The present invention relates to active electronic load circuits for receiving differential input signals and, more particularly, to a balanced differential load for providing a single ended ouput signal in response to a differential input signal applied thereto and a method for filtering unwanted frequency signals at the output thereof.
Many integrated circuits (IC's) such as mixers, detectors and low power FM receivers require filtering and differential to single ended signal conversion. For example, the MC3359 is a low power FM-IF receiver, manufacturered by Motorola Inc., which provides FM-IF to audio signal translation that requires filtering of unwanted high frequency signals from the single ended audio output signal.
A problem can arise in prior art receivers such as the MC3359 due to the fact that the differential FM-IF signal is converted to the single ended audio output signal within the IC and because the higher unwanted frequency signals are filtered to ground. The filtering of the IF signal, as well as any other unwanted higher frequency signal, is provided by a filtering capacitor, either internal or external to the IC, that has one electrode thereof connected to ground. It has been observed that the unwanted signals which are shorted to ground through the capacitor can be injected or fedback to intermediate stages of the receiver to be amplified. This can cause oscillations to occur at the output and/or degradation of the output signal: both conditions of which are highly undesirable.
Hence, a need exists for a balanced differential load that can be used in an IC to provide amplification and differential to single ended output of a desired output signal while filtering unwanted higher frequency signals without producing ground currents due to the filtering.