# A 2.4 GHz 20 W 8-Channel RF Source Module with Improved Channel Output Balance
## 2 Materials and Methods
### Power Amplification Part  
The delivered power from the RF source generation/DC control part is amplified over 53 dBm passing through the power amplification part. In the previous module, the power amplification part was implemented using gallium arsenide and GaN devices as a 2-stage PA and applied to each channel. In the proposed module, however, two types of GaN HEMT packaged transistors were used to implement a single 2-stage PA. Therefore, the number of transistors used is much smaller than that of the previous module; thus, this module has an advantage in terms of cost and DC control part size. The 2-stage PA consisted of a 25 W transistor, CGH40025F, for the drive PA and 2-way combined 120 W transistors, CGH40120F, for the main PA. To verify the performance of each transistor, test units were manufactured as shown in Figure 2. The DC bias of the drive and the main PA used V\({}_{\text{MAIN\_GG}}\) = V\({}_{\text{DRV\_GG}}\) = \(-2.7\) V and V\({}_{\text{MAIN\_DD}}\) = V\({}_{\text{DRV\_DD}}\) = 30 V, respectively. The measurement proceeded with continuous wave (CW) under natural convection at a room temperature of 25 \({}^{\circ}\)C. Figure 3a shows the measured RF characteristics of the drive PA test unit. The measurement results used swept input power from 0 to 33 dBm at 2.4 GHz. An output power of more than 40 dBm was achieved with more than 70% drain efficiency. Figure 3b shows the measured RF characteristics of the main PA test unit. Using a gain block, the input power was swept from 30 dBm to 43 dBm at 2.4 GHz. Output power of up to 50 dBm was achieved with a drain efficiency of over 50%. From the measurement results, it was confirmed that sufficient system target performance could be secured. Figure 2: Photographs of (**a**) the drive and (**b**) the main PA test unit. Using the above two types of transistors, a 2-stage PA test unit was fabricated, as shown in Figure 4a. Isolators, ADI240 CET, were used at the input/output terminal of the 2-stage PA to avoid damage to the power amplification part caused by the reflected power in case of a mismatch. The PCB used for the power amplification part was a RT/duroid(r) 6010.LM (Rogers Corporation, Chandler, USA). The 2-stage PA was designed considering stability to ensure reliability. The stability is determined by [PERSON]'s factor, K, using network scattering parameters [12]. The stability factor of K is required to be larger than 1 at least to prevent oscillation; thus, it was confirmed that is K > 1 in the design and measurement. Moreover, the negative resistance was checked whether it existed at the input and output nodes or not. The DC bias of the drive and the main PA used V\({}_{\text{MAIN\_GG}}\) = V\({}_{\text{DRY\_CG}}\) = -2.7 V and V\({}_{\text{MAIN\_DD}}\) = V\({}_{\text{DRY\_DD}}\) = 30 V, respectively. As the measurement of the 2-stage PA was fabricated as a test unit, liquid cooling was not available, so it proceeded under a duty cycle of 10% pulsed wave. Figure 4b shows the measurement results. The output power reached over 53 dBm at 2.4 GHz and the adjacent frequency. The average value of the measured gain was 23 dB, and an average drain efficiency of over 50% was achieved. GaN-based high-power amplifiers have the advantage of high-power density, but they have a disadvantage of a high amount of heat density as well. Therefore, burnout or performance degradation, such as drain efficiency, can occur without a proper cooling system [13; 14]. Therefore, it was required to consider thermal management when designing the power amplification part [15; 16]. The previous module was implemented with a large water jacket, and the surrounding area was waterproofed using plate silicon and screws, as  
Figure 4: The 2-stage PA test unit: (**a**) a photograph; (**b**) RF characteristic with a duty cycle of 10%. Figure 3: Simulated and measured RF characteristic of (**a**) the drive and (**b**) the main PA test unit. shown in Figure 5a.