What's the proper operation of switching transistor in an active mixer?

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hi
in active mixer single or double balunced

is the proper operation of switching transistor is in saturation ?

should bias it to be normally [cutoff ?or saturation ]when no Local oscillator applied ?or doesn't matter?
thanks in advance
 

Re: mixer question?

You will get maximum gain if the LO signal produces hard limiting. You will get the minimum of mixing products if the LO signal produces more linear variations.
 

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    Hooda

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Re: mixer question?

flatulent said:
You will get maximum gain if the LO signal produces hard limiting. .
Click to expand...
thanks ;
but do you mean squar wave LO or sine wave is better??

2nd what bias condition should be for switching transistor..sat. or cutoff normally ?
thanks in advance
 

Re: mixer question?

Since it is difficult to make square waves at microwave frequencies, it is common to use the sine wave of several times the amplitude needed to run the transistors from off to full on.

The bias depends upon the topology. If you are using the six transistor switching topology, you have no choice but to have them in the linear condition (ID finite value and Vce a few volts) . If you are using some other topology, use whatever bias is convenient.
 

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    Hooda

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Re: mixer question?

Hello

The best way is to sweep on the LO bias and find which value is best for your target i.e "gain,linearity or noise"

My opinion: when you keep the transistors off for more time, you will get better noise figure" specifically lower switch noise" , but the linearity and gain may suffer.

check the work of Hooman Darabi on mixers and mixer noise...
 

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