An Efficient, Broadband SiGe HBT Non-Uniform Distributed Power Amplifier Leveraging a Compact, Two-Section λ/4 Output Impedance Transformer

An efficient, broadband SiGe HBT cascode nonuniform distributed power amplifier (NDPA) is presented for low-cost, fully integrated Si-based phased arrays. Optimum load impedances at each SiGe HBT cascode in a four-stage NDPA core are obtained by scaling the characteristic impedance (
$Z_{0}$
) of the collector transmission lines (TLs) and tapering the SiGe HBT emitter area simultaneously. A novel compact, lumped-element two-section
$\lambda $
/4 output impedance transformer (OIT) is proposed to lower the NDPA load impedance (
$Z_{L}$
) from 50 to 25
$\Omega $
over more than one decade bandwidth (BW). Each
$\lambda $
/4 impedance transformer is realized by four cascaded
CLC

$\pi $
-networks integrated into a single three-turn symmetric inductor in order to achieve compact size, high passive efficiency, and high
LC
cutoff frequency (
$f_{c}$
). The systematic design approach of a lumped-element
$\lambda $
/4 impedance transformer with an arbitrary
$Z_{0}$
is described in detail. The prototype NDPA was fabricated in 0.13-
$\mu \text{m}$
SiGe HBT BiCMOS technology. The proposed SiGe HBT cascode NDPA supports both high linearity (HL) and high gain (HG) modes, each suited to a specific application. The NDPA attains a peak power gain of 10.3/12.5 dB, a saturated output power (
$P_{\mathrm {out}}$
) of 21.3/21.5 dBm, and a power added efficiency (PAE) of 12.2%/12.5%–21.6%/22.0% for HL/HG modes, with a 3-dB BW from 1.5 to 24.0 GHz. The NDPA delivers 13.0-dBm average
$P_{\mathrm {out}}$
with a PAE of 10.0% at 6-Gbit/s data rate 64 QAM modulation.