AC flowing through the supply source of an amplifier

[Otto_]

Hello.
Excuse me for my whitespaces in the electronics theory. Let us consider an amplifier whose input is AC and which is supplied with DC from a battery.
From a thread on another forum (https://forum.allaboutcircuits.com/...C portion of,back to their respective sources. -- the circuit shown there) I have found out that AC flows also through the DC source.
If this is true, can AC affect negatively (like acute damage or service life shortening) the battery? From the answer of Jony130 to the aforementioned thread, AC flows through the DC source only during its negative half-cycle, so I suppose there would not be any problems with the polarity of the battery, but what is with the alternating voltage which could be too high for the battery (but not too high for the amplifier itself)?

I thank You in advance for answers.

 

[rjenkinsgb]

As long as the AC input signal is within the linear range of the amplifier, its effect is to modulate the base bias.

That base current can never be zero (or negative), or so high the transistor is fully on, as long as the amp is in its linear operating range.

The average current through the base bias resistor will be close to its no-signal value and a fraction of the load applied by the collector circuit.

I cannot see any possible way the input signal could adversly affect the battery, other that applying voltages vastly beyond the normal signal range.
With the linked example circuit the transistor would likely fail before a high level input signal had any undue effect on the power source!

 

[Nigel Goodwin]

Sorry, but I've no idea what you're even asking - it makes no sense?.

The input AC doesn't 'flow through the DC source' - and an amplifier doesn't even 'amplify' the incoming AC, it basically makes a larger copy of it - think of it as a electronic version of an Artists Pantograph.

 

[JimB]

Otto, be aware that on most* discussion forums there are a few contributors who talk a lot of nonsense.
As much as I hate to disparage a forum where I am not a member, in this case I think that you have found one of the nonsense talkers.
JimB

* Even here on ETO.

 

[Tony Stewart]

If the battery did last as long as you expected, and if rechargeable was left drained for too long (rapid aging) or has expired its useful life are the most common reasons.

 

[Tony Stewart]

I like your term "whitespaces" which reminds me why the military started publishing " This page is intentionally blank" in documents which reminds me of a political joke.

 

[ahsrabrifat]

Hello.
Excuse me for my whitespaces in the electronics theory. Let us consider an amplifier whose input is AC and which is supplied with DC from a battery.
From a thread on another forum (https://forum.allaboutcircuits.com/threads/ce-amplifier-ac-current-path.120965/#:~:text=The input AC portion of,back to their respective sources. -- the circuit shown there) I have found out that AC flows also through the DC source.
If this is true, can AC affect negatively (like acute damage or service life shortening) the battery? From the answer of Jony130 to the aforementioned thread, AC flows through the DC source only during its negative half-cycle, so I suppose there would not be any problems with the polarity of the battery, but what is with the alternating voltage which could be too high for the battery (but not too high for the amplifier itself)?

I thank You in advance for answers.

Here you may get more info https://www.eevblog.com/forum/chat/does-current-flow-through-a-battery/

 

[MrAl]

Hello.
Excuse me for my whitespaces in the electronics theory. Let us consider an amplifier whose input is AC and which is supplied with DC from a battery.
From a thread on another forum (https://forum.allaboutcircuits.com/threads/ce-amplifier-ac-current-path.120965/#:~:text=The input AC portion of,back to their respective sources. -- the circuit shown there) I have found out that AC flows also through the DC source.
If this is true, can AC affect negatively (like acute damage or service life shortening) the battery? From the answer of Jony130 to the aforementioned thread, AC flows through the DC source only during its negative half-cycle, so I suppose there would not be any problems with the polarity of the battery, but what is with the alternating voltage which could be too high for the battery (but not too high for the amplifier itself)?

I thank You in advance for answers.

Hi,

This is actually a little bit of an interesting question, but the short answer is that it does not matter if AC flows though the battery because that's what the battery is there for.

The worry about damage to the battery because of an extra high voltage is without warrant also. That's because the output capacitor does not act with the output transistor as a voltage doubler.

The worry about AC current though the battery is also nothing, because there must be AC current through the battery in order to produce an AC output. It can be very high too, 1 amp, 10 amps, 20 amps. The only requirement is that the battery has to be rated for that particular amplifier. If the battery is not rated correctly then it can be damaged that's true, but usually this is well thought out and the correct battery size and type is covered in the manual or is already built it.

There is a little bit of a catch, but not too much. That is, the current may not be completely sinusoidal. That's because for that one transistor amplifier when the transistor is conducting heavily, the output comes mostly from the capacitor which has stored energy from the previous half cycle. Some of that energy is transferred to the output and during that time there would be less current through the DC supply. It can still be significant however, because the collector resistor has to be of a small enough value to keep the output impedance low in most cases. That means there can still be significant current through the DC supply although a bit lower.

These single transistor circuits are interesting because they are very useful for educational purposes.