AoE BJT quiescent collector current

[todorp]

Hi all, I am trying to proceed thourough the BJT chapter in AoE and also doing the labs in LAoE.
In both books when talking about bjt followers and amplifiers the authors always fixe a startng quiescent collector current and than proceed from this to determine the biasing network values, etc.
For example for small signal transistors (2N3904) they often use 1mA or 0.5mA as a given.
My question is how do you choose such a value? Why not 10mA or 0.1mA. What tradeoffs are involved?
Besides none of these values correspond to maxima in the collector current vs hfe plots for this type of transistors.
Why not choose a quiescent collector current at which the bjt has maxium current gain? I understand that hfe is not a reliable parameter but still this would be a way to set the quiescent current using a method and not guesswork...

Thanks a lot,
    Todor

[David Hess]

Why not choose a quiescent collector current at which the bjt has maxium current gain?

Power dissipation limits the collector current to sane values even at the expect of current gain and as you point out, current gain varies widely so cannot be relied on anyway.  A 200 milliwatt 200 milliamp 2N3904 cannot be operated much above 10 milliamps in a linear circuit.  But there are other reasons to select a specific collector currnet.

The ratio of input current noise and input voltage noise depends on the collector current so in a low noise application, it might be a specific value.  Or the collector current might be selected for minimum hre (voltage feedback ratio) for best DC performance, about 2 milliamps on a 2N3904.  Often the minimum collector current is determined by the requirement to preserve small signal response over a given operating range; if I have a transconductance amplifier with an output current range of +/-1 milliamp, then the collector current better be at least an order of magnitude higher to preserve linearity, or 10 milliamps.

Differential stages in operational amplifiers select collector current to minimize input bias current or transconductance depending on the circumstances.

[todorp]

@David Hess, thanks a lot for the detailed explanation, now things are a little bit clearer.
    Todor

[T3sl4co1l]

Current depends on circuit requirements.  Of course.  Minimum current is set by required gain, load impedance, bandwidth, noise factor and so on.  Maximum current is set by overall power consumption or dissipation limits, input impedance, and bandwidth again to a certain degree.

Generally, with increasing current: noise falls, impedance falls, gain rises, bandwidth rises, and power rises.

As you approach the current rating of a given device, bandwidth falls, due to a combination of the low impedances being more significant against device parasitics (spreading resistance, lead inductance), and high level injection (fT and hFE fall as you approach Ic(max)).

So at some point you choose a larger device, which unfortunately has more capacitance, may have lower fT, etc., but if those aren't the limiting factor, and you aren't subject to other restrictions (like power consumption), that's what you do.

This also applies to MOSFETs, except for injection of course as they are majority carrier devices, and except for input impedance which is more or less independent of current (i.e., mostly gate capacitance + spreading resistance).

Tim