Emitter Follower

HEllo Guys,
I am trying to design a Emitter Follower. I am trying to give the load
resistance as 50 Ohm.
Could some one tell me how to calculate the Biasing resistance on the
Base circuit of the transistor.I am assuming that the Short circuit
current gain (Beta = 100). I am using a npn 2n2222a transistor for my
purpose. Voltage supply on the collector is 12V. Voltage supplied to
the base is from the output of a sound card or a signal generator.So
please help me in getting the answers
Regards
Nitin

 

I'm curious- why a 50 Ohm load? Is this a college homework assignment by any
chance?

Dave

 

Try http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/npncc.html

Arfa

 

Dave;

I really hope it's not college level, this should be in high school.
Common collector amps are the simplest to design.

Actually I'm a bit squeaky when it comes to common emitter and
especially common base stages' actual input impedance. In general in
those stages I use a heavy handed approach, make it damped (shunted)
and linear.

Anyway, now that you got me on the subject, people have looked at me
funny for saying this; All bipolar transistors operate as common
emitter. No exceptions.

This was in response to the inability of a buddy of mine in trade
school to understand the concept. I had to draw it about 15 different
ways but he finally got it. I almost went into sign language !, that is
I was to the point of holding up three figners and saying sternly "OK,
my thumb is the emitter ok ?" I didn't have to resort to that but it
was close.

It was a success, and he actually doesn't make more money, but he moved
from 90 hours a week to 40 hours a week to make about $50,000. Not bad,
it's just about a whole dollar every day.

To the OP;

In common collector (emitter follower) the resistance presented by the
base is approximately the load resistance times the current gain of the
transistor. This current is applied (V / RL X hfe) in the direction of
whatever is biasing the emitter. As long as the collector is solidly
connected to the Vcc, and input is reference to a propely bypassed Vcc
return this is close enough. When the collector voltage is allowed to
vary, like by a resisor, this is not accurate.

I am not abject to giving help on school projects, but if it is please
say so. For one, this cheap and dirty formula dfoesn't work with
anything else. If you don't learn it the right way you will fall flat
on your face later.

Also, if RL=50 ohms, your wimpy little transistor will have to
dissipate at the very least 720 mW even during quiescence. A 2N2222 is
going to have a problem with that, you will at least need a heatsink.
In fact you might need Freon !

If you're talking about buffing a soundcard output, this is not the way
to go. If you simply get an older soundcard that doesn't require
powered speakers you are fine. They will feed 50 ohms no problem. A sig
generator, OK, mine for example is 30 volt @ 600 ohms output. But then
what would stop you from just feeding that to the soundcard ?

JURB

 

It's not the correct transistor to drive a 50 ohm load.

 

Erm, thanks- but you're replying to the wrong post ;-)

Dave

 

"" bravely wrote to "All" (16 Feb 06 13:11:27)
--- on the heady topic of "Re: Emitter Follower"

ZZ> From:
ZZ> Xref: core-easynews sci.electronics.repair:358523



ZZ> Dave;

ZZ> I really hope it's not college level, this should be in high school.
ZZ> Common collector amps are the simplest to design.

ZZ> Actually I'm a bit squeaky when it comes to common emitter and
ZZ> especially common base stages' actual input impedance. In general in
ZZ> those stages I use a heavy handed approach, make it damped (shunted)
ZZ> and linear.

ZZ> Anyway, now that you got me on the subject, people have looked at me
ZZ> funny for saying this; All bipolar transistors operate as common
ZZ> emitter. No exceptions.

ZZ> This was in response to the inability of a buddy of mine in trade
ZZ> school to understand the concept. I had to draw it about 15 different
ZZ> ways but he finally got it. I almost went into sign language !, that
ZZ> is I was to the point of holding up three figners and saying sternly
ZZ> "OK, my thumb is the emitter ok ?" I didn't have to resort to that but
ZZ> it was close.

ZZ> It was a success, and he actually doesn't make more money, but he
ZZ> moved from 90 hours a week to 40 hours a week to make about $50,000.
ZZ> Not bad, it's just about a whole dollar every day.

ZZ> To the OP;

ZZ> In common collector (emitter follower) the resistance presented by the
ZZ> base is approximately the load resistance times the current gain of
ZZ> the transistor. This current is applied (V / RL X hfe) in the direction
ZZ> of whatever is biasing the emitter. As long as the collector is solidly
ZZ> connected to the Vcc, and input is reference to a propely bypassed Vcc
ZZ> return this is close enough. When the collector voltage is allowed to
ZZ> vary, like by a resisor, this is not accurate.

ZZ> I am not abject to giving help on school projects, but if it is please
ZZ> say so. For one, this cheap and dirty formula dfoesn't work with
ZZ> anything else. If you don't learn it the right way you will fall flat
ZZ> on your face later.

ZZ> Also, if RL=50 ohms, your wimpy little transistor will have to
ZZ> dissipate at the very least 720 mW even during quiescence. A 2N2222 is
ZZ> going to have a problem with that, you will at least need a heatsink.
ZZ> In fact you might need Freon !

ZZ> If you're talking about buffing a soundcard output, this is not the
ZZ> way to go. If you simply get an older soundcard that doesn't require
ZZ> powered speakers you are fine. They will feed 50 ohms no problem. A
ZZ> sig generator, OK, mine for example is 30 volt @ 600 ohms output. But
ZZ> then what would stop you from just feeding that to the soundcard ?


JURB,

If his card has only Line-Outs then he would be better off with a pair
of amplified speakers. If he insists on DIY then perhaps an 8-pin itty
bitty LM386N etc, pair might do. All the preamp gain he needs and it
will drive 1 watt into 8 ohms quite happily without too much fuss.
Just don't use much gain because the little bastard has a noisy input.

If you insist on just 3-pins, then checkout the application note for
the TL431 precision voltage reference which makes a radical use of it
as a 400 milli-Watt phono-amplifier. However it uses a small audio
transformer 330:8 ohms. The beauty of using a transformer is that at
idle it dissipates the maximum power it ever will. So if it is too hot
then bolt it to a big heatsink and it will be safe.

A*s*i*m*o*v

.... A fail-safe circuit will destroy others.

 

Dave - I was referring the OP to that site, and not replying specifically to
your question about the 50 ohm load. The site has information relevant to
his query about how to calculate the values of bias resistor for a common
collector stage, with BIGGGG simple diagrams ...

Sorry if it confused - I was just tacking onto the bottom of the thread for
the benefit of those following it, rather than sticking my bit in the
middle.

Arfa

 

No worries, Arfa, we've all done it!

Dave