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Capacitors in a class A amplifier
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russnash37:
I've been trying for a few days now to get a simple class A amplifier working on a breadboard and I'm not having much luck!  :palm:

My latest attempt was using the design schematic from this article:

https://www.electronics-tutorials.com/amplifiers/small-signal-amplifiers.htm

I didn't have any 0.83uF capacitors, so used some 1uF electrolytic caps that I had in my stockpile.  For some reason that I cannot work out the circuit, when connected up to 12v from my bench supply, pulls 0.0000 amps and seems to do the complete opposite of an amplifier when I connect up some music to it from my cell phones output jack, the volume is greatly diminished as opposed to when I connect it directly to the speaker being used.  I've switched out the caps and tried another 2N2222A transistor, but still cannot change the end result.

Can anyone please offer any insight into where I could be going wrong?  I'm wondering if the slightly larger or different type of caps is causing an issue, but I cannot see that that would cause a significant problem as even the article mentions that larger value caps and even electrolytic caps could be used.  I'm loosing my hair over this one and there isn't much left to go!  :-DD

Thanks in advance for any help.

Russ.
Benta:
Did you read the headline of the page you're linking to? "Small signal amplifiers". Not suited for driving loudspeakers and the like.
russnash37:

--- Quote from: Benta on March 19, 2019, 02:05:04 pm ---Did you read the headline of the page you're linking to? "Small signal amplifiers". Not suited for driving loudspeakers and the like.

--- End quote ---

I did, this was just driving a small (like 2" speaker) but I was mainly observing my oscilloscope to compare the input to the output and would of expected to see some gain in the output signal.
janoc:

--- Quote from: russnash37 on March 19, 2019, 12:38:04 pm ---I've been trying for a few days now to get a simple class A amplifier working on a breadboard and I'm not having much luck!  :palm:

My latest attempt was using the design schematic from this article:

https://www.electronics-tutorials.com/amplifiers/small-signal-amplifiers.htm

I didn't have any 0.83uF capacitors, so used some 1uF electrolytic caps that I had in my stockpile.  For some reason that I cannot work out the circuit, when connected up to 12v from my bench supply, pulls 0.0000 amps and seems to do the complete opposite of an amplifier when I connect up some music to it from my cell phones output jack, the volume is greatly diminished as opposed to when I connect it directly to the speaker being used.  I've switched out the caps and tried another 2N2222A transistor, but still cannot change the end result.

Can anyone please offer any insight into where I could be going wrong?  I'm wondering if the slightly larger or different type of caps is causing an issue, but I cannot see that that would cause a significant problem as even the article mentions that larger value caps and even electrolytic caps could be used.  I'm loosing my hair over this one and there isn't much left to go!  :-DD

Thanks in advance for any help.

Russ.

--- End quote ---

Post the schematic and maybe few pictures of what have you tried to build.

However, these amplifiers, in the configuration described in the article, are unlikely to be able to drive a regular speaker. A speaker will have too low impedance and present too high load for a transistor in that kind of configuration. If you want to drive a speaker you will need to add a power stage of some sort. These amplifiers are meant to amplify weak signals e.g. in a radio, not to drive a loudspeaker.

The fact that it draws 0A on your power supply possibly indicates that you have something wired wrong. A class A amplifier will draw quite large quiescent current even with no signal present because the transistor needs to biased to be conducting in order to not "chop off" half of the input signal.

However, if you have build the amp in figure 4, that one has quiescent current of about 700uA - unless your power supply has an extremely accurate meter with a low burden voltage, you are very unlikely to be able to measure that. Most supplies don't have better resolution than about 10mA (3 digit displays), with a 4 digit one you could theoretically measure 1mA but this amplifier draws less than that. So it would show as 0 on the PSU meter. Use a multimeter, check the voltages across the resistors and compare with what is on that website instead.

aneevuser:

--- Quote from: russnash37 on March 19, 2019, 02:13:38 pm ---
--- Quote from: Benta on March 19, 2019, 02:05:04 pm ---Did you read the headline of the page you're linking to? "Small signal amplifiers". Not suited for driving loudspeakers and the like.

--- End quote ---

I did, this was just driving a small (like 2" speaker) but I was mainly observing my oscilloscope to compare the input to the output and would of expected to see some gain in the output signal.

--- End quote ---

The output of an amplifier can be modelled as an AC voltage source in series with a resistance R_out. For the amplifier that you've built, R_out will be about the value of the collector resistor (I think, off the top of my head..) so say about 6K ohms.

You're trying to drive a speaker of say 8 ohms impedance from the o/p of that amp. So when you connect that 8 ohms in series with the R_out of the amp and ground, you have a voltage divider with about 6K at the top and 8 ohms at the bottom - almost all of the o/p voltage of the amp will be developed across the R_out, and very little across the 8 ohms - in other words the o/p resistance of the amp is very high compared to the load - very little current will flow in the speaker.

To fix this, you need a extra stage that is capable of supplying a larger current to the small speaker resistance i.e. you want a current amplification stage (or to put it another way: you want a stage with a low o/p resistance, so most of the voltage is developed across the 8 ohm speaker). Such a stage is usually based on an emitter follower, and often two of them arranged in a so-called push-pull arangement, which you can look up.
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