Newbie transistor biasing

Phenlight · Post by **Phenlight** » 16 Mar 2013, 20:27

Hi all I'm a 43 yr old newbie trying to get my head around basing lol. Please help lol I have blah blah transistor and want to bias it using a standard 9v battery what do I need off the data sheet I know I need hfe but don't know the where to go to start calculations lol I've read I need half the voltage please say in lay mans terms. I know I have a bc108c I could use. Thx in advance
Phenlight

Phenlight · Post by **Phenlight** » 16 Mar 2013, 23:13

Please a little help as not to pull out the last hairs I have left on my head lol

CHEEZOR · Post by **CHEEZOR** » 17 Mar 2013, 07:36

I'm sure someone can help, but you have to be patient. The forum doesn't have the kind of traffic that other topics might draw. Someone will get to your question, I'm sure. I would, but I don't know enough to help. Sorry.

Oh, and welcome to the forum!

Phenlight · Post by **Phenlight** » 17 Mar 2013, 08:33

Thx I await eagerly.

Nocentelli · Post by **Nocentelli** » 17 Mar 2013, 12:23

Phenlight wrote:I have blah blah transistor and want to bias it using a standard 9v battery what do I need off the data sheet I know I need hfe but don't know the where to go to start calculations lol I've read I need half the voltage please say in lay mans terms. I know I have a bc108c I could use

By "half the voltage", I'm assuming this is the oft-repeated advice that the collector should be idling (i.e. no input at the base) at around half the supply voltage: This means for a 9v circuit, when you measure the voltage on the collector, you would be trying to get around 4.5 volts: This is usually achieved with a fixed resistor of a value appropriate to the specified transistor for that particular circuit. The fuzzface Q2 transistor should supposedly be set for around 4.5v, but it's all down to personal taste since the different sounds achieved by different bias points appeal differently to different people. In a fuzz circuit, for example, we often don't necessarily want a nice clean sound.

I've been building pedals for a number of years now, and have yet to delve deep into the theory side of transistor biasing (nor have I particularly felt the need to). I tend to build variations of existing circuits, and whilst I might tweak the values of capacitors and resistors in the signal path to alter tonal characteristics, range of control, or clipping characteristics, I tend to leave the bias resistors as specified, since it has been established they work well. An exception might be in the above-mentioned fuzzface or similar, where an external "bias" pot (variable resistor on Q2 collector) can yield some interesting different sounds, from more firm and clean (and loud), to super-saturated, through gated and glitchy until the transistor no longer passes any signal.

If you have a particular circuit, or type of circuit in mind that you wish to build, I would search for information related to the biasing of that particular circuit. If you want a step-by-step guide of how to bias any transistor in any circuit for any application, I've yet to find one. There are a few online guides to specific circuits that cover biasing, but often they assume the reader has a decent background understanding already - An example on the fuzzface is RG Keen's "Technology of the Fuzzface":http://www.geofex.com/article_folders/f ... fffram.htm

There are simpler explanations online, Beavis Audio has done a nice simple guide to a nice simple single transistor circuit (EHX LPB-1) that would be a suitable testing ground for any experiments you wish to try with your BC108cs on the breadboard: http://www.beavisaudio.com/techpages/HIW/HIW.png

If you want to approach biasing from a mathematical perspective, I came across this tutorial blog which covers a good amount of the theory side of biasing:

http://www.electronics-tutorials.ws/tra ... ran_1.html

Well worth a read.

Do you have a particular circuit, or type of circuit in mind, or are you hoping to design something original from the ground up?

Phenlight · Post by **Phenlight** » 17 Mar 2013, 19:25

Hi thx for your reply. I was given a load of transistors from a old guy who repaired tvs, to many to mention zxt300 bfy50 and many more and I just wanted to say grab a bfy50 and build a circuit boost or whatever. I've read some of the links previous except one I'm going to look at now.

Nocentelli · Post by **Nocentelli** » 17 Mar 2013, 19:59

I would think the BFY50s are rather low gain for a booster circuit: Datasheets seem to suggest very low hFe for this application - I can't find any reference to the ZXT300. BC108c on the other hand would work fine anywhere that calls for a medium-ish gain silicon npn (it's similar to common types like 2N5088, BC548, 2N2222, 2N3904, 2N4401), they can often be used interchangeably in something simple like a single stage boost, or fuzzface/tonebender type fuzz.

friedtransistor · Post by **friedtransistor** » 18 Mar 2013, 01:15

Personally, I like a 2n2222 with a 10k from positive to collector, 100k from collector to base, 15k or 22k from base to ground, emitter to ground, 0.1u from in to base, and 4.7u from collector to out. Just a suggestion

Phenlight · Post by **Phenlight** » 20 Mar 2013, 20:48

friedtransistor wrote:Personally, I like a 2n2222 with a 10k from positive to collector, 100k from collector to base, 15k or 22k from base to ground, emitter to ground, 0.1u from in to base, and 4.7u from collector to out. Just a suggestion

thx I will give that ago