V RC is the potential difference between V CC and V CE and V CE cannot go below

# V rc is the potential difference between v cc and v

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V RC is the potential difference between V CC and V CE , and V CE cannot go below V BE in order for the transistor to be in active region. T C C v V R I A T RC v V V A T BE CC v V V V A
CHAPTER 5: BJT as Amplifiers 30 Tradeoff between Voltage Gain and Headroom
CHAPTER 5: BJT as Amplifiers 31 I/O Impedances of CE Stage When measuring output impedance, the input port has to be grounded so that V in = 0. r i v R X X in C X X out R i v R
CHAPTER 5: BJT as Amplifiers 32 CE Stage Trade-offs
CHAPTER 5: BJT as Amplifiers 33 Inclusion of Early Effect Early effect will lower the gain of the CE amplifier, as it appears in parallel with R C . O C out O C m v r R R r R g A || ) || (
CHAPTER 5: BJT as Amplifiers 34 Intrinsic Gain As R C goes to infinity, the voltage gain reaches the product of g m and r O , which represents the maximum voltage gain the amplifier can have. The intrinsic gain is independent of the bias current. T A v O m v V V A r g A
CHAPTER 5: BJT as Amplifiers 35 Current Gain Another parameter of the amplifier is the current gain, which is defined as the ratio of current delivered to the load to the current flowing into the input. For a CE stage, it is equal to . CE I in out I A i i A
CHAPTER 5: BJT as Amplifiers 36 Emitter Degeneration By inserting a resistor in series with the emitter, we “degenerate” the CE stage. This topology will decrease the gain of the amplifier but improve other aspects, such as linearity, and input impedance.
CHAPTER 5: BJT as Amplifiers 37 Small-Signal Model Interestingly, this gain is equal to the total load resistance to ground divided by 1/g m plus the total resistance placed in series with the emitter. E m C v E m C m v R g R A R g R g A 1 1