Feedback Mixers
Updated 19 March 2003

    The application of negative feedback to active mixers provides a significant improvement in intermodulation performance over the typical six-transistor commutative mixer, wrongly referred to as the Gilbert Cell. A variety of topologies using both series/shunt feedback and lossless feedback are possible:
    
    
  1. Type I Series/Shunt Feedback Active Mixer. By using a resistive signal combiner at the output, this configuration of the Series/Shunt Feedback Active Mixer provides a high linearity mixer (+27dBm IIP3) that is especially suitable as a demodulator as the output may be direct coupled to baseband circuitry. The LO drive level is 0dBm, and the noise figure performance is comparable to that of the standard six-transistor commutative mixer.
    
    
  2. Type II Series/Shunt Feedback Active Mixer. Replacing the resistive signal combiner of the Type I Series/Shunt Feedback Active Mixer with a pair of hybrid transformers improves the conversion gain of the Series/Shunt Feedback Active Mixer and results in a high linearity active mixer that is suitable for use in receivers and transmitters for frequency conversion as well as a modulator as the input may be direct coupled to baseband circuitry. A PDF file is available that provides a schematic diagramme plus notes for building a Type II Series/Shunt Feedback Active Mixer.
    
    
  3. Lossless Feedback Active Mixer. The application of lossless feedback results in an active mixer having slightly better intermodulation (+30dBm IIP3) and noise figure performance over the Type II Series/Shunt Feedback Active Mixer, and which is the basis for high linearity and low noise configurations. A PDF file is available that provides a schematic diagramme plus notes for building a Lossless Feedback Active Mixer.
    
    
  4. Augmented Lossless Feedback Active Mixer. By adding augmentation to the Lossless Feedback Active Mixer, IIP3 performance approaching +40dBm is achieved with no increase in biasing or LO power, resulting in an active mixer that is highly suitable for receiver, transmitter, and CATV applications where a high order of linearity is a primary consideration. A PDF file is available that provides a schematic diagramme plus notes for building an Augmented Lossless Feedback Active Mixer.
    
    
  5. Low Noise Lossless Feedback Active Mixer. By removing the driver transistors from the Lossless Feedback Active Mixer, the noise figure is reduced to less than 6dB while retaining the high linearity performance of the Lossless Feedback Active Mixer. A PDF file is available that provides a schematic diagramme plus notes for building a Low Noise Lossless Feedback Active Mixer.
  6. 
    Information on these and other feedback mixers is available from various publications, some of which are available on this web page.
    
    
           If you have any comments to send concerning
           feedback mixers, please Email them to 
           christrask@earthlink.net
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