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The Designer's Guide Community Forum › Design › RF Design › Resistors' values of a Current Reference for a LNA??? (Thomas H.Lee's Book)
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Resistors' values of a Current Reference for a LNA??? (Thomas H.Lee's Book) (Read 655 times)
jianke
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Resistors' values of a Current Reference for a LNA??? (Thomas H.Lee's Book)
Apr 05th, 2012, 10:00am  
Hi everyone! I am learning to design a CMOS LNA using Thomas H.Lee's book (The Design of CMOS Radio-Frequency Integrated Circuits). But I am not sure about how to choose the resistors' values of a "Current Reference" (Page 386)??? The values of the two resistors are 1kohm and 2kohm.

The bias current is 5mA in his example. But, if it is changed to 6mA or another value, then how to correspondingly change the values of the resistors of the "Current Reference".

I read some books about current mirror and current reference. But the Current Reference used in Thomas H.Lee's example seems not a general structure explained in these books. Looking foward to your answer. Thanks!
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raja.cedt
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Re: Resistors' values of a Current Reference for a LNA??? (Thomas H.Lee's Book)
Reply #1 - Apr 5th, 2012, 10:11am  
hello,
it is general ckt only. See 1K is mainly to set the bias current, but 2k is to isolate bias as well as signal from the input. So you have to reduce 1k to increase the current.

Thanks,
raj.
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jianke
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Re: Resistors' values of a Current Reference for a LNA??? (Thomas H.Lee's Book)
Reply #2 - Apr 5th, 2012, 11:27am  
Hi raja, thanks for your reply. The bias current is 5mA.
So how can I calculate the resistor value (1kohm), using this current value (5mA)? Is there some equations about it?
(sorry, what does the acronym "ckt" represent for)
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raja.cedt
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Re: Resistors' values of a Current Reference for a LNA??? (Thomas H.Lee's Book)
Reply #3 - Apr 5th, 2012, 1:41pm  
hello,
ckt means circuit. you can use the following formula

Vdd =I*R+(Vth+Vov)...and it is very difficult to calculate Vov as you know mosfet is no longer square law and mobility and cox are also unknown. So i would suggest you to play with simulator.
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