Feeding the input of a simple class A amplifier with its output..?

Thread Starter

dyn_o

Joined May 29, 2017
35
Is it possible to feed the input of a simple class A amplifier with its output to have a perpetual current motion in the all system ?

The transistor needs bias voltage (CC), so we could feed (with a part of the output signal) a full-wave bridge rectifier and then a filter and a smoothing capacitor to obtain a CC voltage that we could use to bias the transistor.

So no need for an external CC voltage source to bias the transistor or an external AC voltage source for the input, maybe only at the beginning to start the amplifier but as soon we get an output signal, we use it as I have just explained, so we turn off the external voltage sources.

I suppose this is not possible but why ?
 

crutschow

Joined Mar 14, 2008
24,295
You can do that.
If the feedback between input and output is positive, it's called an oscillator.
Has been unintentionally built by many engineers trying to design an amplifier :rolleyes:
 

Thread Starter

dyn_o

Joined May 29, 2017
35
Ok, but for the VCC source to bias the transistor ? Is it possible to use a part of the output signal, feed a full-wave bridge rectifier, then filter and smooth to obtain a CC voltage that we use to bias the transistor ? So no external source of power at all ?
 

MrChips

Joined Oct 2, 2009
20,161
Ok, but for the VCC source to bias the transistor ? Is it possible to use a part of the output signal, feed a full-wave bridge rectifier, then filter and smooth to obtain a CC voltage that we use to bias the transistor ? So no external source of power at all ?
Yes, we do that all the time. We take the output from the transistor's collector and feed it back to bias the base. It is called self-biasing with negative feedback.
We can also apply positive feedback to an amplifier. Then the amplifier would saturate, collapse and oscillate in a continuous cycle.

You can also take a water pump and pump water uphill. Then use the water to drive a water turbine which supplies power to the water pump. Presto! No external power required!:D

Or you can shine a high efficiency LED on a high efficiency solar panel, generate electricity and power the LED. Same effect.:rolleyes:
 

Bordodynov

Joined May 20, 2015
2,491
There were amplifiers with a similar effect. When there is no signal through the transistor, a small current flows. As the signal increases, the bias current increases. The increase is achieved with a diode rectifier. The signal to the rectifier is fed from the amplifier output. Then, through the resistor, the rectified signal is applied to the base of the transistor. But everything works from a miniature battery! The transistor works in class A for any signal. Such amplifiers were used in hearing aids.
Do you want such an amplifier?
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