A Monolithic U-Band InP HBT Stacked Power Amplifier With On-Chip Active Biasing

U-BAND PA DESIGN

The unit emitter area of the high speed HBT is 0.5 × 10 μm². For a cascade active core, 18 fingers (18 × 0.5 × 10 μm²) are used for both the CE (Q₁) and CB (Q₂) transistors (see Figure 3). To regulate the DC bias current of Q₁ over process, temperature and DC supply variations, Q₃, D₁ and D₂ form a voltage compensation bias network at the base of Q₁. The active base bias transistor, Q₃, employs eight fingers of a high speed HBT (8 × 0.5 × 10 μm²). Input and output matching circuits are composed of MIM capacitors and on-chip microstrip with top thick metal lines. For compact size, spiral inductors are not used in the matching circuits.

The output power of the n-stacked PA can be expressed as⁴

where n is 2 for this PA. R_{opt_PA}, V_ce, V_k and I_d are the optimum output load impedance, collector-to-emitter voltage, knee voltage and DC supply current, respectively. R_{opt_PA} is

where R_L is the load impedance of each stacked transistor. From Equation 9, R_{opt_PA} increases with R_L. Since the DC currents across each transistor of the stacked PA are all the same, the output power can be enhanced by stacking the transistor.

A constant base voltage has been suggested for obtaining both high output power and high efficiency in an HBT PA.⁸ To achieve this for Q₁, the simulated minimum inductance value of L₁ is 3 nH; however, a high L₁ inductance increases the size of the PA MMIC. Alternatively, a shunt capacitor, C₁, is used to reduce the MMIC size by lowering the value of L₁. Based on a load-pull simulation, two-section LC elements are designed for the output matching network. A one-section LC element is employed for the input matching network of the stacked PA, and an RC network is added to the base of Q₁ for circuit stabilization.

Figure 4 shows the simulated output power (P_out) and PAE of the U-Band PA at 52 GHz with and without capacitor C₁. For the simulation, 0.8 nH for L₁ and 0.1 pF for C₁ are used. Without C₁, RF leakage to the bias transistor Q₃ would cause a voltage drop through the base and emitter in the negative direction, resulting in a base voltage increase for Q₁ with increasing input power. The increased collector current of Q₁ would then unnecessarily lower the P_out and PAE of the power amplifier in the high output power region.

MEASURED RESULTS

The fabricated U-Band PA MMIC (see Figure 5) contains input and output matching circuits, an active bias circuit and probing pads and is laid out in an 0.88 mm × 0.67 mm (0.59 mm²) area. The PA MMIC is biased with V_cc = 5 V and V_ref = 2.5 V and draws a quiescent current of 120 mA. Figure 6 shows the measured small-signal performance of the PA from 45 to 60 GHz. The measured linear gain is about 14.2 dB at 52 GHz, with more than 35 dB isolation. At 52 GHz, the measured 1 dB compression output power (P_1dB) and saturated output power (P_sat) are 21.2 and 25.97 dBm, respectively, as shown in Figure 7a. The measured PAE versus input power is shown in Figure 7b. At 52 GHz, the peak PAE is 28.7 percent. The measured performance of the U-Band PA is in good agreement with the simulation shown in Figure 4.

A comparison of U-Band InP HBT PAs is shown in Table 2.^9-11 The output power density of 670.1 mW/mm2 is the highest demonstrated for an InP HBT PA.

CONCLUSION

A compact U-Band PA with an on-chip active bias circuit using a commercial 0.5 μm InP HBT process has been demonstrated. The fully matched PA MMIC exhibits an output power density of 670.1 mW/mm², which, to our knowledge, is the highest value reported for a U-Band InP HBT PA.

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