Capacitor between collector and emitter help

[michael1978]

can somebody tell me, how and what is doing capacitor between the capacitor and emitter,

i read in the internet that the capacitor is input to the emitter, but i dont know how is working
can explain me please

thnx

 

[Audioguru]

You forgot to post the schematic of whatever you are talking about. A capacitor between collector and emitter is used for positive feedback in a simple FM transmitter.

 

[michael1978]

You forgot to post the schematic of whatever you are talking about. A capacitor between collector and emitter is used for positive feedback in a simple FM transmitter.

thnx man, so only for that used of also for something else also

 

[Audioguru]

A common base transistor amplifier has its input at the low impedance emitter and its output at the collector. It oscillates if a capacitor feeds some collector signal to the emitter..

 

[michael1978]

A common base transistor amplifier has its input at the low impedance emitter and its output at the collector. It oscillates if a capacitor feeds some collector signal to the emitter..

For example this circuits
the collector through the capacitor send an signal in the emitter, and that signal is input at the emitter, the input signal is oscillating taken from input of tuned party, but how you calculate, wich is formula , do you know?

 

[Audioguru]

Your circuit has the emitter of the transistor connected to the circuit's ground, then the emitter is not an input. If you connect a resistor in series with the emitter and ground then the circuit will be an audio oscillator, not a radio.

But the earphone might have a resistance too low then the high DC current might destroy it and the transistor.

Since the variable capacitor has a very high capacitance, are you treying to make an AM radio? Then you need an RF amplifier, not an audio oscillator and you also need an AM demodulator that is usually a diode.
But in your other thread you were trying to make an FM radio that is completely different.

 

[michael1978]

Your circuit has the emitter of the transistor connected to the circuit's ground, then the emitter is not an input. If you connect a resistor in series with the emitter and ground then the circuit will be an audio oscillator, not a radio.

But the earphone might have a resistance too low then the high DC current might destroy it and the transistor.

Since the variable capacitor has a very high capacitance, are you treying to make an AM radio? Then you need an RF amplifier, not an audio oscillator and you also need an AM demodulator that is usually a diode.
But in your other thread you were trying to make an FM radio that is completely different.

thnx Audioguru, to tell you the truth, i just want to know how its work, because now i know that dont oscillate,
because i find strange this connection, so you say just to protect from dc current, nothing else ok?, but how do you calculate that capacitor how many farads? and also they call decoupling capacitor

 

[Audioguru]

An earphone should never have DC current in it.
You must learn about basic electronics to calculate capacitor values. It looks like Swagatam Innovations in India also knows nothing about electronics.

 

[michael1978]

An earphone should never have DC current in it.
You must learn about basic electronics to calculate capacitor values. It looks like Swagatam Innovations in India also knows nothing about electronics.

but whu you dont tell mee, if i know i will not be here

 

[Audioguru]

This is not a school. On this website and others we already learned electronics in school, university or college. We do not come here to teach people like you who know nothing about electronics, instead we come here to chat with people like us about how circuits work and how to make them simpler or work better.

 

[michael1978]

This is not a school. On this website and others we already learned electronics in school, university or college. We do not come here to teach people like you who know nothing about electronics, instead we come here to chat with people like us about how circuits work and how to make them simpler or work better.

i did not know that, one friend of mine, he say go and ask to this forum for help, secondly wat cost to helpy??????????, i learn also in school private, but i stop it

 

[Audioguru]

There are some real teachers who come to these forums and there are some wannabe teachers. Sorry, I am not a teacher. Also, you posted a horrible schematic that will not work because it is full of errors.

 

[BobK]

Also, you posted a horrible schematic that will not work because it is full of errors.

It may not be a good circuit, your point about running current through the earphones is well taken, but I suspect it will work. It is basically just a crystal radio with the transistor serving both as a detector and amplifier.

Bob

 

[Audioguru]

I see how it can be a simple AM crystal radio. The website for Swagatam Innovations is www.homemade-circuits.com in India. I could not find this project in their list of hundreds of circuits.

 

[michael1978]

There are some real teachers who come to these forums and there are some wannabe teachers. Sorry, I am not a teacher. Also, you posted a horrible schematic that will not work because it is full of errors.

I just was searching for fm receiver in google and i pic up this one.....i think somebody who make, he dont make it with error

 

[Audioguru]

Somebody else found the circuit of an FM transmitter on the internet, built it and asked on a forum why it didn't work.
I looked at its circuit and saw 4 errors that I fixed:
1) Its mic preamp transistor was biased wrong when the battery was new and again when the battery was old so I corrected its biasing and added a low dropout voltage regulator IC.
2) Its radio frequency changed when something moved towards or away from its antenna so I added an RF amplifier to separate the tuned LC circuit from the antenna.
3) Its radio frequency changed as the battery voltage ran down so I powered its oscillator from the voltage regulator I added.
4) It sounded muffled with no high audio frequencies so I added pre-emphasis like all FM radio stations have.

I built it compactly on stripboard with all the strips cut as short as they can be and tested it. It worked perfectly and sounded great. Here it is:

 

[michael1978]

Somebody else found the circuit of an FM transmitter on the internet, built it and asked on a forum why it didn't work.
I looked at its circuit and saw 4 errors that I fixed:
1) Its mic preamp transistor was biased wrong when the battery was new and again when the battery was old so I corrected its biasing and added a low dropout voltage regulator IC.
2) Its radio frequency changed when something moved towards or away from its antenna so I added an RF amplifier to separate the tuned LC circuit from the antenna.
3) Its radio frequency changed as the battery voltage ran down so I powered its oscillator from the voltage regulator I added.
4) It sounded muffled with no high audio frequencies so I added pre-emphasis like all FM radio stations have.

I built it compactly on stripboard with all the strips cut as short as they can be and tested it. It worked perfectly and sounded great. Here it is:

i dont understand, why the people put circuits with errors!!!!!!!!!!!!!!!!!!!!!!!!

 

[AnalogKid]

For example this circuits

That circuit is a simple AM radio receiver. The transistor is acting as a rectifier with gain, and the capacitor is a lowpass filter to recover the audio signal. It works with the impedance of the earphone to form a single-pole R-C filter.

ak

 

[Audioguru]

i dont understand, why the people put circuits with errors!!!!!!!!!!!!!!!!!!!!!!!!

Because they did not learn the basics of electronics.

I am not a doctor because I never learned the basics of medicine and diseases but I do not put medical errors on the internet like some "quack" wannabee doctors do.
There are even people who know nothing about mechanics putting errors about fixing cars on the internet.

 

[michael1978]

That circuit is a simple AM radio receiver. The transistor is acting as a rectifier with gain, and the capacitor is a lowpass filter to recover the audio signal. It works with the impedance of the earphone to form a single-pole R-C filter.

ak

AHhh lowpass i know what is lowpass, thank you..........