In the interest of making an oscillator, I made the above circuit after reading about gain, current flow, active region, and how an NPN transistor works.
I then put the circuit in LTSpice and note the input and output voltages.
The input is in blue, the output is in green.
I see that there is gain. For the oscillator, where all I think I need is some amout of positive gain, this should probably work. I then started thinking about microphoens and speakers and how to dial in an exact amount of gain given a mic.
So, my question is this, lets say I have a signal that oscillates += .005 volts and I want it amplified to swing between +9 volts and 0 volts, what equations would get me the appropriate resistor values. I would set the middle of the active region to be Vcc/2 as a Vce reference. This would be 4.5 volts. How do I best determine the resistors to get to this level?
Lets say, I then change the input swing between += .2, how do I reset the resistors to get to the appropriate swings in the output region?
When I say swing, I mean above and below a DC bias voltage on the base.
I feel I should be able to adjust this to exactly to whats needed by choosing the appropriate values of resistors. Thanks, Jeff
Best Answer
The maximum voltage gain from a single bipolar is VDD / 0.026.
Thus your 9v example has max gain of 9/0.026 or 9 * 39 ~~ 360.
Yet to convert 0.005 volt into 9vpp you need gain of 9/0.005 = 9*200 = 1,800.
Summary: a single bipolar is not the answer.