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# Biasing a Transistors with resistor values?

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#### walters

##### Advanced Member level 2
biasing resistor

1.) How do u find out what "resistor values" to use when biasing a transistor?

Fuzz Face schematic:
http://www.geocities.com/teleman28056/fuzz_face_schematics.html

2.) Why did they choose these "resistor values" to biasing the transistor in the fuzz face?

3.) How did the designer get these "resistor and capacitor values"?

Maestro Fuzz schematic:

4.) How did the designer get these "resistor and capacitor values"?

5.) Why did they choose these "resistor values" to biasing the transistor in the Maestro fuzz?

lowrey organ schematic

Hi,

If you look at the circuit's you can see that the resistors, resistances of the BJT's and the capacitors will create filters, there is a HPF at the input of the fuz face. The designer just chose component values that would allow the frequencies of interest through whilst meeting the other design criteria (gain, etc.).
To fully understand the choice of resistor/cap, one would have to fully analyze the circuit's (R1 is a feedback resistance in the fuz face, the size of it will have been chosen with reference to R7 and R4) .

If you have a version of SPICE, give the circuit a whirl and change some values around whilst noting there effect on the response.

germanium fuzz schematic

What other design criteria?

Was the Resistance Vaules for the transistor is based on the GAIN?
but how did the designer get these Resistance values?

When changing the Resistance values of the transistor bias ,what does it change?
a.) the Gain?
b.) the circuit design criteria? like what?
c.) the circuit design response? like what?
d.) The bias symmetry?

resistors biasing

The cicuit is a distortion or fuz unit, the design criteria is for the circuit to act as one through the audible range of frequencies. The designer would have followed the standard design pattern of chosing a couple of sensible values (resistors of a few K, instead of 10M or something), then working out the rest from there.
I can't answer the question on what exactly the design criteria were as I didn't design it, and have no way of knowing what the designer was asked to produce/wanted to produce.
When I am talking about the response of the circuit, I am refering to the ac resopnse and general behaviour of the circuit. Simulate it with a sine wave at the input and look at the ac responce, and voltages in the trasient domain
Distortion is commonly created with diode clipping, as in the Maestro (i assume, that's how it operates), this cut's off the peaks of signal giving the raspy distorted tone. The fuz face I assume doeas similar by driving the signal to the rails. Run it through SPICE and you'll very quickly find out how it works.

You can download the ORCAD demo from here, it should allow you to simulate the fuz-face, (I think, I can't quite remember how many nodes it allows).

you could try winSpice or any of the free simulators.

transistor biasing fuzz face

General design rules:
R1 and R2 are the base bias resistors, setting the bias point.
R3 is the collector load resistor.
R4 is the emitter stabilising resistor.
C3 is the emitter decoupling capacitor.
C1 and C2 are coupling capacitors which allow ac signals to pass but block dc.

1.) But how do u get the "values" for the resistors/capacitors?

2.) (a sine wave at the input and look at the ac responce, and voltages in the trasient domain) What am i looking for in the Transient domain?

V
Points: 2
changing resistor value in transistor

Audio Electronics by John Linsley Hood, is a decent book on the subject. It's a few years old now, but It has some good stuff if you're interested in audio circuits.

Added after 16 minutes:

walters said:
General design rules:
R1 and R2 are the base bias resistors, setting the bias point.
R3 is the collector load resistor.
R4 is the emitter stabilising resistor.
C3 is the emitter decoupling capacitor.
C1 and C2 are coupling capacitors which allow ac signals to pass but block dc.

1.) But how do u get the "values" for the resistors/capacitors?

2.) (a sine wave at the input and look at the ac responce, and voltages in the trasient domain) What am i looking for in the Transient domain?

You get the "values", by having a precise idea of what you want the circuit to do, chosing a realistic value of a component and from that designing the rest of the circuit. Other values will work. C1 is decoupling the emitter.
This is more than just a common emitter amplifier. There is no easy answer to what value to pick, electronics just isn't that easy.
In the transient domain, look at voltages throughout the circuit particularly the Base, Collector and Emitter of Q2, the input and output. Change a resistor (anyone) value and look at the effect it has on tthe waveforms. Do this for every component to find out how they effect the circuit. Also look at the frequency domain to see what happens to the bandwidth. This will quickly give you an understanding of how the circuit operates. Then you should have a good idea of how to design such a circuit, including choosing the component values.

biasing transistors with one resistor

What does the feedback resistor do?

Does the feedback resistor change the bias? or symmetry?

(Germanium forward bias is .3 volts and silicon forward bias is .7 volts)

Is it true that forward bias is based on saturation?

Whats the difference between .3volts forward bias VS .7volts forward bias?

whats the difference in saturation or other criteria?

lowrey organ schematics

REFER EDC BY MILLAMN HALKIAS BOOK FOR THE DESIGN. EQUATIONS ARE GIVEN .USE THOSE.

how to find base resistor of transistor

walters said:
What does the feedback resistor do?

Does the feedback resistor change the bias? or symmetry?

(Germanium forward bias is .3 volts and silicon forward bias is .7 volts)

Is it true that forward bias is based on saturation?

Ans:- Certainly not

Whats the difference between .3volts forward bias VS .7volts forward bias?

Ans:- Difference is because of material i.e. germanium and silicon repectively

whats the difference in saturation or other criteria?

Ans:- A simple saying of saturation is full on i.e. emitter and collector and other is cutoff which is full off

over biased germanium transistor

i mean saturation as distortion

I still don't understand how these designers get these component values
and they would know what voltages they need

nte102a transistor fuzz

I still don't understand how these designers get these component values
and they would know what voltages they need

Walters,
This is something that is learned in a classroom. For me it was in second or third semester of college. It blows my mind that people on this board are running spice simulations but have NO IDEA about the theory behind the electronics in their simulations. I am not necessarily talking about you,Walters, I am speaking in general about the many posts that I have read on this board over the past few months. Anyway, Back to transistor biasing, its very easy and if you have at least a half a year of basic electronics (Classroom) theory under your belt, you should be able to figure it out on your own.

pspice germanium transistor

I still don't understand how these designers get these component values
and they would know what voltages they need

can u tell me what i need to know to get this information ?

What are some good tips or short cuts i can use to get this information please?

how to bias a maestro fuzz

The important thing in this circuit is the signal which is passing through. Work out by hand or simulation how the DC biasing effects the signal on it's path through the circuit. There are three ways to analyse a circuit,
1 - pen and paper
2 - simulation
3 - staring at it
3 generally only works when you've practised 1 and 2 for many years.

Understanding is fought for and won, nobody can give it to you!

nte102 fuzz

Yes But What is the standard DC biasing operating voltages for the base,collector,emitter? what are the rules for these?

The emitter Resistance value changes the GAIN the most right?

What are the guide lines for operating voltages for the base,collector,emitter?
what are the voltage ranges?

fuzz face theory

walters said:
Yes But What is the standard DC biasing operating voltagesfor the base,collector,emitter? what are the rules for these?

The emitter Resistance value changes the GAIN the most right?

What are the guide lines for operating voltages for the base,collector,emitter?
what are the voltage ranges?

There really isn't such a thing. In general you want the output to be biased in such a way as the output can go up as much as it can go down. But that depends on what the purpose of the circuit you're designing is.

biasing a germanium transistor

ya i know that so the AC waveform doesn't Clip
but still the resistor values for base,emitter,collector change alot from circuit to circuit

(Germanium forward bias is .3 volts and silicon forward bias is .7 volts)

Is it true that forward bias is based on distortion saturation?

Whats the difference between .3volts forward bias VS .7volts forward bias?

bias for germanium transistor

walters said:
ya i know that so the AC waveform doesn't Clip
but still the resistor values for base,emitter,collector change alot from circuit to circuit

(Germanium forward bias is .3 volts and silicon forward bias is .7 volts)

Is it true that forward bias is based on distortion saturation?

Whats the difference between .3volts forward bias VS .7volts forward bias?

The forward bias is the 'turn on' voltage of the transistor. the difference between 0.3V and 0.7 volts is that a transistor with a smaller 'turn on' voltage will operate at a lower voltages. The turn-on voltage may have some bearing on the distortion. In diode clipping (most common form of distortion circuit) it is the forward drop that causes the clipping. I don't think that's what's happening here, but I haven't anayzed these circuits. The Meastro fuz is almost definately utilising diode clipping.
Simulate the circuit, and find out how it works.
The reason resistor values change greatly between different circuits is that there are no right and wrong values. You can set up an amp to behave pretty much identically with vastly different values. It is the behaviour of the circuit you need to design and not the resistor values. The relative values of the resistors are important, not the actuall values.

R6 controls the volume.
R7 controlls the distortion.

If you simulate this, make R7 2 resistors (which add up to 1K) vary the ratio and look at the effect on the output. try changing the feedback resistance to see if you can get the same behaviour as changing the r7 ratio. Have a look at what the other resistors do. Try changing all values to get the circuit to behave in the same manner as it does now. Once you have done that you should understand how the circuit works.

Analogue electronics is an art, you can't follow a set of rules. This is why you don't get software to design analogue circuits from transfer functions etc.

edit:
In this circuit you want the ac waveform to clip! That is what gives you the distortion.

germanium transistors silicon bias resistor

So if the values of the resistors don't mean anything , then how would i know what each resistor or capacitor does?

I think the main point of this is about the RATIO's of the overall biasing resistor values right?

(This is why you don't get software to design analogue circuits from transfer functions)

What u mean by this? i thought algorithms take a analog circuit into transfer functions , into software algorighms

The Germanium Fuzz face using Germanium transistors giving it a fuzzy distortion sound , VS the silicon fuzz face using Silicon transitors gives a Harsher , nasel distortion sound why is that? the resistor/capacitor values are the same design just the transistor materials are different and the power supply polarity also

What does the Germanium transistor distort/saturate different than the silicon transistors? is this because of the forward biasing junction? or breakdown junctions?

Does the Biasing resistor values for emitter,base,collect set the GAIN or just the biasing voltages?

nte102 germanium transistors

The Maestro Fuzz (Germanium Transistors) that you have listed in your original post is BULLSH_T. That is a Lowrey Organ Co. schematic and the transistors (and diode) in that circuit are silicon, NOT GERMANIUM. Do You Want The Original Maestro (Satisfaction) Fuzz Schematic the one the rolling Stones used on "I Can't Get No, Eh-Eh, SATISFACTION" . Well kid, you not only have no clue about electronics, you apparently don't know how to use the internet. Thats a sad state for someone who wants to build any type of electronic related equipment. Can you say "FIRE" .
The real schematic For the Original (Germanium) Maestro Fuzz (Satisfaction) Box is attached. This is a schematic for the original (model 1) and the revision (Model 1A). The transistors used (for model 1) 2N270 crosses to a NTE102 and (for model 1A) 2N2613 crosses to a NTE102A. These transistors are both germanium.

how to find resistor values for germanium

1.) They make TWO different versions of the FUZZ FACE a silicon and germanium one
Why do they sound different??

((The Germanium Fuzz face using Germanium transistors giving it a fuzzy distortion sound , VS the silicon fuzz face using Silicon transitors gives a Harsher , nasel distortion sound why is that? the resistor/capacitor values are the same design just the transistor materials are different and the power supply polarity also))

What does the Germanium transistor distort/saturate different than the silicon transistors? is this because of the forward biasing junction? or breakdown junctions?

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