current biased collector

gorge

Member
2016-04-27 6:59 am
With a common emitter amplifier, is it preferable to use a current mirror to bias the collector with the appropriate current? Or is it more common to just have a voltage supply -> collector resistor -> BJT collector pin?

If it's preferable to use a current mirror, then why is this approach not also employed at the base? I mean instead of the usual Voltage supply -> base resistor -> BJT base pin?
 

rayma

Member
2011-04-29 8:37 pm
If it's preferable to use a current mirror, then why is this approach not also employed at the base?
I mean instead of the usual Voltage supply -> base resistor -> BJT base pin?

The beta of bipolar transistors is poorly controlled, so the base is normally voltage-driven
instead of current-driven. The biasing resistors at the base are sized to allow a bleeder current
much larger than the base current, so the DC base voltage is fixed and independent of the
variability in the bjt. Then the DC emitter current can be set by the base voltage minus Vbe (0.65V),
divided by the emitter resistor value chosen.
 
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With a common emitter amplifier, is it preferable to use a current mirror to bias the collector with the appropriate current? Or is it more common to just have a voltage supply -> collector resistor -> BJT collector pin?

If it's preferable to use a current mirror, then why is this approach not also employed at the base? I mean instead of the usual Voltage supply -> base resistor -> BJT base pin?

Bias is voltage by definition, so "bias the collector with the appropriate current" does not really make sense.

BJT is a voltage-controlled device. You bias the base (referenced to emitter) in order to have certain collector current.

What you really mean, I believe, is the way collector load is organized. It can be static (resistor) or dynamic (constant current source or bootstrap). Dynamic load provides higher dynamic impedance of the load, reducing distortion.

There's a lot of information on the web or in the well known books on the amplifiers design - look for "dynamic load".

Cheers,
Valery
 

rayma

Member
2011-04-29 8:37 pm
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gorge

Member
2016-04-27 6:59 am
Perhaps the key to answering my question of whether a current driven Vbe would be better than a voltage controlled one is understanding why an active load causes the amplifier to be independent of variability in Beta. Does anyone know why that is?
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Forget terminolgy that refers to current drive. You cannot drive current into anything here, only gate it to flow according to signal + bias voltage, Beta, power supply limitations, current limiting and load. If the base or collector is starved of current by high circuit resistance, you may well have non-linear operation but that is not the issue you are concerned with.

The issue is about the load lines and maintaining the BJT operating point in a linear range at all times but the full answer won't come in a glib few words like a Q&A reply. It deserves a formal study of transistor operation before you can visualize and understand how a load affects transistor operation in many areas. As Valery pointed out, a dynamic load such as a current sourced or bootstrapped amplifier stage, presents less load on the preceding amplifier stage than a static one. That alone is enough to reduce the need for current (less Beta) and improve linearity.

A voltage amplifier stage in an audio amp, using Douglas Self's various analyses makes a good introduction to the requirements there.