Common Collector

Thread Starter

chaosweapon

Joined Nov 14, 2008
15
I am trying to design a Common Collector but I have 2 problems:
1. The negative part of the ac signal is clipped unless I use a very high dc voltage (such as 100V).
2. The circuit only works fine at high capacitances.

Here's a copy of the circuit
http://forum.allaboutcircuits.com/attachment.php?attachmentid=8527&stc=1&d=1238940076

By the way, are there any rules of thumb for this type of circuit? I used the R2= (beta * RE) / 10 but I'm not sure whether it is valid since there is no RC.

Thanks
 

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thatoneguy

Joined Feb 19, 2009
6,349
A Common Collector amplifier only provides current gain, are you measuring voltage?

The base is biased at 10.2V, the signal source looks to be 2.5V.

Is the capacitor inline with the signal source 1 Farad?
 

Audioguru

Joined Dec 20, 2007
11,251
It looks like the load resistor is only 10 ohms. Then the output voltage will be very low.
The 1uF output capacitor value is much too low for 50Hz feeding 10 ohms.
The max allowed current in a BC547 transistor is only 100mA.

To drive 10 ohms you need a PNP transistor and an NPN transistor in series.
My circuit will work but its input impedance is fairly low.
 

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Thread Starter

chaosweapon

Joined Nov 14, 2008
15
I am actually working on a class A amplifier. The purpose of the Common Collector is as a buffer between the Common Emitter and the load (10ohm resistor). The capacitors are 1Farad each, else the circuit doesn't work correctly. Maybe I'll use a Darlington Pair to raise the input resistance. Common Collectors are good for driving low loads. I still can't figure out why the negative part of the input signal is clipped.
 

KL7AJ

Joined Nov 4, 2008
2,208
I am trying to design a Common Collector but I have 2 problems:
1. The negative part of the ac signal is clipped unless I use a very high dc voltage (such as 100V).
2. The circuit only works fine at high capacitances.

Here's a copy of the circuit
http://forum.allaboutcircuits.com/attachment.php?attachmentid=8527&stc=1&d=1238940076

By the way, are there any rules of thumb for this type of circuit? I used the R2= (beta * RE) / 10 but I'm not sure whether it is valid since there is no RC.

Thanks

Hi Chaos:

Almost certainly you have to adjust your bias....emitter followers are particularly picky about this, because the current gain is so high. With no input signal, measure the voltage right at the emitter...it should be exactly half your supply voltage. If not, adjust your biasing resistors until it is.

Eric
 

thatoneguy

Joined Feb 19, 2009
6,349
The circuit has too much gain. Increased the base drive by changing the bias resistors lower by a factor of 10 stopped the clipping (5.6k/4.8k). The 10Ω load pulls the emitter low pretty quickly.

--ETA: Teach beat me to it.
 
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