Effect of bypass caps

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Effect of bypass caps

Postby RnFR » 14 Nov 2008, 05:57

can someone please explain how parallel caps to ground affect gain and biasing in BJT or FET gain stages? i have heard a lot of things, like "will increase distortion", "will affect bass", etc. but have never found a concise explanation of what is going on here. anyone want to field this one?
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Re: affect of parallel caps to ground

Postby analogguru » 14 Nov 2008, 11:29

A cap in parallel to the "working2-resistor will always affect TREBLE and depending on size even short the signal totally.

If the signal is taken at the emitter as shown this is -without the cap - only a buffer.

(Only) if you take the signal in fig. 2 from the collector the (bypass-)cap will increase tha AC-gain frequency- depending on its size.

The following considerations are only for the situation when the signal is taken from the collector:

So if in the modified configuration is a 2k2 emitter resistor and the cap is 22 µF the reactance (= AC-resistance) of the capacitor at 80 Hz (lowest note on guitar) is approx. 100 Ohm.

Compared to the value of the emitter-resistor (2k2) the reactance of the capacitor is much lower, therefore the emitter resistor is neclectable for AC-gain considerations concerning the audio-spectrum in doubt.

If the value of the cap is only 100n it is different:
at 80 Hz the cap-reactance is about 20k therefore not influencing the gain very much.
At 800 Hz the reactance is 2k therefore this has to be considered to be in parallel to the 2k2-resistor resulting in a total value of 1k at the emitter, at 8kHz the resulting value is only 200 Ohm.

Since the emitter resistor acts as a current-feedback reducing the gain of the stage, it easily can be seen that the gain is different depending on the frequency - the bypass cap will result in a higher gain at higher frequencies, therefore it can be seen as a treble-boost.

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Re: affect of parallel caps to ground

Postby RnFR » 14 Nov 2008, 17:14

ah yes- BYPASS caps.

thank you for the explanation, AG. i think i understand a bit more now. i'll have to go over this a bit more later and see if i have any questions.
if a mod wants to put bypass caps in the title that might help too. any other info is welcome.

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Re: affect of parallel caps to ground

Postby earthtonesaudio » 14 Nov 2008, 18:00

AG is totally correct.

Here's another way of looking at it (this is how I remember it):

Caps let AC current through them, but not DC. Small caps let through only high frequencies, bigger ones let through progressively lower frequencies. Keep adding more and more capacity and you have a battery, but that's getting off the path a bit...

Anyway, in the common emitter configuration, you CAN put the emitter directly at ground, but the resulting circuit is difficult to bias and varies a lot with hFE, temperature, etc etc. Not particularly fun. (like Q1 in a Fuzz Face)

The way to a more stable circuit is to introduce some degenerative current feedback at the emitter, usually with a resistor (like Q2 in a Fuzz Face). The problem is that AC gain is reduced by this, and the solution to this problem is to make the resistor appear smaller to AC signals by 'bypassing' the emitter resistor with a capacitor to ground (like the gain pot in a Fuzz Face).
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Re: Effect of bypass caps

Postby RnFR » 14 Nov 2008, 18:32

i get it. very helpful. thanks guys.

i really dig this FAQ section. if i come up with more good questions that could use some explaining, even if i happen to know the answer, i'll post.

if anyone else has anything to add to this subject, feel free.


oh yeah- thanks for changing the title- and my spelling :wink:.
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Re: Effect of bypass caps

Postby RnFR » 15 Nov 2008, 05:35

ok, is there some way to determine which frequencies are being pushed depending on bypass cap and Re values?
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Re: Effect of bypass caps

Postby Fuzzer » 19 Nov 2008, 04:20

There are some calculations involved, you find the low-cutoff frequency for the amplifier. Usually, the most important highpass filter in a common emitter amplifier is the one determined by the bypass cap, more than the coupling caps.
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