[SOLVED] applying a dc offset to a small signal ac input

[krazyfencer]

Hello,

I am trying to create a really basic pre-amplifier as a first step of constructing an AM receiver. The first problem that I have encountered is adding a dc offset to the small signal ac input to bias the transistor in the pre-amplifier into the linear range. I hope that made sense, I'm really just getting into electronics. I read in a previous thread (circa 2005) by tofinhk that you want to add a capacitor between the ac input and the dc source.
Here is what I've currently got (where R3 would actually be the base of the transistor amplifier):


My questions:
1) is this a reasonable way to add a dc offset to an ac source?
2) is the capacitor there to prevent the dc current from going into the ac source?
3) are the ac source (in my case an antenna) and dc source supposed to share a common ground?
4) do you need an inductor to prevent the ac source from going into the dc source?

I hope this doesn't read too long, I am as new to forums as electronics. Thanks for the help!

 

[chuckey]

1. yes
2. yes
3. yes
4. No
Frank

 

[jiripolivka]

In transistor amplifiers we need to adjust the DC voltage and current through the transistor to get an optimum operating point. This is done by the resistors in your picture and the structure is called the "DC bias network".
When this is done, you need to add an AC or audio or RF signal to transistor base without disturbing the DC setting.
One way is shown in your schematic- use a capacitor coupling. In a similar way you can handle the output, use a capacitor coupling.
Another way is to use a transformer; you can see both methods used in "old" AM/FM radio schematics quite clearly.

 

[krazyfencer]

Thanks, that was just what I was looking for!