# what is the significance of the transit frequency and maximum frequency of oscillatio

1. ## what is the significance of the transit frequency and maximum frequency of oscillatio

Hi I know the definitions of these two parameters: transit frequency (fT) and maximum frequency of oscillation(Fmax). fT: Frequency at which current gain become 0 dB, Fmax: Frequency at which power gain becomes 0 dB.
What I don't know is that which one to use when..is it like I should look at Fmax while designing power amplifiers and fT is for voltage/current amplifiers.
Also fT and Fmax ,are they defined only for single transistor or can they be calculated for any circuit, which has many transistors in it. Thanks

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2. ## Re: what is the significance of the transit frequency and maximum frequency of oscill

Originally Posted by circuitslave
fT: Frequency at whi Fmax: Frequency at which power gain becomes 0 dB.
No.

Show me your power gain definition.
Can you surely understand power gain ?

Originally Posted by circuitslave
What I don't know is that which one to use when..is it like I should look at Fmax while designing power amplifiers and fT is for voltage/current amplifiers.
No.
fT is useful for frequency divider at most.

Originally Posted by circuitslave
Also fT and Fmax ,are they defined only for single transistor
Yes.

However you can define 3dB bandwidth for any circuit based on any gain which you define.

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3. ## Re: what is the significance of the transit frequency and maximum frequency of oscill

fT : The Frequency where the Short Circuited Current Gain drops to Unity of Active Device ( BJT or MOS etc.)
fmax: Maximum Available oscillation Frequency that is defined by Intrinsic Elements of the Active Device.( see. Guillermo Gonzales )

Neither fT nor fmax. is a Direct Design Metric of an Power Amplifier. OK, fT should be as much as possible but it's not enough.There are other intrinsic and extrinsic elements to be considered.

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5. ## Re: what is the significance of the transit frequency and maximum frequency of oscill

they are only roughly related.
More important...is the negative resistance of your active device higher in value than the resistance of your resonant tank circuit.

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6. ## Re: what is the significance of the transit frequency and maximum frequency of oscill

fmax is the frequency where the maximum gain of a transistors (maximum gain --> conjugate matching at BOTH input and output) becomes zero. Beyond this point you will be getting lesser power output than the power you put in.
fT is the transit frequency where the current gain becomes zero. There are relationships between fT and fmax and higher fT of course means higher fmax. Goes without saying that for PA design, the most important metric is fmax.
fT is more useful for a bipolar amplifier than for a MOSFET amplifier (unlike fmax which is independent of the choice of the particular type of device).

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