雷達電子防護關鍵技術研究---總計畫

Abstract

進行之研究方法與個人專長：本人專長為射頻，微波，及毫米波電路設計及量測。此 14GHz 功率放大器之第一級電路為一個高增益中功率的微波積體電路，因此0dBm 的輸 入訊號可被放大至20dBm 以上。接下來依序串接多個單一功率電晶體(例如東芝的功率 電晶體)，使最後輸出功率可達20W。因為在固定偏壓情況下這些電晶體的功率增益多 隨著環境溫度的增加而降低，因此必須設計一個溫度回授機制，適時自動調整電晶體 的偏壓狀態，使其增益盡可能穩定。當然,在極大功率的狀況下，隨著溫度升高而增加 偏壓電流的這種方式，有可能造成電晶體的進一步升溫，而造成正回授的情形。為了 預防這種情形，勢必酌量降低電晶體的偏壓的溫度補償。此時，剩餘的功率增益變化 可由第二層回授機制來消除。這裡我們在電路的前緣，加上一個可變衰減器。當溫度 升高時，此衰減器的衰減量會降低，因而容許更多的信號流入。是以雖然功率放大器 的增益雖有降低，最後的輸出信號之功率可藉此維持一定。當然，也可經由偵測輸出 功率的方式，來控制此衰減器的衰減量。這樣，藉著自動調整電晶體的偏壓狀態，及 衰減器的衰減量，可使功率放大器輸出為最穩。這裡溫度的偵測是靠功能類似一個(齊 納)二極體的積體電路來達成(可由國家半導體購得，其反向偏壓的溫度係數為 10mV/K)。功率的偵測，及信號的衰減，則各有適當的二極體以專司其職。而偏壓電路 及電晶體周圍的高頻被動電路的設計，務必使電晶體遠離振盪。此項研究牽涉到微波 電路，元件物理，類比控制電路，熱力學，及金屬機械加工。

My expertise is on the RF, microwave and millimeter-wave circuit design and measurement. The first stage of this 14GHz power amplifier is made of a high-gain medium-power microwave integrated circuit, so that 0dBm input signal can be amplified to render an output of more than 20dBm. Then we cascade several monolithic power transistor (for example, the Toshiba’s Power FET’s) in the goal of reaching a final 20Watt output power. Since given a fixed voltage bias condition, each transistor’s power gain tends to reduce as increasing ambient temperature, thus a feedback mechanism must be implemented so as to automatically adjust the bias, therefore stabilize the power gain. Of course, under the extreme circumstance of very large power dissipation, it is possible that the above-mentioned approach will cause a self-heating of the circuit and then inadvertently a positive feedback. With this in mind, the temperature compensation of the bias for constant gain needs to be modest at most. The residual gain variation can now be eliminated with the implementation of a second feedback mechanism, i.e., the implementation of a variable attenuator at the input port of this power amplifier. By decreasing the amount of attenuation when temperature increases, thus allows more signal to be fed into the amplifier, the total output power can be sustained to a fixed value, even though the overall amplification of the power amplifier itself will decrease with ambient temperature. Alternatively, this attenuator can be manipulated not be the temperature but the detected output power of the amplifier. Either way, by automatically adjusting the bias of the amplifier and fine-tuning the variable attenuator, it is achievable maintaining a constant output power for the amplifier. Here the temperature sensing is made possible by the use of a Zener-diode-like integrated circuit (IC), such as the one made by National Semiconductor with 10mV/Kelvin temperature coe3fficient. The power detection and attenuation can also be carried out using specific diodes. It is important to have the transistor away from oscillation in our design of the biasing circuit and the high-frequency passive circuits surrounding the transistor. This research involves the knowledge of microwave circuit, device physics, analogue control circuit, thermo-physics, and high-precision machining.