I request you to consider the questions/doubts:

Positive Feedback

In a positive feedback the feedback signal is on phase with the input time varying signal (if any). If there is no input signal, the FB signal is in phase with the noise signal. Hope this concept is right.

1. If the amplifier’s voltage gain >1, the input volt. + FB volt. will be amplified to a higher value. If this trend goes on for several times, we will get an output voltage free! This obviously is against the laws of physical sciences, since we can’t get anything worth out of nothing. This could mean (a) somewhere volt. Is getting saturated (i.e., it can’t increase after a few ‘iterations’) or (b) if output voltage goes up, the electric current at the output terminal will decrease, thus the law of conserv. of energy is valid or (c) some other thing happens.

If both (a) and (b) are wrong/partially correct, what is the correct inference?

Power Amplifier

2. What is power ‘amplification’? Again the law of conservation of energy cannot be violated!

Thanks a lot in advance for spending valuable time on reading the text. My e-mail is... Mod Deleted email address... and request you to kindly answer the above. I humbly wish to get physical insights into what’s going on there.

 

Welcome to AAC!
If the amplifier gain is high enough then the positive feedback would cause the output to get driven to either supply rail (or close to it). It can't go beyond that unless inductive/capacitive transients are involved. No physical law is broken.

 

Here is an example of an amplifier which has a signal fed into it
which tries to create a V higher than the supply rails of the amplifier.

In other words when Vin x G >= Vsupply than output of amp
can go no further, it does not create more energy or potential.

This example does not have feedback, but principal the same
for case with feedback.

Regards, Dana.

 

2. What is power ‘amplification’? Again the law of conservation of energy cannot be violated!

The designation is perhaps somewhat of a misnomer.
A power amplifier is simply an amplifier whose signal power output is significantly more than its signal power input, such as an audio speaker amp. The power comes from the supply voltage(s) of course.

But what would you think is a better name for such an amplifier?

 

You understand one concept correctly (law of conservation of energy) but applying this concept incorrectly in applying this to a power amplifier.

Let us examine two analogies of power and energy.

Let us look at the simple lever principle.


In a lever, a small weight M2 can lift a larger weight M1 if the length b is greater than the length a.
The lever is balanced when,
M1 x a = M2 x b

That is, we can amplify the effect of a small weight to lift a heavier load, but the work done is still the same.

An AC transformer is the second analogy more closely related to your power amplifier question.

A step-up transformer amplifies voltage. In the diagram shown above, there are 4 times more turns on the secondary winding than on the primary winding. The input voltage is amplified 4 times to give 4 times the voltage output at the secondary. Note however that the current is reduced by a factor of four. Power is conserved. For a transformer with 100% efficiency, the input power (P = V x I) equals the output power.

Now let us consider the power signal amplifier.
The power of the input signal is the input voltage multiplied by the input current (P = V x I).
The output power is the output voltage multiplied by the output current. In a power amplifier, the output power can be much greater than the input power, and yet the amplifier does not violate any law of conservation of energy or power. Why?

That is because the output power comes from the power supply, not the input signal. The input signal is used to effect a control valve that allows more electrical power to flow from the power supply. If you want to balance the power equation in order to verify the law of conservation of energy, you need to look at the power being taken from the power supply, not the input signal.

 

On the concept of positive feedback, in social issues we call this a "vicious circle" or "vicious cycle" where something you do makes matters worse.

Global warming is a classic example though there are people who would vehemently deny that this is happening.

Another example is using your credit card to pay the interest on your credit balance. Very soon you will hit your credit limit and eventually may have to declare bankruptcy.

When a signal from the output is fed back to the input in-phase, it causes the output to rise exponentially until the amplifier saturates. The output reaches the power supply limits and can rise no further. If there is a phase shift between the output and input there will be a delay between the input and output and this will lead to oscillations.

 

The initial error was assuming that a power amplifier creates power. The reality is that it boosts power by adding power from a separate source. And the useful output is never quite as much as the total input power. Positive feedback will indeed result in an output increasing to some limit, but there is no free power, not ever.