questions on analog study

[chu]

hi sir, i'm currently studying analog circuit design (Gray/Meyer's book) and need some help for further understanding, thanks in advance.

1) the resistance when looking into the source of MOS is 1/gm, does this apply to BJT too ?

2) for simple common-emitter amp. with resistor Rc tied to Vcc, Rout=ro || Rc ... how about for common-base amp ? since ro is resistance for collector-emitter, so Rout for common-base amp should be Rout=Rc only ... is this right?

3) from chapter 3, can see that C-B and C-C config 's RL & ro has effect to total input resistance(Rin) , and Rs has effect on total Rout ... this is some kind of feedback effect ... so it's only happen at BJT and never MOS ... am i right?

4) voltage gain Av=GmRout (Gm for the amp circuit) and Ai=GmRin ... so this two apply globally for all the BJT circuit?

5) for Common-Source degeneration, roughtly (ro ignored) 1/Gm = Rs + (1/gm) (is this right?) ... how about C-E degeneration circuit .. use same mehod to get Gm?

Thank you very much.

rgds,
chu

 

[flatulent]

A general answer to your question is that bipolar and mos transistor small signal circuit characteristics are similar. The equivalent circuit values may be different but the equations of single stage amplifier performance are the same.

 

[Vamsi Mocherla]

I agree with the above post. The major difference in the bipolar and the mos models is that between the Gate and other terminals, there is an open connection, while in a bipolar, there is a base resistance rb which will connect it to the collector terminal. Capacitances like Cπ and Cµ are replaced by CGS and CGD. So, the small signal modelling for both bipolar and MOSFET are similar. Remember that MOSFET is a Voltage Controlled Current source and the bipolar a Current Controlled Current source. So, there is only a resistor (rb in bipolar), which really makes the difference in order to get the transconductance.

 

[chu]

Thank you very much for the reply, i appreciate it very much.

best rgds,
chu

 

[layes2]

1/gm is also for mos and bjt