18-05-2017, 02:02 PM
(This post was last modified: 18-05-2017, 02:03 PM by jimmc101.
Edit Reason: punctuation
)
Maybe I should modify my assertion to say that a bipolar transistor without any external feedback is fundamentally a current amplifier.
Even a common base stage will be horribly non-linear if fed from a voltage source (ie a low impedance).
Adding a series resistance to the input will reduce the non-linearity but then the resistor is effectively a voltage to current converter.
To generalise for a common emitter transistor, adding external negative feedback can stabilise either the current or voltage gain as desired.
To stabilise the voltage gain add an emitter resistor (series derived series feedback), the limiting case for this is the emitter follower with 100% voltage feedback. (not 100% voltage gain).
To stabilise the current gain add a resistor from collector to base (shunt derived shunt feedback), the limiting case for this is common base with 100% current feedback. (not 100% current gain).
Certainly I would regard a common emitter transistor with an emitter resistor as Jeffrey described i.e. a voltage amplifier with a parasitic input current but I would not apply this to the transistor on its own.
Jim
Even a common base stage will be horribly non-linear if fed from a voltage source (ie a low impedance).
Adding a series resistance to the input will reduce the non-linearity but then the resistor is effectively a voltage to current converter.
To generalise for a common emitter transistor, adding external negative feedback can stabilise either the current or voltage gain as desired.
To stabilise the voltage gain add an emitter resistor (series derived series feedback), the limiting case for this is the emitter follower with 100% voltage feedback. (not 100% voltage gain).
To stabilise the current gain add a resistor from collector to base (shunt derived shunt feedback), the limiting case for this is common base with 100% current feedback. (not 100% current gain).
Certainly I would regard a common emitter transistor with an emitter resistor as Jeffrey described i.e. a voltage amplifier with a parasitic input current but I would not apply this to the transistor on its own.
Jim
