Collector vs Emitter

[ele1200]

Hey,<p>Is there much difference is placing a load in the collector path or emitter path of a transistor. This is a basic situation where the collector is tied to Vcc and the emitter to ground, with the base voltage driving the on/off state of the transistor. I am not talking different current values in the two loops but if the transistor is driven saturation and cutoff.<p>[ November 11, 2002: Message edited by: newguy ]</p>

[L. Daniel Rosa]

Putting the load in the collector leg of the circuit will not require the base driver to go rail to rail, but the emitter leg will. This can (but not necessarily will) improve switching time.

[billdar]

Puting the load off the collector simplifies
the base current calcultions.<p>They are also amplifying different quantities.
common collector amplifies voltage whereas
common emitter amplifies current.

[Ron H]

Billdar, you got these backwards:<p>"They are also amplifying different quantities.
common collector amplifies voltage whereas
common emitter amplifies current."<p>Common collector is an emitter follower, and has current gain, but the voltage gain is slightly less than one. Common emitter is so called because the emitter is common to the input and output circuits. The output is taken off the collector, and, in general, this configuration will exhibit voltage gain.<p>Ron

[billdar]

oh yeah, sorry <p>My error correction unit between the brain and
typing fingers temporarily malfunctioned. Thanks
for catching that.

[bodgy]

Next time Billdar use closed loop feedback - open loop is just no good!<p>Ah yes if you put a load eg:. a LED in the collector and the bias resister in the emitter leg you will have a rough and ready constant current source.<p>bodgy

[russlk]

Another point to consider is that the transistor input impedance is low when the emitter is grounded and the load is at the collector. The input impedance is high (beta*load) when the load is in the emitter circuit.

[Bernius1]

Just for a little clarification....
** com.base, V-gain (Av)>1 , current gain <1
(only volyage gain)
** com.coll, current gain (Hfe)>1, but Av<1
(only current gain w/load in emitter)
** com.emit, Av, AND Hfe. Most gain.
They also all have different Zin, & Zout levels.
I think the most important thing to remember in
non-switch-type ckt's is that a high Rc (collector
resistor) yields higher voltage gain, limits quescient current drain, and forces more of the emitter's current through the output cap. Must
differentiate between SIGNAL , and BIAS VOLTAGE.
Thank you for enduring my tirade.Regards...

[MrAl]

Hello there,<p>In addition to the other great posts, i'd like
to add the following.<p>One of the important things that has to be
taken into consideration when deciding whether
to use an emitter load or a collector load is
whether or not the output has to see the whole
supply voltage when you only have a drive
signal that can only go as high as the
supply voltage. This kind of drive is very typical unless you have a special driver or
something. Even more typical is a drive voltage
that is already less then the supply voltage
and this makes the situation even more critical
in some apps.<p>If you put the load in the emitter leg you can
only get an output voltage equal to the supply
voltage (Vcc) minus the base emitter voltage
(approx 0.7 volts). This might not seem like
much, but for battery operated equipment it
can mean the difference between the circuit working
well and the circuit not working at all.
If you put the load in the collector leg, you
have the ability to drive the output (with
a good quality transistor) to almost full supply
voltage with a base drive voltage as little as
0.7 volts. Typical saturation voltages of good
transistors are less then 0.2 volts, so this
gives you an extra 0.5 volts from the battery
that will make it to the output.
The drawback to putting the load in the collector
circuit is that the base current doesnt get to
the output, so you have that small percentage
lost which reduces efficiency. With very high
currents this has to be taken into consideration
also and the benefits/drawbacks weighed together.
With a good quality transistor this might not
be so much of a problem though, because the gain
is usually also pretty high, and this means only
a small percentage of current is lost thus keeping
the efficiency high.<p>Most of these considerations are mainly of importance
when dealing with battery operated equipment,
but any application which has a limited supply
voltage to work with might feel the difference
also.<p>Good luck with your circuits,
Al