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transistor input impedance, output impedance, and output drain configuration, Q fact

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sys_eng

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1)why is 50ohm impedance? why not 10, 20, 60, 80? what so special about this number?

2)input to the gate of the transistor which very high input impedance, would that overload the signal source at the gate?

3)why open drain configuration is popular? if you connect a load to it then it's unmatched in terms of impedance.
I mean, the load is 50 ohm, but the output impedance of the open drain is Not 50 ohm, so they are unmatched.

4)I read somewhere if its oscillator circuit, the Q factor is improved by having output impedance increased. Is having higher Q factor means having higher resonance frequency therefore higher bandwidth?

5) do you want to operating at resonance frequency? I mean imaginary impedance gone and only the real one exist, so it's most efficient. So you want operating at the most efficient point.
 

1) Impedance matching is usually only important in high frequency digital or RF applications where you need to match impedances to avoid reflections and standing waves in the transmission lines.
Here's a short history for why 50 ohms was selected as a common value for the characteristic impedance but other values such as 75 and 93 ohms are also common.
- - - Updated - - -
2) If the gate is a high impedance why would you think it would load the source?
Just the opposite is the case.

3) There's no reason to match the drain impedance to the load for most applications.

4) Q affects the stability of an oscillator circuit, not its frequency.
A higher Q means a lower bandwidth for a tuned circuit.

5) Operating at the resonant point of an LC circuit is not typically about efficiency, although it is the point where the inductive reactance is cancelled by the capacitive reactance.
It is the point when an LC circuit wants to oscillate.
 

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