enter image description here

enter image description here

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

  • \$\begingroup\$ en.wikipedia.org/wiki/Bipolar_transistor_biasing \$\endgroup\$ – Bruce Abbott Aug 8 '17 at 18:04
  • \$\begingroup\$ Microphones often have special requirements. So you will want a pre-amp stage. You need to specify the microphone type, at least. Also, a speaker similarly has it's own special requirements. So you will want a power output stage for the speaker. In between these, you could consider your simple common emitter voltage stage, I suppose. But you aren't going to get all this work done with a single BJT. \$\endgroup\$ – jonk Aug 8 '17 at 18:33
  • \$\begingroup\$ What do you mean by pre-amp? Something that takes a .05 volt swing and turns it into a .8 volt swing. This being followed by another stage that takes a .8 volt swing and turns it into a 2.5 volt swing. I was wondering if it can be done with one bjt with the appropriate resistors. I was playing with values and suspect that to do this the resistances become very large and the collector current because extremely small \$\endgroup\$ – Jeffrey Edward Messikian Aug 8 '17 at 19:52
  • 2
    \$\begingroup\$ I think they wrote a song about blue on black. Very hard to see. \$\endgroup\$ – Blair Fonville Oct 28 '17 at 19:35
  • \$\begingroup\$ @JeffreyEdwardMessikian Didn't see your comment until today. You didn't use my name in the comment. Sorry about that. The pre-amp is designed for the transducer itself. Transducers have a wide variation of physical behaviors -- their purpose is to convert some physical phenomena into electrical signals (or the reverse, as the case may be.) Special considerations are often required when designing a circuit, which must take into account the physical details of the transducer. The pre-amplifier takes note of these details, appropriately. It's not just a matter of gain. \$\endgroup\$ – jonk Feb 17 '20 at 7:36

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.


After some digging I found that the following link helped me out the most.


In particular, the article describes the gain of the above circuit to be Rc/R4. The article related collector current to base current to emitter resistor size to gain.

Saying that the gain is Rc/R4 in my above circuit is all I think I needed.


A single transistor with a voltage gain more than about 100 will have horribly high distortion. 50 years ago two transistors made a reasonable mic preamp. Today an opamp is used.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.