CMOS Self-Biased Cascode Power Amplifier
I am studying the article "A 2.4-GHz 0.18-um CMOS Self-Biased Cascode Power Amplifier" (attached). Here are what I am confused. Please help. Thank you.
1-In order to give max. swing to M1, Vdd has been used.Otherwise Vds of M1 will be lower so consequently swing will also be low.
2-RF chokes are useless in RFIC circuits.Instead using a resistor will also supply wanted gate voltage.Because there is practically no DCcurrent flows.
3-Ls inductor is used to able to play around input impedance to get or approach to optimum impedance for lowest NF.
Thank you, BigBoss.
I see it now. Can I ask another question about choosing the bias voltage?
Here is a section in Design of Analog CMOS integrated circuits by Razavi. I am confused about how to determine the valid range for bias voltage.
Please help me with the question in the picture below.
Well, I have mistaken for a long time. Why RF choke can"t be use in RFIC? Because its big size? I have seen a lot of RF power amplifier at uF technology use inductors to bias for transistor. Are these inductors RF chokes?
About using resistor:
+ In DC mode: There is no DC current => No losses.
+ In AC mode: RF signal (current) will flow through this resistor and causes losses. If so why resistor is used?
I thought it is a parasitic parameter.
The use of this inductor will affect the operation of power amplifier in AC mode. I think we have a trade-off here, right?
Vin is NOT AC signal level, Vin is Vgs1 so it's bias voltage.If we assume that the transistors are identical ( so it's true) Vgs1=Vgs2 for the same current.
Therefore there isn't any unknown variable in this calculation.Vout has some option to make these transistors both in saturation or one of them.So you should carefully select Vb so that Vout will satisfy the inequality in eq.1 (look at the explanation, he talks about a variable operating point that will cause a compromise )
2-RF Chokes can be used for discrete Linear Amplifiers/Power Amplifiers but not on the silicon because of their huge size.Instead, RF Chokes may be placed extrenally on off-chip.If you have sufficient space, you can of course use them but it won't be very practical.
3-Ls inductor will play a role to obtain optimum NF by compensating Cgs Cgd and the others.Because each Low Noise Transistor has intrinsically determined a Optimum Noise Impedance.Ls is for that..
Thank you, BigBoss.
Can you help me explain the sentence in the first picture?
In the cascode configuration, transistor M1 has a smaller drain–gate voltage swing. This is because the voltage at D1 is always lower than voltage at G2 by an amount equal to the gate–source voltage of G2.
I really can't figure out why!