# Multisim common collector confusion

#### jaydnul

Joined Apr 2, 2015
175
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

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#### wayneh

Joined Sep 9, 2010
17,475
The 10Ω load is far too large for the 100µF capacitor to make any difference. Try raising C1 and R5.

#### jaydnul

Joined Apr 2, 2015
175
So for this circuit to drive a 4 ohm speaker I would need a giant capacitor? Like .1 Farads?

#### crutschow

Joined Mar 14, 2008
33,367
So for this circuit to drive a 4 ohm speaker I would need a giant capacitor? Like .1 Farads?
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.

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#### wayneh

Joined Sep 9, 2010
17,475
Your guess is only 50x higher than crutschow's calculated value. I've seen worse.

#### Picbuster

Joined Dec 2, 2013
1,045
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