# emitter follower (common collector amp)

Discussion in 'General Electronics Chat' started by donut, May 26, 2012.

1. ### donut Thread Starter Member

May 23, 2012
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Every book has a different circuit drawn for the emitter follower circuit. My question is why is R1 and R2 needed in Figure 1 and why is it not included in figure 2.6?

What is R1 and R2 doing for the circuit in Figure 1?

What is V^- in Figure 1?

What is rin and rout?

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2. ### bretm Member

Feb 6, 2012
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They're the same core circuit, it's just that the second circuit just focuses on the transistor action and leaves some things implied.

One of the implied things is the DC bias voltage required for V_in, which is what R1 and R2 establish.

V+ and V- determine the range of the voltage divider established by the transistor and the emitter resistor.

r_in and r_out are the input and output impedences.

3. ### Audioguru New Member

Dec 20, 2007
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R1 and R2 are a voltage divider that set the DC base voltage at half the supply voltage so that the emitter can swing up and down.

4. ### tpny Member

May 6, 2012
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this is because the signal is ac. you don't have to do this when the signal is dc and you just want the transistor to turn on and off. Come to think of it do people ever need to use transistors to amplify ac (given that op amps exist in the world).

5. ### donut Thread Starter Member

May 23, 2012
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Seems like you are implying that transistors used as amplifiers are more than likely going to be used in DC applications?

Opamps are better suited for AC amplification applications?

So which circuit attached to this thread would be used in a AC application?

SO which circuit attached to this thread would be used in a DC application?

6. ### ErnieM AAC Fanatic!

Apr 24, 2011
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You do realize that an op amp is a collection of several transistors, right?

7. ### crutschow Expert

Mar 14, 2008
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Yes, if you want to amplify GHz signals or drive a speaker with high power AC, for example.

8. ### tpny Member

May 6, 2012
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Yep. I'm just thinking in terms of practice, i would never use a transistor (as a single component) to amplify ac - just use an op amp IC. (Who would in a world where there is op amp?) But they put so much emphasis on that in the textbooks which made the material more inaccessible than it should, at least for me it was..

In my opinion yes.

The left one with all the resistors and capacitors. The capacitors are there to decouple the dc in the signal at the input and recouple it into the amplified waveform at the output.

The simple one on the right

9. ### ErnieM AAC Fanatic!

Apr 24, 2011
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First off, you're not doing practice, you are doing education. It would be a disservice to you not to teach you how the fundamental elements work.

Next, there was one time my company was given a module to build as our customer just couldn't get it to work, and it was expensive for what it did. It seemed to be designed with a "need function, look up an IC" mentality. I was able to make several chips dissapear completely, and 2 other chips got replaced by 2 transistors. So I cut the cost and size in half without breaking a sweat.

So yeah, you should know this stuff. Even in 2012.

10. ### MrChips Moderator

Oct 2, 2009
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Why would anyone do arithmetic by hand when you can just use a calculator, computer, spreadsheet?

Tpny, do you realize someone has to design the opamp from scratch using transistors?

There are many situations where a transistor is a better choice than an opamp.
I want to see you design a 10A switch mode power supply or a 100W audio amplifier using opamps and no transistors.

Last edited: May 28, 2012
11. ### tpny Member

May 6, 2012
216
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I was thinking along the lines of why would you use a vanilla npn transistor (like a typical one in a textbook) to amplify ac when you have op amp in the real world. But indeed, high current applications do call for high current transistors, op amps won't do. (Or gigahertz signal according to crutchow but I don't know why, never done high frequency..)

12. ### MrChips Moderator

Oct 2, 2009
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Because, first of all, one has to learn how all configurations of a single transistor amplifier works before one can appreciate all the properties of an op-amp.

Secondly, there are many low power AC applications where a single transistor amplifier is the better choice. In many preamp circuits, the very first active stage is a single transistor front end for the best possible performance.

13. ### tpny Member

May 6, 2012
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Ok, that's interesting. But why is the performance better? Thanks!

14. ### MrChips Moderator

Oct 2, 2009
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Because one can choose the transistor with the desired specs that one is looking for. It is not always possible to do that with an op-amp. Many sensitive applications call for a low noise, low input capacitance, high input impedance, wide band-width amplifier that only selected FETs can provide. It is common to test and hand select such transistors.

15. ### upand_at_them Active Member

May 15, 2010
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Don't the ham guys build their radio circuits with transistors, not op-amps?

16. ### MrChips Moderator

Oct 2, 2009
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True, I'm a ham also.

Also did you know you can build a simple LED flasher with one transistor. No need for a 555 timer IC?

Or a DC-DC up converter with one transistor?

17. ### upand_at_them Active Member

May 15, 2010
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Feedback via a capacitor, to produce the oscillations?

18. ### Mike33 AAC Fanatic!

Feb 4, 2005
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Reason for Transistors #1) Many audio-shaping circuits sound "sterile" or "different' when tried with opamps. Note the venerable "Fuzz Face" guitar circuit which made Hendrix a legend, or the Tube Screamer (SRV), which uses a BJT buffer on the front end....there are literally DOZENS of audio circuits that DO NOT sound the same if you replace their BJTs with opamps.

I won't even go into the use of 'classic' Germanium transistors...the holy grail of tone....they actually cost DOLLARS each now, for the NOS type.

Reason #2) you can jam 2 of them in the space taken up by 1 IC. And you don't need a socket...so the cost is like 5 cents vs. \$1 for the chip...and a 2n5088 is about as quiet as a TL071, IMO. No reason to not like transistors.

Reason 3) If you keep a bunch of BJTs around, you can do many things with them (amplify, switch, all that stuff). Making an opamp do such versatile things may take a lot more 'reinventing' than many of us want to deal with...

That said, I use both...the transistor where space/parts count matters, the opamp for easy pop-in 'black box' stuff. Just my 2 cents...