An active balun circuit is provided for single-ended to differential RF signal
conversion with enhanced common-mode rejection which suppresses common mode signal
and which achieves phase and amplitude balance without sophisticated tuning or
compensation methods. The circuit has a single-ended input and balanced output
with phase and amplitude balance error less than 2 and 1.2 dB, respectively,
measured from 1.5 GHz to 1.8 GHz at 5V supply. When supply voltage drops down to
1.5V, its phase and amplitude balance error remains within 5 and 2 dB, respectively.
The circuit achieves a balanced output via an output network which behaves as an
impedance matching network for differential mode signal and is grounded for common
mode signal. As a result, common mode signal is suppressed and 180-degree phase
balance at output is achieved. The circuit has high-linearity (P1 dBin=5
dBm, IIP3=16.6 dBm) and low residual phase noise (<-;155 dBc/Hz at 100 kHz and
above) which make it suitable as an active balun/buffer amplifier between LO and
balanced mixer for base station receiver applications.
