The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> BJT Common-Collector (CC) and Common-Base (CB) at RF Frequency ?
https://designers-guide.org/forum/YaBB.pl?num=1302887345

Message started by jianke on Apr 15^th, 2011, 10:09am

Title: BJT Common-Collector (CC) and Common-Base (CB) at RF Frequency ?
Post by jianke on Apr 15^th, 2011, 10:09am

With the help of the low-frequency h-parameter model, we know that a Common-Collector (CC) amplification circuit amplify current (i.e. voltage gain is about “1”), and a Common-Base (CB) amplification circuit amplify voltage (i.e. current gain is about “1”).

At RF frequency (e.g. 2GHz), RF small-signal model has to be used for the junction capacitance and feedback effect of the BJT transistor. I want to know at this frequency, whether the CC or CB amplification circuit has the same function, just like at low frequency. For example, at 2GHz frequency, a CC amplifier amplify current (voltage gain is about “1”), and a CB amplifier amplify voltage (current gain is about “1”).

I checked several references but can't find the analysis of CC or CB amplification circuit at RF frequency. Doesn't anyone have an idea or any references? Thanks

Title: Re: BJT Common-Collector (CC) and Common-Base (CB) at RF Frequency ?
Post by buddypoor on Apr 15^th, 2011, 11:03am

jianke wrote on Apr 15^th, 2011, 10:09am:

You are right with the above statements, however, inorder to avoid misunderstandings you should realize that you, of course, could have circuits that amplify currents as well as voltages. Therefore, your formulation in brackets "i.e." ("that means") is not correct and can be misleading.

Title: Re: BJT Common-Collector (CC) and Common-Base (CB) at RF Frequency ?
Post by jianke on Apr 15^th, 2011, 7:32pm

Yes, we can design a circuit to amplify current as well as voltage, just like the Common-Emitter (CE) configuration. Maybe my description is a little confused, please allow me clarify my question here.
(1) At 2GHz frequency, whether the Common-Collector (CC) circuit can amplify the current, and its voltage gain is about “1”. -- Yes or No ?
(2) At 2GHz frequency, whether the Common-Base (CB) circuit can amplify the voltage, and its current gain is about “1”. – Yes or No ?

Could you please also give me some reasons? Best Regards!

Title: Re: BJT Common-Collector (CC) and Common-Base (CB) at RF Frequency ?
Post by buddypoor on Apr 16^th, 2011, 1:37am

Without detailed and specific investigations I would answer your question as follows:
*Even for lower frequencies some simplifications, minor effects and parasitics are neglected, thereby allowing your statements "about 1" (for the voltage or current gain, respectively.
* In principle, this approximation holds also for higher frequencies, however, the error by neglecting all these parasitic influences will be certainly larger.
* In summary, it depends on the error that can be accepted by you resp. your application if such statements are appropriate or not.
(Example: 0.99 can be regarded as app. "1"; does this hold also for 0.95 ? I don`t know).

Title: Re: BJT Common-Collector (CC) and Common-Base (CB) at RF Frequency ?
Post by purplewolf on Apr 17^th, 2011, 2:04pm

jianke wrote on Apr 15^th, 2011, 7:32pm:

The answer is yes....The fundamental principle are always obeyed unless you do some mistake in simulation envirnonment or in ur schematic