# Common-Collector (CC) and Common-Base (CB) at RF Frequency

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#### jianke

##### Member level 3
BJT Common-Collector (CC) and Common-Base (CB) at RF Frequency ?

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

#### vfone

Both, CB and CC amplifiers do not suffer from the Miller effect, so their frequency response is similar.

#### jianke

##### Member level 3
Thank you for your reply. Besides the Miller effect, we should also consider other factors at RF frequency, such as Early effect and collector-emitter resistance. I am a little confused with “their frequency response is similar”. Please allow me clarify my question here.
(1) At 2GHz frequency, whether a Common-Collector (CC) circuit can amplify the current, and its voltage gain is about “1”. -- Yes or No ?
(2) At 2GHz frequency, whether a 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!

#### vfone

If a transistor in CC configuration can amplify (in current) a 2GHz signal, the same transistor in CB configuration can amplify (in voltage) a 2GHz signal.

#### chuckey

As Vfone said, the input impedance of a CB is say 50 ohms, its output impedance 5 K, so same input and output current but voltage gain can be high (V= i X Z). The base operates as a shield between the input and output circuits. It is the best way of "stretching" the operating frequency of a transistor.
Frank

tony_lth

### tony_lth

points: 2

#### FvM

##### Super Moderator
Staff member
Even with microwave transistors having e.g. 10 GHz fT, the said gain values at 2 GHz will differ from "about 1". Without specifying a transistor type, the question is almost useless.