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Synchronous Modulator
Versions 1 and 2
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This unique circuit uses a very small amount of battery current and has excellent carrier suppression.  Its only shortcoming is that you must limit the input signal to 0.7 Vp-p in order to prevent distortion.  To use higher input voltages, replace Q3 with a JFET.  A MOSFET doesn't do as well as an NPN transistor.

More:  Operational AmplifiersModulators

 

Click on the Schematics to view PDFs

Version 1


Version 2

This is an opamp version of the above circuit.  It has more consistent performance over varying power supply voltage than the discrete version, but it still has the 0.7 Vp-p limitation of the maximum input voltage.  You should use a JFET or a CMOS switch in place of Q1 if you need to accommodate a high input voltage.  Also a low pass filter needs to be adder after the output of the circuit, where the discrete version had one built in.

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