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Electronics => Beginners => Topic started by: Simon123 on December 28, 2013, 12:03:14 pm

Title: Amplifier
Post by: Simon123 on December 28, 2013, 12:03:14 pm
Hello!

I built this https://lh3.googleusercontent.com/-zE1Eh1U18MU/UTyhJzfwVEI/AAAAAAAAAGU/Qrq0BWrF6YA/s1600/2013-03-08+17.12.50.jpg
amp a year ago, but it seems, that something is missing here.
When i look at the other schematics i see emiter capacitor and resistor, but i dont have any.
That can make my amp unstable, so i dont know if i should improve anything?

Thanks.
Title: Re: Amplifier
Post by: mrkev on December 28, 2013, 04:11:45 pm
...
When i look at the other schematics i see emiter capacitor and resistor, but i dont have any.
That can make my amp unstable, so i dont know if i should improve anything?...

Is it working? If yes, don't bother :).

In your solution, you want to have like 6V at the output 270R resistor with 0V~ input. (So when the input voltage is negative, your transistor have some space to close.) This means that bias current trough AC176 should be 6V/270R = 22,2mA.
By a datasheet, h21e (=gain) of AC176 should be in range 50 - 250. And your adjustment of bias point (by the 100k trimmer) is amplified by this number too.

Since the voltage drop over BE of transistor and h21e can slightly change over time and with temperature, this amp can move from it's bias point quite quickly (it won't be unstable, but it will disort one half of the signal).

The resistor in emiter actively reduce the h21e. So even with temperature change, bias point will stay at right place. This would ofc. reduce gain of whole amp. That is why you also add an capacitor, which acts like "short" for AC signal. Anything with a higher frequency will have the full h21e, but DC bias will have reduced h21e.
Title: Re: Amplifier
Post by: filip_cro on December 28, 2013, 05:25:22 pm
Like mrkev said you could use RE and CE for negative feedback.
Title: Re: Amplifier
Post by: Simon123 on December 28, 2013, 08:09:04 pm
Thanks!
Only thing is, i dont know what you mean by is bias current, when you are talking about current threw 270R resistor?
And the amp is working.
Title: Re: Amplifier
Post by: mrkev on December 28, 2013, 09:09:39 pm
Thanks!
Only thing is, i dont know what you mean by is bias current, when you are talking about current threw 270R resistor?
And the amp is working.
Sorry if that wasn't clear. I started with C-E bias current, which is supposed to be about 22,2mA (since with 0 input, you should get 1/2vcc at output). For that, you need cca 50 - 250 (gain of transistor) lower B-E bias current. Since that gain can change (with time&temperature), your bias point (which means the state of transistor with 0V input) can shift somwhere else.
It's just a basic circuit and it will work for a wide range of bias points, so it's not that hot problem... If you wanna know more, just google up: 'class A amplifier'.
Title: Re: Amplifier
Post by: Simon123 on December 28, 2013, 09:29:40 pm
Thanks