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Projects => Beginner Project => Topic started by: tonepoet on February 01, 2004, 03:46:35 PM

Title: Biasing transistor...................how?
Post by: tonepoet on February 01, 2004, 03:46:35 PM
I probably shouldn't have waited so long to ask this, but my work has kept me from spending very much time on the computer. I built my NPN Boost, and the bypass and the light worked, but I was getting a crummy gated and very weak overdrive. Not really an overdrive, but like an Orange Squeezer in a very bad way. I flipped the transistor around, didn't matter. So I ordered the one stated in the project from Small Bear and got the same response. I am pretty sure that it is an unbiased transistor. What do I do? I know that I have to measure the legs or something, but what does that tell me?
Title: Biasing transistor...................how?
Post by: smoguzbenjamin on February 01, 2004, 04:04:45 PM
Well, first of all it's a misbiased transistor ;) unbiased would be... well :? I dunno. Anyway, in an NPN transistor, (if I am correct, not sure though this is a tough question) the collector sould be high, the base should be at a diode drop above the emitter (0.6v) and the emitter should be 0v. Try using a trimpot between Base and Collector...
Title: Re: Biasing transistor...................how?
Post by: KR Audio on June 07, 2020, 11:19:10 PM
A pic of the schematic would help... is this a circuit you designed or is one from the net? Biasing a transistor's base at half the voltage of the collector is a good rule of thump.
Title: Re: Biasing transistor...................how?
Post by: bluebunny on June 08, 2020, 04:57:00 AM
Why the numerous necro-posts?  This one must win a prize!  :icon_eek:
Title: Re: Biasing transistor...................how?
Post by: antonis on June 09, 2020, 05:22:23 AM
Biasing a transistor's base at half the voltage of the collector is a good rule of thump.

If ONLY it's an Emitter follower..  :icon_wink:
(which I presume it isn't the case here..)

But, let thread rest in peace.. :icon_cool:
Title: Re: Biasing transistor...................how?
Post by: KR Audio on June 09, 2020, 11:04:23 AM
No, it will also work for a grounded emitter amplifier also. Some SPICE Pics attached
(https://i.postimg.cc/fkSH7pD6/2020-06-09-11-01.png) (https://postimg.cc/fkSH7pD6)

(https://i.postimg.cc/v4hXHdt6/2020-06-09-11-02.png) (https://postimg.cc/v4hXHdt6)
Title: Re: Biasing transistor...................how?
Post by: KR Audio on June 09, 2020, 11:06:03 AM
No, it will also work for a grounded emitter amplifier. Some SPICE Pics attached
(https://i.postimg.cc/fkSH7pD6/2020-06-09-11-01.png) (https://postimg.cc/fkSH7pD6)

(https://i.postimg.cc/v4hXHdt6/2020-06-09-11-02.png) (https://postimg.cc/v4hXHdt6)

Biasing a transistor's base at half the voltage of the collector is a good rule of thump.

If ONLY it's an Emitter follower..  :icon_wink:
(which I presume it isn't the case here..)

But, let thread rest in peace.. :icon_cool:
Title: Re: Biasing transistor...................how?
Post by: antonis on June 09, 2020, 03:10:04 PM
Correct me if I'm wrong but Base voltage sets Collector current, hence Collector voltage..

So, you can't bias Base in advance of Collector voltage.. :icon_wink:
Title: Re: Biasing transistor...................how?
Post by: KR Audio on June 09, 2020, 04:34:32 PM
I wouldn't say it "sets" the collector current but allows current to follow i.e. biasing the transistor.
The Collector resistor has some influence but the emitter resistor has more effect in terms of biasing, current follow, and voltage gain.  It is indeed possible to bias a transistor by using half of the collector voltage as a benchmark when making a common emitter amp. Both Common Emitter amps and Emitter followers use a voltage divider at the base for this purpose. Though I see what you mean in a temporal sense that you have to have base bias before you know what the voltage at the collector will be but that's why its a rule of thump... its a concept to guide you in setting the amp up to function in its most basic form, then you can tweak it from there. You don't need to know the specfic collector voltage because the voltage divider will render half or near half of that as a general rule whiich wil bias the circuit.