transistor amplifiers im special

[the cracken]

hi,
When using a resistive divider to bias the base of the transistor you need to have sufficient current flowing thru the divider in order to avoid the small changes of current into the base from changing the bias point voltage.

Its common to choose between 5 and 10 times the required base current to flow thru the biasing resistor divider.

Its not guess work, its working with an acceptable range of divider currents.

if we use 82K for R1 and 39K for R2 we would get a base voltage of

(**broken link removed**)

it seems they picked two resistances out of no where. how do i choose the resistors i need?

im guessing that the input resistance is affected by R2 and the transistor?

 

[mneary]

The value of the base resistors should be high enough so that we can have the desired input impedance at the input (considering the base resistance is in parallel).
The base resistors should be low enough so that the base current doesn't cause a major shift in the divider output.

 

[the cracken]

The value of the base resistors should be high enough so that we can have the desired input impedance at the input (considering the base resistance is in parallel).
The base resistors should be low enough so that the base current doesn't cause a major shift in the divider output.

so if i have an in put voltage of 140 milivolts peak to peak how do i work out the base resistors?
thanks

 

[mneary]

Input voltage is not a factor in selecting base resistors.

 

[the cracken]

Input voltage is not a factor in selecting base resistors.

so what is?

 

[ericgibbs]

so what is?

I have answered your question in post #19, regarding the selection of the bias resistors.

Also 'agu' and 'mneary' have explained the requirements of the divider.

I would suggest you draw a simple circuit and post it with your calculated values, this will give us an idea of which part of the circuit is causing you a problem.

Did you run the LTS.asc file I posted for you.?

 

[the cracken]

the hole circuit is the problem. how im i ment to know the needed input impedance??

i have read the link that you gave me and im starting to read the other parts

 

[the cracken]

o and yes i have run the file it works perfectly, one question how do you change the results you get up? gain voltage frequency...
can i get it to come up like an oscilloscope?

and do you know any good oscilloscopes that i can buy? probably only going to use it for audio so it doesnt need to go to very high frequency's!

 

[Nigel Goodwin]

the hole circuit is the problem. how im i ment to know the needed input impedance??

i have read the link that you gave me and im starting to read the other parts

As I pointed out at the very beginning of this thread, that crude single transistor is totally useless for a guitar preamp - so you're completely wasting your (and our) time. You 'can' do it with multiple transistors (and it was done that way before opamps), but it's so trivial to do correctly with an opamp (at a tiny fraction of the cost) that it's not done any other way.

 

[ericgibbs]

the hole circuit is the problem. how im i ment to know the needed input impedance??

i have read the link that you gave me and im starting to read the other parts

hi,
To save me typing out detailed explanations I have located these 3 links that should help you with regarding input/output impedances.

If you plan to buy a scope I would look thru e-Bay for 'working' second hand scopes, for audio only a 10mHz dual trace would be fine.

Input impedance - Wikipedia, the free encyclopedia

**broken link removed**

Impedance and Reactance

 

[audioguru]

how im i ment to know the needed input impedance??

Two links were posted for guitar preamp circuits designed by experts. One has opamps and an input impedance of 1M ohms. The other uses a Jfet and says the minimum input impedance for a guitar preamp is 1M ohms and its input impedance is 3M ohms.

You cannot easily make a transistor circuit have an input impedance as high as 1M ohms.

 

[the cracken]

Two links were posted for guitar preamp circuits designed by experts. One has opamps and an input impedance of 1M ohms. The other uses a Jfet and says the minimum input impedance for a guitar preamp is 1M ohms and its input impedance is 3M ohms.

You cannot easily make a transistor circuit have an input impedance as high as 1M ohms.

Iv tried building a op amp circuit off the internet and it didn't work. so i would rather gain an understanding than build random circuits that "might" work.

I want to build amp that are class a only as they have no cross over distortion even though they are very inefficient!

Do you know any good interactive simulators so i can play to get an understanding?

 

[audioguru]

Opamp circuits work perfectly if they are designed and built correctly.

Only the lousy old LM358 and LM324 low power opamps have crossover distortion. TL07x opamps have distortion of only 0.003% and are class-AB, not class-A.

A single class-A transistor has distortion of up to 40%.

 

[Nigel Goodwin]

Iv tried building a op amp circuit off the internet and it didn't work. so i would rather gain an understanding than build random circuits that "might" work.

Opamp designs work perfectly, FAR, FAR, easier to design than a transistor one - you would be hard pushed to find a design that didn't work, more likely it was built wrong.

I want to build amp that are class a only as they have no cross over distortion even though they are very inefficient!

As AG said, your stage is VERY distorted - and as we've all repeatedly said, it's absolutely useless for a guitar anyway.

 

[the cracken]

Opamp circuits work perfectly if they are designed and built correctly.

Only the lousy old LM358 and LM324 low power opamps have crossover distortion. TL07x opamps have distortion of only 0.003% and are class-AB, not class-A.

A single class-A transistor has distortion of up to 40%.

i don't really want to make amps i want to make guitar affects pedals i don't think that would make much difference but i though i would mention it.

 

[the cracken]

Opamp designs work perfectly, FAR, FAR, easier to design than a transistor one - you would be hard pushed to find a design that didn't work, more likely it was built wrong.



As AG said, your stage is VERY distorted - and as we've all repeatedly said, it's absolutely useless for a guitar anyway.

and i tough i would need more than one transistor but i wanted to start with understanding just the one at this point. having multiple transistors would mean i have less distortion would it not?

 

[audioguru]

An opamp has at least 30 transistors so its voltage gain is 200,000 or more. Then a lot of negative feedback is added which reduces the gain to a manageable amount and also reduces distortion to almost nothing.

 

[Nigel Goodwin]

and i tough i would need more than one transistor but i wanted to start with understanding just the one at this point. having multiple transistors would mean i have less distortion would it not?

Having multiple transistors in a correctly designed circuit would reduce distortion, increase input impedance, and could make a reasonable job of it. However, a single opamp would do a FAR better job, and take no designing at all - opamps are THAT easy to use.

I designed and built a two channel mixer for a sudden disco I had to do - took perhaps 30 minutes, from been asked to do the disco to having a working mixer in my hand (in a 'case' as well).

 

[the cracken]

Having multiple transistors in a correctly designed circuit would reduce distortion, increase input impedance, and could make a reasonable job of it. However, a single opamp would do a FAR better job, and take no designing at all - opamps are THAT easy to use.

I designed and built a two channel mixer for a sudden disco I had to do - took perhaps 30 minutes, from been asked to do the disco to having a working mixer in my hand (in a 'case' as well).

so you've convinced me but how do i go about choosing the right opamp. i know how to set the gain its the ratio of the input and feed back resistors right?

 

[audioguru]

An opamp with an input resistor is an inverting circuit. The input impedance is the input resistor's fairly low value resistor.

Another type of opamp circuit is the non-inverting type that has a very high input impedance. It also has a feedback resistor but has a resistor from the inverting input in series with a capacitor to ground.