching circuit component values, to improve the gain and input matching circuit. to note is: before optimization, must be as complete as possible Output circuit model, then its optimal operating frequency to achieve the best match with the RL. Figure 4 Simulation results for the amplifier circuit, Figure 5, the final optimization results for the circuit.

Digital TV Transmitter Power Amplifier

3 test results

After many experiments and over and over again, the measured results shown in Figure 6. The driver amplifier working in the linear state. By the gain curve of Figure 6 can see the entire band gain flatness, as about 12dB, and the simulation results are similar. Back wave loss is less than 15dB, VSWR less than 1.3 band. input power of 2 watts, with power meter measured the output power of 25W, the signal amplitude stability, and its intermodulation suppression is less than-35dB. the indicators to meet the system requirements Digital TV Transmitter with similar foreign targets in the amplifier close to the cost of greatly reduced numbers for the future development of domestic production of TV transmitters basis.