SKY65120

application note SKY65120: WCDMA PA Bias Method For Lower Junction Temperature Introduction This application note describes how SKY65120 may be used with reduced bias control to obtain better thermal performance. It is especially useful for customers who may not have a suitable heat sink available directly below the PA module. To achieve high gain (>23 dB), the SKY65120 consists of two stages of amplification. The base current of each amplification stage is controlled by an on-die active bias circuit. Each active bias circuit has its own reference voltage supply, Vref1 and Vref2. The bias voltage (Vc_Bias) is shared by the two active bias circuits. This application note describes the active bias circuit used by SKY65120. The note explains the use of external supply voltages (Vc_Bias and Vref) and setting limits available to the customer. Test results of key parameters are also given over temperature. VREF1 (5 V) RREF1 VC_BIAS (5 V) VREF2 (5 V) RREF2 Active Bias Circuit Circuit Description The active circuit is designed to maintain a constant supply current to the base of its respective transistor as supply voltage and temperature is changed. Each active bias circuit consists of three HBT transistors and three resistors, connected as shown in Figure 2 below. VC_BIAS R1 Q1 Q2 R2 I_Supply (to Base of PA) Q3 R3 VREF RREF (On PCB) Active Bias 1 Active Bias 2 RF In Input Match Stage 1 VCC1 (5 V) Interstage Match Stage 2 VCC2 (5 V) Output Match RF Out Figure 2. Schemati...