A bipolar voltage variable attenuator for radio frequency applications

In this work, a new concept for a voltage-controlled variable attenuator, using bipolar transistor technology, is experimentally demonstrated. The input signal is first transformed from an unbalanced to a balanced signal using an active balun. The balanced signal is subsequently fed to a pair of common-emitter (CE) circuits. The attenuated output signal is obtained by summing the output at the collector of each common-emitter circuit. An emitter-coupled differential pair with an active current source was used to control the gains of the above two CE circuits and thus achieved variable attenuation. Experimental results using this circuit reveal an attenuation range from 0 dB to 25 dB. The experimental results agree very well with simulation.