How to set the bias voltage of the BJT in Gilbert mixer
working region of BJT would make difference to the function or index of mixer?
Thank you very much.
More DC current means better linearity (IP3), but always will be a compromise between power consumption and linearity.
Bias current in Gilbert mixers also affect the noise figure. But because the noise figure is pretty high in this kind of mixers (9-10dB), power consumption and linearity are the main parameters that need to be tuned.
Gilbert cell is a linear multiplier. If you are using small signals set BJT region into normal active domain, don't operate it in saturation, so it will be faster and non-desired mixing products will be smaller.
With large local signals for example the conversion gain will be higher but the BJT will leave the normal region and doesn't matter where is the operating region.
But what is the index of a mixer? I don't understand this.
Yes，I am using small signals to set BJT region.But you mean Gilbert cell would be faster when operate it in saturation,right?I don't understand this.could you please explain it in detail?Thank you very much.
In addition,the index I mention is conversion gain,NF,and so on.
I have just said with large signal amplitudes on the RF or the LO input the non-linear mixing products will be such high it doesn't matter where the transistor operates.
But with smaller signal amplitudes it can be better if you don't let the BJTs to leave the normal active region, they shouldn't operate in saturation if the high linearity is the target.
And I told the opposite, if the BJTs operate in saturation the mixer will be slower and won't handle well higher frequencies. BJT is not as good switch like MOSFET, it needs more time to come out from saturation as I know, but parasitic capacitances are smaller so with linear operating quite high frequencies are reachable.
OK，I get it.
Thank you.