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BJT voltage divider biasing - how to decide on the value of collector current?

atferrari

Well-Known Member
After revising the procedure to calculate values for voltage divider biasing of a BJT amplifier, I couldn't find a concrete indication of how to decide on the collector current.

The "too much" limit is obvious, but, what about the other extreme of a reasonable minimum?

My question: how to decide on the Ic for:

amplifiers
buffers
audio oscillators
RF oscillators?

Thanks for any help you could provide.
 

audioguru

Well-Known Member
Most Helpful Member
Obviously, the collector resistor value depends on the maximum load current and the power supply voltage.
Using a 9V supply, a transistor with a 1k collector resistor can drive a 100k load very well.
 

atferrari

Well-Known Member
Obviously, the collector resistor value depends on the maximum load current and the power supply voltage.
Using a 9V supply, a transistor with a 1k collector resistor can drive a 100k load very well.
Hola audioguru

Thanks for replying.

Yes but, how to decide on the Ic to go ahead for my calculation? Guessing and rule of thumb seems a poor criterion for a proper design. And even then, I did not see any clearly stated for the four cases I mention.
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
Hola audioguru

Thanks for replying.

Yes but, how to decide on the Ic to go ahead for my calculation? Guessing and rule of thumb seems a poor criterion for a proper design. And even then, I did not see any clearly stated for the four cases I mention.
Because they can't be any clearly stated values for such vague cases.

As AG has already said, they depend mostly on external factors.

'Guessing and rule of thumb' are probably as good as anything else - there's no real 'optimum value', unless you VERY accurately specify everything around it, there's more of a 'large range' where performance is perfectly satisfactory.

Often in these types of 'calculations' you have no option but to decide on a particular value to start with for one component, you can then calculate the other values based on that starting point.
 

atferrari

Well-Known Member
Because they can't be any clearly stated values for such vague cases.

As AG has already said, they depend mostly on external factors.

'Guessing and rule of thumb' are probably as good as anything else - there's no real 'optimum value', unless you VERY accurately specify everything around it, there's more of a 'large range' where performance is perfectly satisfactory.

Often in these types of 'calculations' you have no option but to decide on a particular value to start with for one component, you can then calculate the other values based on that starting point.
I see Nigel. I got your concrete reply but other than feeling free to try whatever comes to my mind, I am not any closer to choose a reasonable value. :(
 

gophert

Well-Known Member
Most Helpful Member
atferrari , ignore Nigel, he has trouble with abstract thoughts.

I think it is safe to say you should design collector current a minimum of 10x to (preferably) 100x ABOVE the transistor cutoff current (see "collector cutoff current" on datasheet - usually listed next to "base cutoff current"). An ON Semi datasheet for 2N3904 lists 50nA as the cutoff current for both cutoffs so I would design the minimum current at 5uA.

But... like said, it USUALLY depends on other components and you design amplifiers around them. But if you are trying to design for micro power you have to examine questions like you ask here, do the same for each component or module and design around your most pressing constraints.

Also, you need to look into how:
- downstream modules load (draw current) from that collector, and
- other current leakages, like reverse currents, or dark currents that can dilute your collector signal with noise.

I would recommend you trying to track down the "Art of Electronics" and review the section on low current circuits.
 
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gophert

Well-Known Member
Most Helpful Member
atferrari , ignore Nigel, he has trouble with abstract thoughts.

I think it is safe to say you should design collector current a minimum of 10x to (preferably) 100x ABOVE the transistor cutoff current (see "collector cutoff current" on datasheet - usually listed next to "base cutoff current"). An ON Semi datasheet for 2N3904 lists 50nA as the cutoff current for both cutoffs so I would design the minimum current at 5uA.

But... like said, it USUALLY depends on other components and you design amplifiers around them. But if you are trying to design for micro power you have to examine questions like you ask here, do the same for each component or module and design around your most pressing constraints.

Also, you need to look into how:
- downstream modules load (draw current) from that collector, and
- other current leakages, like reverse currents, or dark currents that can dilute your collector signal with noise.

I would recommend you trying to track down the "Art of Electronics" and review the section on low current circuits.
But to clarify...

You will easily run into problems with distortion, clipping and noise issues I you expect a transistor to act like a class A amplifier over several powers of 10 (several orders of magnitude) in current flow. also, your method of controlling that current to a very low level, either by very low base current or limiting current at collector will cause their own distortions as base current limiting means you will be in the logarithmic unsaturated region of the V/I curve. Also, The Early Effect, and... and... and... will all cause distortions or unplanned performance at very low output of you expect a very wide dynamic range on one transistor in your circuit.
 

unclejed613

Well-Known Member
Most Helpful Member

throbscottle

Well-Known Member
Maybe I'm thinking of the wrong thing here, but when I went to college we had one class involving printed charts of collector currents, photocopied from datasheets I guess, so you could put your ruler across at the opposite angle so it crossed in the middle of your desired line - the spot where you get the most linearity - and the I and V on the graph's axes give you the value of collector load you need.

But I'm sure you already know that! Anyway in case you didn't, there it is.

Doesn't answer the question though, does it? I suppose the thing to do is think of the load resistor in terms of it being the input source for the next stage, and go from there.
 

atferrari

Well-Known Member
Maybe I'm thinking of the wrong thing here, but when I went to college we had one class involving printed charts of collector currents, photocopied from datasheets I guess, so you could put your ruler across at the opposite angle so it crossed in the middle of your desired line - the spot where you get the most linearity - and the I and V on the graph's axes give you the value of collector load you need.

But I'm sure you already know that! Anyway in case you didn't, there it is.

Doesn't answer the question though, does it? I suppose the thing to do is think of the load resistor in terms of it being the input source for the next stage, and go from there.
After revisiting the so many concepts as per recent suggestions I realized that I actually knew about Q and the load line. It seems that I am still good at forgetting things.
Thanks for replying.
 

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