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

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jianke

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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

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

jianke

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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

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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

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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
 

FvM

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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.
 

jianke

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Thanks for all of your reply! "Without specifying a transistor type"...
What does the "transistor type" means? BJT / FET ? Or others ?
Do you know which type of transistor I should choose to realize my work. That is, CC configuration mainly amply current, and its voltage gain is "1". Or, CB configuration mainly amply voltage, and its current gain is "1".
 

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