Local NFB via capacitor

Thread Starter

pdavis68

Joined Nov 27, 2013
42
I've seen a lot of audio circuits where you'll have a transistor in say a common emitter setup and then a cap, say 150pf between the collector and the base. I'm assuming this is a local NFB to attenuate higher frequencies. If that's the case, how do you calculate the effect of that? What's the frequency? What's the attenuation? I've googled and maybe I'm just wrong about what this is, but I can't find anything on it.

Pointers to further reading on the topic would be appreciated as well.

Thanks.
 

Thread Starter

pdavis68

Joined Nov 27, 2013
42
The capacitor would attenuate low frequencies and pass high frequencies. Can you show an example?
I was unclear in what I meant. Yes, the capacitor will pass the high frequencies, but as it's inverted at the collector, it would thus act as NFB, attenuating the high frequencies. This would be an example.

1604423589582.png

So how would I calculate the effect?
 

Papabravo

Joined Feb 24, 2006
17,041
I was unclear in what I meant. Yes, the capacitor will pass the high frequencies, but as it's inverted at the collector, it would thus act as NFB, attenuating the high frequencies. This would be an example.

View attachment 221355

So how would I calculate the effect?
Capacitors don't know anything about the inversion of the signal polarity; they are passive components.
 

Thread Starter

pdavis68

Joined Nov 27, 2013
42
Capacitors don't know anything about the inversion of the signal polarity; they are passive components.
I never suggested the capacitor "knew" anything. You seem to be more interested in picking apart what I'm saying than actually answering the question I've posted twice. If you're not interested in helping, please just don't reply.
 

Papabravo

Joined Feb 24, 2006
17,041
I never suggested the capacitor "knew" anything. You seem to be more interested in picking apart what I'm saying than actually answering the question I've posted twice. If you're not interested in helping, please just don't reply.
You're suggesting that because a CE configuration will have an output that is an inverted replica of the input that suddenly the capacitor will invert the way it works and attenuate high frequencies. I'm telling you that notion is nonsense. You can ignore the replies, but you don't get to choose who can reply.

2nd point: As usually understood negative feedback implies a phase shift of 180° and a capacitor, by itself cannot provide that. So to answer your question it is not providing negative feedback.
 
Last edited:

Thread Starter

pdavis68

Joined Nov 27, 2013
42
You're suggesting that because a CE configuration will have an output that is an inverted replica of the input that suddenly the capacitor will invert the way it works and attenuate high frequencies. I'm telling you that notion is nonsense. You can ignore the replies, but you don't get to choose who can reply.
No, you're clearly misunderstanding what I'm saying. I'm not an EE. I've never taken an electronics class, so if my language in imprecise, I apologize. I'm basically asking 2 questions:
1> Is my understanding correct and if so, how do I calculate the effect (frequency and attenuation)? And if my understanding is incorrect, what is the purpose of the capacitor in the circuit.
2> Where can I find further reading on the subject.

My understanding is:
1> It's configured as a common emitter amplifier
2> The output of the emitter is the amplified and INVERTED signal from the base.
3> The capacitor is acting as a high pass filter to send that INVERTED high frequency portion back to the base, as such, it would be acting to REDUCE the high frequency output of the amplifier. Also known, I believe, as local negative feedback or LNFB.
 

Papabravo

Joined Feb 24, 2006
17,041
No, you're clearly misunderstanding what I'm saying. I'm not an EE. I've never taken an electronics class, so if my language in imprecise, I apologize. I'm basically asking 2 questions:
1> Is my understanding correct and if so, how do I calculate the effect (frequency and attenuation)? And if my understanding is incorrect, what is the purpose of the capacitor in the circuit.
2> Where can I find further reading on the subject.

My understanding is:
1> It's configured as a common emitter amplifier
2> The output of the emitter is the amplified and INVERTED signal from the base.
3> The capacitor is acting as a high pass filter to send that INVERTED high frequency portion back to the base, as such, it would be acting to REDUCE the high frequency output of the amplifier. Also known, I believe, as local negative feedback or LNFB.
1> A common emitter amplifier takes it's output from the collector. The emitter is the common node between the input and the output circuit.
2> The emitter is grounded and is the reference point for the rest of the circuit
3> At high frequencies the base and the collector are effectively shorted together by the capacitor. What is the impedance of that capacitor for frequencies over the audio range?
 

Audioguru again

Joined Oct 21, 2019
3,680
The capacitor feeding the collector signal to the base does not cancel high frequencies if the input signal source has a low output impedance.
 
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