In a common-emitter amplifier, the input ac signal source is 0.5 Vpp, 1000 Hz. T
ID: 1921504 • Letter: I
Question
In a common-emitter amplifier, the input ac signal source is 0.5 Vpp, 1000 Hz. The output ac signal is 1.88 Vpp. If voltage gain is equal to Vout/Vin, then 1.88Vpp/0.5Vpp = 3.76. I believe this should be -3.76 because there is a 180 degree phase reversal between the input and output ac waveforms. I also calculated the dynamic emitter resistance using the formula r'e = 25mV/emitter current. The emitter current is 1.9 mA, so r'e = 13.16 ohms. Also in a textbook I saw that voltage gain is approximately equal to -Rc/r'e. If the collector resistor is 2 kohms, does this mean that the voltage gain is also found by -2ohms/13.16 ohms = -151.976? If so, I'm confused about the different voltage gain answers I'm finding using Vout/Vin, and the other formula -Rc/r'e.Explanation / Answer
The voltage gain Vout/Vin that you calculated is at f=1000Hz The formula to calculate the voltage gain Av = - Rc/re hold good for mid-freuqency range, where the gain is constant. So you could be finding gain at two different frequencies (low and mid frequency) , thats why two different answers