# Multisim common collector confusion

Discussion in 'General Electronics Chat' started by jaydnul, Nov 5, 2015.

1. ### jaydnul Thread Starter Member

Apr 2, 2015
91
0
I am trying to do a basic current gain amplifier. Everything works until I place the capacitor in series with the 10 ohm load to remove the DC offset:
I have no clue what's going on here. I tried playing with C1 value but it didn't do much. Shouldn't the output just be the input at the origin? What am I doing wrong?

Thanks

Edit: the transistor is an ideal NPN

Last edited: Nov 5, 2015
2. ### wayneh Expert

Sep 9, 2010
12,391
3,246
The 10Ω load is far too large for the 100µF capacitor to make any difference. Try raising C1 and R5.

3. ### jaydnul Thread Starter Member

Apr 2, 2015
91
0
So for this circuit to drive a 4 ohm speaker I would need a giant capacitor? Like .1 Farads?

4. ### crutschow Expert

Mar 14, 2008
13,503
3,376
Guessing at part values does not generally work well for electronic design.
If you do the math for the -3dB point [C = 1/(2πRf)] it requires nearly a 2,000μF capacitor to give a -3dB rolloff into a 4 ohm load at f = 20Hz.

Last edited: Nov 5, 2015
5. ### wayneh Expert

Sep 9, 2010
12,391
3,246
Your guess is only 50x higher than crutschow's calculated value. I've seen worse.

6. ### Picbuster Member

Dec 2, 2013
391
53
Well your problem is simple to explain.
the current (Emit + Basis) goes via 10 ohm.
open then sw then it goes via 1K so Ue goes up and when above 0,6V The xtor is switched off.
adive: measure Ue when sw is open and alter the 1K resistor

7. ### jaydnul Thread Starter Member

Apr 2, 2015
91
0
Haha ya, .1F was an exaggerated estimation. I'm still thinking about this problem though; if I have a DC biased audio signal and I want to send it into a 4 ohm speaker, is the only simple option a giant capacitor or is there some technique that I am missing?