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BJT Selection for Gilbert Cell

Hetfield

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i want to design a double-balanced gilbert mixer circuit where the LO 5 Ghz and the RF 4.9 Ghz and the IF 0.1 Ghz and , want to know that what should i care while choosing the right transistor for the good result. can you give me some recommendations. thanks in advence
 

d123

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Hi,

I know nothing about Gilbert cells/mixers, sorry... In my ignorance, I would guess that at 5 GHz you might want to look at RF transistors? I read they are usually made with MOSFETs nowadays, not sure if it meant at IC level, 'though.
 

Hetfield

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Hi,

I know nothing about Gilbert cells/mixers, sorry... In my ignorance, I would guess that at 5 GHz you might want to look at RF transistors? I read they are usually made with MOSFETs nowadays, not sure if it meant at IC level, 'though.
is this because the frequency limitation, should i use RF transistors i mean the bandwidth of the transistor must be higher that the operation freq. of the mixer right?
 

d123

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Hi,

I would have thought so, but don't take my word for it, I know as much as you do. Isn't it (exactly) the same as an op amp in that respect: gain falls (to 0) with increased frequency due to finite bandwidth?
 

betwixt

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I sense some confusion here.
I really wouldn't recommend you try to build a 5GHZ mixer using discrete components, especially using BJT devices when ready made ICs are available. Your BJTs would ideally have fT of > 10GHz which limits availability and layout would be extremely critical.
I have used IAM81008 successfully in the past but I'm not sure if it is still available.

Brian.
 

BigBoss

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Look at Infineon website, they have SiGe BJT's to operate GHz range..
 
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