# Emitter-Follower Amplifier

Discussion in 'Homework Help' started by Chinh Thuy Nguyen, Dec 21, 2016.

1. ### Chinh Thuy Nguyen Thread Starter New Member

Dec 21, 2016
2
0
Hi everybody,
I'm meeting a trouble with Emitter-Follower Amplifier circuit. That is in two attached pictures.
What is the meaning of two capacitors that I highlight by the red circle?
As well as, what is the "self-trigger" in Amplifier circuit?
Thank you!

2. ### MrChips Moderator

Oct 2, 2009
18,891
6,051
The first circuit does not have an emitter follower.
The second circuit has an emitter follower in the second stage.

The capacitors you have circled in red change the response at higher frequencies.

Is this homework?

3. ### Chinh Thuy Nguyen Thread Starter New Member

Dec 21, 2016
2
0
Oh sorry, I have a mistake with the picture 1.
Yes this is my homework

4. ### BillB3857 AAC Fanatic!

Feb 28, 2009
2,490
385
Do you know what RL means? In the 2nd circuit, if a capacitor is in series with RL, what can you say about the current seen by RL?

5. ### crutschow Expert

Mar 14, 2008
22,518
6,598
In the first circuit, the common-emitter output stage has the DC bias point (DC gain) determined by R8, and the high frequency AC gain determined by R4 and C.
The low frequency corner for the transition from the DC gain value to the AC gain value is determined the the R4C time-constant.

In the second circuit Ci, Cc, Ce1 and Ce2 are DC block capacitors that pass the AC signal but block the DC bias voltages.
Ce1 bypasses the emitter of Q1 so that Q1's gain is maximum (not reduced by the degeneration effect of Re1).

6. ### MrChips Moderator

Oct 2, 2009
18,891
6,051
In that case, moved to Homework Help.

7. ### Zeeus Member

Apr 17, 2019
376
12
Nice...So in the first circuit, he's trying to invert the signal twice..Although why use current source as input? Isn't Rout of current high?

Same as second circuit..In the second stage, bias resistors (120k||20k) * 10 > beta * Rin(270 + 120 + re)..(This is not bad design?)

8. ### Zeeus Member

Apr 17, 2019
376
12

hmm..Don't understand

9. ### MrAl AAC Fanatic!

Jun 17, 2014
6,233
1,352
Hello,

As a first approximation, look at all the capacitors as a short for AC analysis, and as a DC voltage source for DC analysis.
That will tell you what they are for qualitatively.
To get quantitative results you have to do a full analysis.

10. ### crutschow Expert

Mar 14, 2008
22,518
6,598
Below the R4C corner frequency it series impedance starts increasing and the gain is eventually determined by the value of R8 at DC.
At higher frequencies the impedance becomes equal to R4, which then determines the AC gain.

11. ### Zeeus Member

Apr 17, 2019
376
12
The first image, the cap highlighted in red, please what's polarity?

Thought + side to emitter of q2 but after rethinking, think it's the other way around

what is it?

Thank you

12. ### Ylli Active Member

Nov 13, 2015
700
201
+ on the resistor side.