How to bias a basic circuit?

[Squiggles333]

Hi everyone. I've been trying to get my head around biasing transistor circuits and I'm feeling pretty stumped. I have my multimeter and my formulas, I've read all I can and watched loads of vids online but I'm still stumped about how to go about it myself.

Take the lpb1 for example. I've followed a vid where someone sets the bias of this circuit from start to finish and I understand that the goal is to get the collector voltage as close to 4.5v as possible. Why should this circuit be biased at 4.5v however? Is this based on the transistor used and something you can find from a data sheet? Or is it collective opinion that it just sounds "best" at that voltage?

I'm wondering because I'm experimenting with transistor fuzz and distortion type circuits on the breadboard. My bias technique as of now is to replace resistors with pots and use my ears to find the sweet spot with the most volume, bass and ideal gain. After that I use my multimeter to find out the values and replace the pots with normal resistors. Without a reference for "what the collector voltage should be" I'm unsure of what I should be trying to achieve with bias other than a sound that I like, or how to decided without my ears what would be an ideal starting point for bias.

I'm also a bit unclear on voltage at emitter, the video I watched used a graph from a data sheet ("on voltages") and made an approximation based on the graph to get this value. I suppose I would repeat this for whatever transistor I'm using but I'm unsure what I'm trying to understand from the data sheet at this point.

In the same video it's also assumed that the current is between 300-500 micro amps. Is this an assumption we make for most pedals? Or just the lpb1?

I'm particular I'm trying to learn how this works whilst building a harmonic jerkulator. I've swapped out the stock transistors so it would be really good to feel confident about how the rest of the circuit is working with them, or how to go about deliberately causing a change.

I can appreciate this is whole big can of noob worms, so sorry for all the questions and thanks in advance to anyone that helps! 

[11-90-an]

Sadly I too am a noob, quite worse, even, as I forget my Ohm's Law almost always...

Anyway the only thing i am "sure" about is the 4.5v collector voltage, which is set so that the output can have equal "headroom" on both top and bottom... in other words gives the "maximum amplificiation" or the loudest output...

Probably didn't make sense right? Sorry... not good at explaining things at all...

Here's a small illustration/example

[iainpunk]

i don't think it a good idea to always bias at 4,5V, since the input signal and the emitter voltage takes up part of the headroom.
i rule of thumb use is to bias between 65%-80% of the supply voltage.
some types of circuits sound better biased higher or lower, but that depends on the clipping characteristics. a range master is generally biased at 7 to 7,5V.

cheers, Iain

[Squiggles333]

Sadly I too am a noob, quite worse, even, as I forget my Ohm's Law almost always...

Anyway the only thing i am "sure" about is the 4.5v collector voltage, which is set so that the output can have equal "headroom" on both top and bottom... in other words gives the "maximum amplificiation" or the loudest output...

Probably didn't make sense right? Sorry... not good at explaining things at all...

Here's a small illustration/example

Hey, a good way to learn is to teach! From what else I understand this makes sense to me. Especially as a visual. I have something in my notes about the room for either peak being equal hense the need to bias to 4.5 volts. What I dint understand is why that's the case for this circuit. Take the range master for example, it's recommended biased to 6.9v (abouts, I think). I'd like to know what about my circuit informs me of this!

Thanks for your input

[Squiggles333]

i don't think it a good idea to always bias at 4,5V, since the input signal and the emitter voltage takes up part of the headroom.
i rule of thumb use is to bias between 65%-80% of the supply voltage.
some types of circuits sound better biased higher or lower, but that depends on the clipping characteristics. a range master is generally biased at 7 to 7,5V.

cheers, Iain

No, I wouldn't have thought that was the done thing. I've seen the bias recommendations for the range master. I'm wondering what about the range master or the lpb1 informs you what the ideal bias is. Is it something from the scehm/data sheet? Both these circuits run from a 9v supply correct? So the rule of thumb doesn't apply to at least these two examples? Or am I misunderstanding supply voltage?

Thanks, again, Iain!

[antonis]

It isn't easy to cover "transistor bias" in single thread..

Collector quiescent voltage (for a Common Emitter amp) depends on what we want for transistor to do..

For small signal symmetrical amplification (distorted or not), Collector should sit at (V_CC + V_E)/2 ..
(e.g. for 1k Emitter resistor and 1mA Collector current, Collector resistor should be 5k..)

For Emitter follower (Common Collector or buffer), Emitter should sit around V_CC/2..
(considering signal amplitude (Vp-p) much smaller than V_CC or Emitter resistor much smaller than load for almost equal current sourcing/sinking capability..)

The above stand for symmetrical output waveform and for single (or last stage) transistor amp..

[iainpunk]

both are vintage pedals, not designed to be too linear, but to boost the sound MUSICALLY, so a bit of distortion isn't a problem
the bias if the range master is deliberately high to make it clip quite quickly, this gives even more asymmetry to the wave and warmth to the tone.

the 4.5V isn't actually the most optimal bias point:
- the voltage aver the collector resistor is 4.5V
- 4.5/10000*390=0,1755V at the emitter,
- there is at least 0,35V over CE (about half BE)
- the input signal that could be as high as 0,8 Vpp to calculate hedroom, add 0,4 V to the base voltage
from just under 1V to 9V is the headroom, so 4,5 v isn't ideal, but 5 is closer.
but the transistor is far from linear and 4,5V was probably better sounding to the engineers

to be honest, in pedal building and/or if there is no (global) feedback loop, the ''ideal'' bias voltage is often irrelevant, and its better to bias by ear.

the harmonic percolator should be biased by ear, but a good starting point is to get the node of both emitters and the capacitor as near to 4.5v as possible.

cheers, Iain

[Squiggles333]

It isn't easy to cover "transistor bias" in single thread..

Collector quiescent voltage (for a Common Emitter amp) depends on what we want for transistor to do..

For small signal symmetrical amplification (distorted or not), Collector should sit at (V_CC + V_E)/2 ..
(e.g. for 1k Emitter resistor and 1mA Collector current, Collector resistor should be 5k..)

For Emitter follower (Common Collector or buffer), Emitter should sit around V_CC/2..
(considering signal amplitude (Vp-p) much smaller than V_CC or Emitter resistor much smaller than load for almost equal current sourcing/sinking capability..)

The above stand for symmetrical output waveform and for single (or last stage) transistor amp..

Fair point I'm often finding my questions have answers that all go beyond me still! I hadn't thought to research this based on the function of the transistor, thanks for bringing that to my attention.

Can I just ask please, with the harmonic jerkulator, Q1 is for small signal amplification, and Q2 is the emitter follower?

Thanks for your help!

[Squiggles333]

both are vintage pedals, not designed to be too linear, but to boost the sound MUSICALLY, so a bit of distortion isn't a problem
the bias if the range master is deliberately high to make it clip quite quickly, this gives even more asymmetry to the wave and warmth to the tone.

the 4.5V isn't actually the most optimal bias point:
- the voltage aver the collector resistor is 4.5V
- 4.5/10000*390=0,1755V at the emitter,
- there is at least 0,35V over CE (about half BE)
- the input signal that could be as high as 0,8 Vpp to calculate hedroom, add 0,4 V to the base voltage
from just under 1V to 9V is the headroom, so 4,5 v isn't ideal, but 5 is closer.
but the transistor is far from linear and 4,5V was probably better sounding to the engineers

to be honest, in pedal building and/or if there is no (global) feedback loop, the ''ideal'' bias voltage is often irrelevant, and its better to bias by ear.

the harmonic percolator should be biased by ear, but a good starting point is to get the node of both emitters and the capacitor as near to 4.5v as possible.

cheers, Iain

Iain thank you so much this has helped me to understand the function of bias much better. I wasn't aware of its function with the rangemaster and the musical properties.

I currently have the circuit set up to how I like it by ear, but the nerd in me wants to try the maths to understand it better. Thanks for all your help!

[iainpunk]

Can I just ask please, with the harmonic jerkulator, Q1 is for small signal amplification, and Q2 is the emitter follower?

both transistors are signal amplifiers. an emitter follower ALWAYS has the output at the emitter of the transistor, but Q2 has the output connected to the collector so its a gain stage^{note 1}

the bias of a Jerkulator (where both emitters meet)should be 4.5V, a percolator should be biased around 2V^{note 2}

the math for ''perfect'' bias is only to bias that the maximum swing, this is a good thing if there is feedback to linear-ise the response of the gain stage, but in an non-linear gain stage, bias should be done by ear.
if you want to know the ''correct'' way to bias, check this link:
https://www.electronics-tutorials.ws/amplifier/transistor-biasing.html
there is everything you need to know for transistor bias.

cheers, Iain

edit:
Note 1 : unless there is the same impedance at both the collector and emitter, its is a gain stage, if the impedance at the emitter is bigger, its an attenuator, and if they are equal, its an inverting buffer.
Note 2 : 2V is a good starting point, but bias it to your personal taste

[antonis]

IMHO, Harmonic Per/Jer-culator isn't one of the most "elementary" bias studing circuits..  

[iainpunk]

IMHO, Harmonic Per/Jer-culator isn't one of the most "elementary" bias studing circuits..

true, but if you know how to bias one of those properly, you can bias anything

cheers, Iain

[antonis]

if you know how to bias one of those properly, you can bias anything

To know how to bias one of those properly, you should already know the reason for it..

P.S.
If you like some philosophical debate, plz be my guest..
(but take into account my country of origin..)