Development of Ku-band Multi-stage Power Amplifier

1 Types of multi-stage power amplifier cascades [1]
   There are generally three types of amplifier cascades:
　　(1) Adding an isolator between amplifiers. In this method, each amplifier is first debugged and matched separately, and then connected in series through an isolator. The advantage of this method is that the amplifier chain basically does not need to be debugged after cascading, and the working stability is good. The disadvantage is that the system cost is relatively high, and the volume and weight are large.
　　(2) Direct cascading without adding an isolator between amplifiers. In this method, each amplifier is also debugged and matched separately, but no isolator is added during connection. This method is more of a compromise and is often used in communication systems and some low-cost transceiver equipment. It does not require much volume, but after direct cascading, it needs to be debugged according to the situation.
　　(3) Sharing matching circuits between amplifiers. Its characteristic is that the output matching circuit of the previous amplifier and the input matching circuit of the next amplifier are shared, that is, the output impedance of the previous solid-state power device is directly matched to the input impedance of the next solid-state power device. The advantages of this method are saving volume and weight, but it is difficult to design, has a high debugging risk, and is very easy to damage the solid-state power device. This cascade method is more common in MMIC (Monolithic Microwave Integrated Circuit).
　　Combining the advantages and disadvantages of the three cascades and the system's requirements for the volume of the power amplifier, the third cascade method is selected. This method is difficult and requires accurate parameters of the amplifier tube and accurate matching circuit.
2 Power amplifier indicators
　　The indicators of the power amplifier are as follows: operating frequency is 17.7 GHz~18 GHz; gain ≥23 dB; output power P (1 dB) ≥19 dBm; power fluctuation ≤1 dB; input and output standing wave ratio ≤2.5; size is 25mm×20 mm.
3 Power amplifier die selection
　　The system requires the power amplifier gain to be greater than 25 dB and the output power to be 19 dBm. The power amplifier die AF08P2-000 of Alpha Company is selected. The parameters of the die are as follows: Vds＝5 V, Ids＝140 mA in typical working state; 8.5 GHz gain 9 dB; 1 dB power compression point 24dBm. The source of the die has been connected to the metal on the back through a through hole, so when using it, you only need to directly ground the metal on the back, and there is no need to connect the source to the ground.
4 Design of power amplifier
　　Alpha provides the S parameters of the typical working state of the amplifier die. S12 is very small〔2〕, so a one-way design method can be used. The above S parameters are used as a black box to design the circuit in Ansoft Serenade. Using the S parameter circuit, you can only design the matching circuit of the circuit, but not add bias voltage. However, since the design of general amplifiers is most concerned about the matching circuit of the amplifier (high-power nonlinear amplifiers require large signal models). When designing, the bias circuit part is also designed in the circuit. As long as the filtering performance of the designed circuit is good, the impact is not great. With the help of the matching tool in Serenade, the general framework of the three-stage amplifier can be obtained. Then optimization is performed to obtain the final circuit, as shown in Figure 1.