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?
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.
Last edited:
I can't seem to find any info about bleeder current. What is a bleeder resistor exactly and how is it connected to the base?
I think you need a better understanding of BJTs and their bias before worrying about current mirrors. You can't understand a current mirror unless you fully understand BJTs.
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
Here are a couple of tutorials to get you started. There are several methods of biasing a bjt.
Transistor Biasing and the Biasing of Transistors
Biasing Calculations : Bipolar Junction Transistors - Electronics Textbook
Last edited:
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?
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.
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.
- Status
- Not open for further replies.