BJT amplifier frequency response.

[pooface]

Today I did a lab on BJT amplifier frequency response.

I got the fc1 and fc2,BW and F0 center frequency. The band pass turned out to be asymmetrical.

What i want to know is this.
Input V=40mVpp
output V=2Vpp
cut off V=1.41Vpp

When I turned up the frequency to 2fc2, i got about 0.84Vpp and 4fc2=0.47Vpp.

Theoretically 8fc2 would have yielded 0.235Vpp correct? No problem.

When i did fc1/2 i got 0.58Vpp and fc1/4 0.22Vpp. I guess there are reasons at such a low frequency why i wouldn't get half of 0.58Vpp.

My question is between the 4 values that I have told you, what are the relationships between fc1/2 and 2fc2...and... fc1/4 and 4fc2.

thanks.

 

[Valkarie]

The relationship between fc1/2 and 2fc2 is that they both represent the same point on the frequency response curve. Specifically, they both represent the lower 3 dB point, which is the point where the power has dropped to 1/2 of the maximum power. The relationship between fc1/4 and 4fc2 is similar, in that they both represent the upper 3 dB point, which is the point where the power has dropped to 1/4 of the maximum power.

 

[piercas]

The relationships between fc1/2 and 2fc2, as well as fc1/4 and 4fc2, are related to the bandwidth of the amplifier. In general, as the frequency increases, the output voltage decreases due to the limited bandwidth of the amplifier. This can be seen in your results where at 2fc2, the output voltage is lower than at fc1/2. Similarly, at 4fc2, the output voltage is lower than at fc1/4.

The asymmetrical bandpass response you observed is a common characteristic of BJT amplifiers and is caused by the parasitic capacitances and resistances within the amplifier circuit. These parasitics can affect the overall frequency response and cause it to be asymmetrical.

It is also worth noting that the output voltage at 8fc2 may not necessarily be exactly 0.235Vpp due to various factors such as component tolerances and non-idealities in the amplifier circuit.

In conclusion, the relationships between fc1/2 and 2fc2, as well as fc1/4 and 4fc2, are affected by the bandwidth and parasitic elements in the amplifier circuit. It is important to consider these factors when analyzing the frequency response of a BJT amplifier.