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About Transist Frequency

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Hanumantas

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Hello All,

I have seen lot of activities in this forum so joined recently. I am student in Communication microelectronics, I have some doubts on Transist frequency.

Why Transist Frequency uses small signal analysis?

Why it is used as parameter to compare devices like MOSFET and FET?

Thanks in advance..
 

Hanumantas said:
Hello All,
I have seen lot of activities in this forum so joined recently. I am student in Communication microelectronics, I have some doubts on Transist frequency.
Why Transist Frequency uses small signal analysis?
Why it is used as parameter to compare devices like MOSFET and FET?
Thanks in advance..
Click to expand...

The definition of the transit frequency is based on the ratio of two signals (gain: output/input).
It makes no sense to define a ratio for two signals having a different form (wave shape), which is the case for non-linear transfer functions.
Because all active devices can be considered as linear only for small signals (in fact, that is the definition of "small signal operation") the definition of the transit frequency applies for small signals only.
 
The transition frequency is basically the highest frequency that a transistor can operate with unity gain, ie with amplification capabilities.
If you cannot amplify, you are better off using a resistor divider.
And since we are talking about highest frequency, we can start comparing between different processes.
 
Hello All Thanks for Help.. Please let me know How a circuit works in Small Signal and Large Signal? I mean how the analysis goes.. Why small signal analysis is done inspite all real time circuits are non linear?
 

By learning how to make a linear design, you can simplify and improve the prediction without fancy Sim software.
This is useful for RF gain calculations but not useful for Switch operation where the gain is much less.

For example small signal gain is often a function of impedance ratios in feedback designs such as CB, CE, OA, etc.
This also makes assumptions with variations in hFE, temp and Vcc. In this case monte-carlo tolerance simulation works best.

Here is SS analysis of FETs
 
Last edited:

Hanumantas said:
Hello All Thanks for Help.. Please let me know How a circuit works in Small Signal and Large Signal? I mean how the analysis goes.. Why small signal analysis is done inspite all real time circuits are non linear?
Click to expand...

Yes, you are right - all real circuits are non-linear. However, for most of the tasks in analog signal processing (amplification, filtering, ...) LINEAR operation is desired.
Thus, parts and circuits are operated only within their "quasi-linear" characteristic relationships.
Remember: Electronic basics (laws of Kirchoff and Ohm, superposition, definitions for gain and impedance,...) assume linearity. That means: Application of these rules and laws applies only as long as (quasi-)linearity is ensured.
 

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