# The relationship of C to the frequency response of a BJT

Discussion in 'Homework Help' started by Hiikaru, May 25, 2010.

1. ### Hiikaru Thread Starter New Member

May 25, 2010
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Hey guys, im new here

I was hoping someone could help me on a little matter. i Have been investigating simulations of certain amplifiers. Using PSPICE, i have designed a two stage BJT amplifier and with a AC sweep simulation profile ive produced a frequency response.

The first frequency response, all capacitors were 1e-9F and the second all caps were 10e-9F.

From the first response, the the lower frequency cutoff starts at 12Hz. the second starts at 1.2KHz. Both ending (upper frequency cutoff) at 36MHz.

Some what difference of 100 times the of the first response to the second response by increasing the capacitor 10 times.

So my question is, what relationships does the capacitors have with the two stage BJT frequency response?

If pictures of my investigation are needed for more visual aid, ask, and i will post them asap.

May 11, 2010
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Yes, please post pictures. I am interested to see them.

May 25, 2010
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4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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It's very hard to see the schematic detail but I think I see the cap values are either 1000nF [1uF] or 10nF [0.01uF] in each case.

Your results confirm the expectation - that the higher the coupling capacitance values then the lower the low side 3dB cut-off frequency. The higher capacitance value gives lower coupling reactance values - therefore less signal attenuation at the same frequency.

More to the point ....

Your chosen values are in the ratio of 100:1 [1000nF/10nF], not 10:1 as you state, which is consistent with the observed 100 fold change in the lower 3dB cut-off frequency.

5. ### Hiikaru Thread Starter New Member

May 25, 2010
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oh yeah sorry, i noticed that 100:1 ratio, but i forgot to change it when i posted the photos. i had even named the photos 1000nF and 10nF hahah. thanks for the clarification.