Bypass 'capacitors' on single transistor amplifier (?)

[Willen]

What is the purpose of parallel capacitor to spk and collector capacitor axactly?

I guessed parallel capacitor 10nf is to eliminate high frequency noise (or what?) but 10nf seems little high so won't it short the audio?

I am about to ask another same type of question after solving this one... Thanks

 

[Nigel Goodwin]

The electrolytic is the normal emitter bypass capacitor, preventing negative feedback to give higher gain, and the 10nF is a simple HF roll-off capacitor, to reduce high frequencies and help remove RF from the speaker.

 

[crutschow]

The corner frequency of 10nF across an 8 ohm speaker is about 2MHz, so it will have no effect on the audio.

 

[Willen]

I found on some basic circuit- 10nF as a audio coupling capacitor so it will passes audios. Then won't the audio short through 10nF? I think better to use 100pf of 1nF, isn't it?

 

[WTP Pepper]

The corner frequency of 10nF across an 8 ohm speaker is about 2MHz, so it will have no effect on the audio.

But it will help stop your local taxi firm splattering audio into the circuit.

 

[Nigel Goodwin]

The corner frequency of 10nF across an 8 ohm speaker is about 2MHz, so it will have no effect on the audio.

The speaker is 64 ohm please pay attention!

 

[Nigel Goodwin]

I found on some basic circuit- 10nF as a audio coupling capacitor so it will passes audios. Then won't the audio short through 10nF? I think better to use 100pf of 1nF, isn't it?

No - trust people who know how it works.

If you want to convince yourself, work out at what frequency the reactance of 10nF will equal 64 ohms?.

 

[Willen]

OK i think main question is solved! Another same question about bypass capacitor-

this amplifier has a 100pf bypass capacitor between base to collector. Amp has a negative feedback resistor as base bias.

I've seen various amplifiers which has voltage devider as base bias and I've seen 100pf capacitor between base to emitter.

Is it good to use 100pf between base to collector if amp has feedback resistor as a base bias? (may be childish question? )

 

[simonbramble]

the cap across the bottom resistor on a resistor divider is normally to stop noise from the power supply getting into the dc bias circuit to the base (and is normally bigger than 100pF). The capacitor from collector to emitter is normally high frequency negative feedback to stop oscillations (and taxi cabs) getting through to the output.

Another reason for the cap across the speaker is to reduce the 'thump' on the loudspeaker when the power is switched on

 

[Willen]

...The capacitor from collector to emitter is normally high frequency negative feedback to stop oscillations (and taxi cabs)...

Did you mean 'Collector to Base' like my circuit to stop oscillation? Because collector to emitter feedback cap help to oscillate on VHF oscillator (like 5pf).

And what is taxi cab effect?

 

[Willen]

...The capacitor from collector to emitter is normally high frequency negative feedback to stop oscillations (and taxi cabs)...

Did you mean 'Collector to Base' like my circuit to stop oscillation? Because collector to emitter feedback cap help to oscillate on VHF oscillator (like 5pf).

And what is taxi cab effect?

And 'thump' mean a loud sound effect due to surge voltage when we switch ON?

 

[simonbramble]

sorry - I meant Collector to Base. The Taxi cab effect refers to WTP Pepper's comments above - my strange sense of humour....

 

[unclejed613]

since the original circuit is a super-regenerative receiver, the first transistor oscillates at an RF frequency. the 10nF caps bypass RF from the audio waveform. the 10uF cap allows the 2N3904 a stable DC operating point without sacrificing AC gain.

also, if this is a receiver, there should be an antenna input somewhere....... more specifically it should be capacitively coupled to the base of the oscillator.

 

[crutschow]

The speaker is 64 ohm please pay attention!

Yup, I missed the 64 as applying to the speak impedance. In that case the corner frequency is ≈250kHz.

 

[Willen]

Yup, I missed the 64 as applying to the speak impedance. In that case the corner frequency is ≈250kHz.

Hi, are you talking about Cutoff Frequency of capacitor by saying 'Corner effect'? I tried to search about corner effect but even can't find.

I think Yes because I got 248KHz cutoff frequency by 10nF parallel capacitor followed by 64 ohms series resistor which is LowPass filter (attached figure).

But here in real circuit 10nF cap and 64 ohms both are in parallel, will this form 'LowPass filter'?

 

[unclejed613]

simplified version? yes.... with 10nf in parallel with 64 ohms. the 64 ohms remains 64 ohms at all frequencies (we'll ignore the frequency dependent characteristics of the speaker for the moment, and treat it as a resistance) and call it the load resistance (Rl), but the reactance of the capacitor IS frequency dependent, and it decreases as frequency increases. at 100hz, it's almost 160k ohms, at 1khz it's 16k, at 10khz it's 1.6k, at 100khz it's 160Ω, and at 1Mhz it's 16Ω. so until you get to 100khz, it hasn't much effect, but as it's reactance decreases, it shunts more AC current away from the 64Ω load. the point where Xc and Rl are equal, is about 250khz. since half the current is shared equally, the power dissipated in the 64Ω speaker is about half what it was at any audio frequency. this is the "corner frequency". so for the load, this parallel arrangement has the effect of a low pass filter for the current through the speaker.

 

[crutschow]

Hi, are you talking about Cutoff Frequency of capacitor by saying 'Corner effect'? I tried to search about corner effect but even can't find.

I think Yes because I got 248KHz cutoff frequency by 10nF parallel capacitor followed by 64 ohms series resistor which is LowPass filter (attached figure).

But here in real circuit 10nF cap and 64 ohms both are in parallel, will this form 'LowPass filter'?

Yes, the corner frequency is the "cutoff frequency" (-3dB point) of the circuit.

It's actually the transistor in series with the impedance of the capacitor and speaker in parallel that forms the low-pass filter. The transistor acts like a current source (very high output impedance so it has no significant effect on the filter cutoff frequency), thus the percentage of the transistor current that goes through the speaker at a particular frequency is determined by the relative impedance between the capacitor and the speaker at that frequency.

 

[audioguru]

1) Did you make the extremely simple "regen" FM radio?
2) Was it difficult to tune to one side of an FM radio station for it to "slope detect" the FM since it is actually an AM radio?
3) Was it overloaded by many strong local stations?
4) Was it NOT sensitive to weak distant stations?
5) Was the sound distorted with no high audio frequencies?
6) Did its harmonics cause a nearby airplane to crash?

 

[Willen]

1) Did you make the extremely simple "regen" FM radio?
2) Was it difficult to tune to one side of an FM radio station for it to "slope detect" the FM since it is actually an AM radio?
3) Was it overloaded by many strong local stations?
4) Was it NOT sensitive to weak distant stations?
5) Was the sound distorted with no high audio frequencies?
6) Did its harmonics cause a nearby airplane to crash?

Hi audioguru,
Being interested with electronics and being engage with well known electronics experts (like here) I have been hobbyists. I like amazing but 'basic' circuits of each device which is amazingly smaller. Like basic of FM station= small FM Tx, basic of AM radio= Crystal radio and cat whisker radio or one to three transistor radios and other amazing detectors etc. This is also very basic and amazing thing to me so going to make as an experiment.

My area has been covered by only 10 FM stations with 1 AM station. It is very open area. So may be I have lots of chance to experiment with too much basic circuits. So no chance to crash an airplane. Feeling excited...

Here is no active RF Cops. We are facing an interference on FM band from few years at 104MHz. It has very strong signal so if I searched FM stations using auto search function with cell phone, it detects noise of 104MHz as a station. I don't know which device is producing this extremely huge interference. If it was Canada, it would be a huge problem and would solve within an hour.

 

[crutschow]

Way back when (circa 1960), I built a single vacuum tube (valve) super regenerative FM receiver from Popular Electronics Magazine. Amazingly I found a reprint here "One-Tube FM Tuner" and here's a picture of it. The air variable cap was tuned with a vernier dial to allow fine tuning (the super regen is difficult to tune). I actually got it to work and was able to pick up a few local FM stations. Of course the sound quality was not the best, but it was usable.