Golden Fleece transistor bias

[m7b52000]

the standard version of the schematic has a collector resistor 47k which results in a collector voltage of 2.7V. Is there a theoretical advantage in getting the collector voltage closer to 4.5 V e.g reducing coll resistor to  22k which would result in 4.1V. I appreciate that the gain would decrease but this could be offset by reducing the emitter resistor?

cheers

[Rob Strand]

the standard version of the schematic has a collector resistor 47k which results in a collector voltage of 2.7V. Is there a theoretical advantage in getting the collector voltage closer to 4.5 V e.g reducing coll resistor to  22k which would result in 4.1V. I appreciate that the gain would decrease but this could be offset by reducing the emitter resistor

These things are what they are.  If the bias is off then so be it.  However, that shouldn't stop you playing around. Maybe you can make an improvement.

There's no simple way to compensate for the gain drop.  A lower collector voltage gives more gain.  Within reason the gain difference isn't enormous.

You can tweak the bias three ways:
- decrease the collector resistor Rc
- increase the emitter resistor Re
- increase the base resistor Rb

You might find you need to change the values a *lot* more than a factor of 2 to bias at 4.5V.
If bias to a collector voltage of 4.5V, all pretty much end-up with the same gain.  Changing
the collector resistor will require a larger emitter bypass cap.

[antonis]

the standard version of the schematic has a collector resistor 47k which results in a collector voltage of 2.7V. Is there a theoretical advantage in getting the collector voltage closer to 4.5 V ?

Getting Collector voltage closer to half the supply level (ideally to [V_CC + V_E] / 2) you obtain symmetrical swing..

As it is, Collector can swing all the way up (almost +9V) but only down to V_E + V_CEsat ..

In the former case, your output amplitude should be a +/- diode forward voltage drop (oversimplified approximation but let it be for the time being) where in the later case it might be even lower for input signal waveform positive part..
(it should be already clipped due to Collector to Emitter restricted headroom..)

So, Collector bias level it's clearly a matter of taste..