Simple Diode and Bipolar Transistor Mixers

 

 

 
 

         

The amplitude modulation (AM) process of converting a modulated signal to a higher or lower frequency is known as frequency translation or frequency conversion.  A circuit that performs frequency conversion is known as a mixer. Below are two examples of simple mixers.

   

 

 

 

Figure 1. A Diode Mixer

  

Figure 1 shows a simple frequency converter or mixer whose main component is a diode. An input signal with frequency fs is applied to the diode mixer through transformer T1.  A signal from a local oscillator of frequency fo is also applied to the mixer through capacitor C1. These two input signals (fs and fo) emerge at the output of the diode. A tuned LC circuit at the output of the diode allows either the difference frequency (fo-fs) or the sum frequency (fo+fs) to pass to the mixer's output. 

   

Figure 2. A Bipolar Transistor Mixer

  

Figure 2 shows a simple frequency converter or mixer that employs a bipolar transistor biased in such a way that the collector current does not vary linearly with the base current. An input signal with frequency fs is applied to the base of the transistor through a transformer.  A signal from a local oscillator of frequency fo is also applied to the transistor's base through a capacitor. These two input signals (fs and fo) emerge at the collector of the transistor. A tuned LC circuit at this collector output allows only the difference frequency (fo-fs) to pass to the mixer's output.

   

The mixer in Figure 2 is commonly used in radio receivers for translating a signal to a lower frequency, where it is easier to achieve high gain and good selectivity.

   

See Also:  A Simple Amplitude Modulator

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