Common collector amplifier

  • Thread starter eric336
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  • #1
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Normally in common collector amplifier, Rc (collector resistor) is absent or otherwise it is short circuited by capacitor.
My question is: If Rc is present and not short circuited, does it affect the voltage gain? If it does, how it affects? Appreciate if someone can present clearly in equivalent circuit. Thanks.
 

Answers and Replies

  • #2
uart
Science Advisor
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Normally in common collector amplifier, Rc (collector resistor) is absent or otherwise it is short circuited by capacitor.
My question is: If Rc is present and not short circuited, does it affect the voltage gain?
No.

If it does, how it affects?.
It affects (reduces) the maximum output voltage swing but does not alter the gain.
 
  • #3
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Thanks uart, appreciate it very much.
 
  • #4
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Ask yourself what the resistors do in a transistor amplifier.

Base resistors establish the DC level of the base.

The emitter resistor (if present) raises the voltage of the emitter to some desired level depending upon base bias level.
It also serves to limit the current flow through the transistor.

If there is no emitter resistor the transistor is at risk of excess current flow unless there is a collector resistor.

The above is true of all configurations including a CC configuration - also called an emitter follower.

The voltage gain of the emitter follower is just less than 1. The output voltage swing is developed across the emitter resistor in parallel with the load resistor or just across the load resistor if no emitter resistor is present.
So an emitter resistor affects the current gain as it forms part of the (effective) load. However it does not affect the voltage gain.
 
  • #5
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I knew the role of emitter resistor is to stabilize Q-point.

As far as I concern, the voltage gain of CC should approximately equal to 1, after all CC is just used as buffer, we don't want so much gain. Is it possible it is less than 1, like you said?
 

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