for electronic technicians?

Question

A common-emitter audio amplifier stage has a 0.47 micro Farads coupling capacitor between the previous stage and its base. Which one of the following two audio frequencies will produce the greatest signal level at the transistor's base

___a. 40 Hz
___b. 3KHz
___c. Both frequencies will produce the same signal level at the base
___d. Common-emitter audio amplifiers never use a coupling capacitor.

i am taking an electronics course and i am having trouble with this question. please explain or show how you came to your answer

Answer 1

3Khz. The cap in series, from output of previous stage to input of next, is making a high pass filter. If the cap was going to ground, then it'd be a low pass filter. So, that being said, the higher frequency is going to have a higher amplitude at the base of the amplifier, while the lower frequency will be dropped a little by the reactance of the capacitor.

Some will argue and say C, but the cap is a dead giveaway that you will have reactance which is frequency dependant. Even if the difference between drops is 0.000001V from one frequency to the next, you asked which would produce the greatest signal level. 3KHz is the answer.

Answer 2

Whenever you have a capacitor in series with the signal, a high-pass filter is formed (i.e. DC cannot pass, only 'high'-er frequencies). That is the situation here, so based on that alone I would say the answer is b. 3 kHz.

Based on my experience, 0.47 uF is not that big of a value to be passing low frequencies (40 Hz is pretty low), anyway. Of course to be precise I would have to know more about the circuit to determine if c. is correct -- but b. is probably the correct answer.

.

Answer 3

the equation to use if Xc = 1/(2*pi*f*c)

pi = 3.14159.....
f = your frequency
c = your capacitance
Xc = Capacitave Reactance

The lower the reactance, the better your coupling will be.