Capacitors in a class A amplifier - Page 1
Forum section: electronics / beginners

author: russnash37
author_rank: Newbie
timestamp: March 19, 2019, 12:38:04 pm
content: 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!My latest attempt was using the design schematic from this article:https://www.electronics-tutorials.com/amplifiers/small-signal-amplifiers.htmI 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!Thanks in advance for any help.Russ.

author: Benta
author_rank: Super Contributor
timestamp: March 19, 2019, 02:05:04 pm
content: Did you read the headline of the page you're linking to? "Small signal amplifiers". Not suited for driving loudspeakers and the like.

author: russnash37
author_rank: Newbie
timestamp: March 19, 2019, 02:13:38 pm
content: Quote from: Benta on March 19, 2019, 02:05:04 pmI 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.

author: janoc
author_rank: Super Contributor
timestamp: March 19, 2019, 02:22:19 pm
content: Quote from: russnash37 on March 19, 2019, 12:38:04 pmPost 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.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 19, 2019, 02:29:21 pm
content: Quote from: russnash37 on March 19, 2019, 02:13:38 pmThe 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.

author: russnash37
author_rank: Newbie
timestamp: March 19, 2019, 02:34:27 pm
content: Quote from: janoc on March 19, 2019, 02:22:19 pmYes, it was the amp in figure 4. Thanks for your explanation on the current, that certainly makes sense. My bench PSU is only an entry level type device so I'm sure it cannot read such a small current as 700uA. I will look at the voltages around the circuit to double-check that I hadn't made a mistake that I've missed so far.I'm also wondering if I would be better off using a function generator as a test input for such circuits, I used music as I didn't have anything else, but am expecting a function generator DIY kit in the mail in the next few days.

author: russnash37
author_rank: Newbie
timestamp: March 19, 2019, 02:48:33 pm
content: Quote from: aneevuser on March 19, 2019, 02:29:21 pmI'd never thought of the output stage acting like a voltage divider, that makes so much sense now. Could I expect a similar issue with my oscilloscope? Would that also explain why I was seeing a smaller output current on the scope compared to the input?

author: janoc
author_rank: Super Contributor
timestamp: March 19, 2019, 02:51:42 pm
content: Quote from: russnash37 on March 19, 2019, 02:34:27 pmEven many expensive lab supplies don't go that low. That's what a multimeter is for, the meters on the supply are only informative, don't rely on them.Quote from: russnash37 on March 19, 2019, 02:34:27 pmNot really, music is fine. Just make sure to divide down the input voltage to suitable levels - if you "blast" this type of amplifier with 1V (or more) out of the phone headphones jack (or even a generator), you will overload it and it won't work properly. Again, it is an amplifier for small signals (small as in millivolts!), not necessarily an audio amplifier.This page has a few more practical circuits if you want to experiment with audio amplifiers:https://www.nutsvolts.com/magazine/article/bipolar_transistor_cookbook_part_7

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 19, 2019, 06:20:48 pm
content: Quote from: russnash37 on March 19, 2019, 02:48:33 pmThe scope should not be a problem - it has a very high input impedance (say 1M Ohm or greater) and thus when placed in a circuit, it will draw little current - it won't disturb the operation of the circuit much.If you want to see if your amplifier is working, remove the speaker completely, and leave the scope probe across the o/p - because of its high impedance, almost all of the o/p voltage will be developed across the probe, and will be clearly visible, if it's there.

author: Benta
author_rank: Super Contributor
timestamp: March 19, 2019, 07:59:17 pm
content: Once again "small signal".If you think a class A amplifier with 0.7 mA emitter current will drive an 8 ohm speaker, think again.

author: bionic
author_rank: Newbie
timestamp: March 19, 2019, 08:13:05 pm
content: Class A amplifier with BJT and voltage divider option will drive a speaker. If you bias it right to the max spec for BETA. you will need a darlington pair to juice up the amps at the end stage.I have done it myself, so I know for a fact that it can be done.

author: Benta
author_rank: Super Contributor
timestamp: March 19, 2019, 09:01:27 pm
content: But not at 0.7 mA emitter current. We all know it can be done, but in this case: no.

author: Audioguru
author_rank: Super Contributor
timestamp: March 19, 2019, 10:00:43 pm
content: Yes, the small signal transistor circuit is severely overloaded by a speaker that is only 8 ohms. The transistor amplifier circuit is designed to feed a POWER amplifier that has an input resistance of at least 100 thousand ohms or more.The very low value of the 1uF output capacitor is another problem. It cuts frequencies below 20kHz when it feeds an 8 ohms speaker, then it cuts the levels of ALL audio sounds.

author: james_s
author_rank: Super Contributor
timestamp: March 19, 2019, 11:37:04 pm
content: Easiest to just forget about this circuit and look for a class A audio power amplifier circuit. All that needs to be said here is that this one won't work for what you are trying to do.