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Re: what are the differences between source follower and ...
my friend... Source follower are on FET (field effect transistor) while the emitter follower are on BJT (bipolar unijuction transistor). usually emitter follower is used on current amplyfication, while source follower is commonly used as logic inverter...
...yes ,i konw that ,but it seems impossibe the source follower can be used as a logic inverter. don't they transfer the signal without changing its polarity?
i just want to know why we usually say the buffer effect of ef are better than sf
Re: what are the differences between source follower and ...
Eric G. said:
my friend... Source follower are on FET (field effect transistor) while the emitter follower are on BJT (bipolar unijuction transistor). usually emitter follower is used on current amplyfication, while source follower is commonly used as logic inverter...
Re: what are the differences between source follower and ...
finfoun,
With regard to your comment "i just want to know why we usually say the buffer effect of ef are better than sf ", you must analyse the circuit and the device (bjt or FET) you are using. The desired properties of a buffer are high input resistance, low output resistance.
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For the FET, the input resistance is very high (essentially an open circuit at low frequencies). The output resistance is the reciprocal of the forward transcunductance.
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For a bjt, the calculations are more complicated. The intrinsic base emitter resistance rbe is equal to 26/Ic, where Ic is the collector current in mA. Let Re = the physical emitter resistor. Let Ri = the source resistance presented by the input to the buffer. Let Ro = the output resistance of the buffer. Let hfe = the common emitter current gain of the transistor (Beta). Let Rl equal the load resistance. Then
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Ro = ((Ri/(hfe+1)) + rbe)) || Re, Where || means "in parallel with".
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The input resistance = ((Re||Rl) + rbe) (hfe+1)
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As you can see from the equations, higher hfe increases the input resistance, and lowers the output resistance. In addition the source resistance effects the output resistance. Conversely, the load resistance affects the input resistance.
Regards,
Kral
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