Hello,
I'm trying to build a primitive common-emitter signal amplifier, the textbook one with collector and emitter resistors RC and RE, and then a voltage divider (resistors R1 and R2) to set the voltage for the base. The immediate mission is to set up a circuit with a Q point that keeps VCE to half the source voltage.
But I am finding that, while I believe I am doing the arithmetic correctly, the voltage drop across RE is not 1V like I want, but 0.05V. And VB is around 0.83V, not the 1.6V or 1.7V I was expecting. And VCE is maybe 0.02V.
What I think is happening is, my voltage divider is getting skewed by the transistor, and contrary to my wishes, much more current is flowing through the base than through R2. But that is to be expected, since RE is so much smaller than R2, right? Does this sound like anything anyone else has experienced, or does it sound like I've simply built it wrong? Suggestions, for example using a diode in place of R2 to force the voltage upward?
Here are specs:
Transistor: 2N3904
VCC: 9V
Desired IC: 10 mA
Desired voltage across RE: 1V
Base current that (I believe, per datasheets) will give me the desired Q point (VCE = 4.5V): 40 uA
RC: 350 Ohms (I used 330 Ohms)
RE: 100 Ohms
R1: 16.5k (I used 18k)
R2: 4.2k (I used 4.7k)
I'm trying to build a primitive common-emitter signal amplifier, the textbook one with collector and emitter resistors RC and RE, and then a voltage divider (resistors R1 and R2) to set the voltage for the base. The immediate mission is to set up a circuit with a Q point that keeps VCE to half the source voltage.
But I am finding that, while I believe I am doing the arithmetic correctly, the voltage drop across RE is not 1V like I want, but 0.05V. And VB is around 0.83V, not the 1.6V or 1.7V I was expecting. And VCE is maybe 0.02V.
What I think is happening is, my voltage divider is getting skewed by the transistor, and contrary to my wishes, much more current is flowing through the base than through R2. But that is to be expected, since RE is so much smaller than R2, right? Does this sound like anything anyone else has experienced, or does it sound like I've simply built it wrong? Suggestions, for example using a diode in place of R2 to force the voltage upward?
Here are specs:
Transistor: 2N3904
VCC: 9V
Desired IC: 10 mA
Desired voltage across RE: 1V
Base current that (I believe, per datasheets) will give me the desired Q point (VCE = 4.5V): 40 uA
RC: 350 Ohms (I used 330 Ohms)
RE: 100 Ohms
R1: 16.5k (I used 18k)
R2: 4.2k (I used 4.7k)