EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: techninja80 on January 03, 2025, 09:27:49 pm
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So trying to put a few things I learned together to just to see how things work. So the circuit has no real world application other than to make me smile. I am trying to make a tiny signal into a big one with decent amps. I guess maybe a speaker is the best IRL application for this. Either way. I am using a 1 watt load as X5 as the end goal for my tiny .1V from sig to be amplified to 6v. I am also trying to use things that I have in my lab right now. So that is why these items where chosen. Ultimately every thing seems great until I get to X5 that is where the signal collapses to nothing. My guess is that the transistors Q1, or Q2 is the problem because I can make the transistor amplify the signal as needed if I just use a signal source.
Please explain to me what I am doing wrong. If I need other discreet components please let me know.
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The DC biasing of Q1 and Q2 is inappropriate for a linear amplifier.
Relying on the beta of a transistor for biasing fails due to the beta for an individual part being very uncertain.
See any elementary textbook for examples of useful bias circuits.
What is X5 in that drawing?
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https://www.youtube.com/watch?v=9SclQIWkOtk (https://www.youtube.com/watch?v=9SclQIWkOtk)
Try this, very basic op-amp amplifier circuit:
[attachimg=1]
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What is the purpose of the emitter follower (Q1) in that circuit?
The opamp already has a very low output impedance.
Why not just connect the load (the 100Ω resistor) to the opamp output?
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The DC biasing of Q1 and Q2 is inappropriate for a linear amplifier.
Relying on the beta of a transistor for biasing fails due to the beta for an individual part being very uncertain.
See any elementary textbook for examples of useful bias circuits.
I tried replacing X5 with R7, but I kept getting a collapse of stg3 signal. That is why I used the voltage follower and it improved things but I lost half of my amplitude. What I don't understand is if I just use a siggen, and replicate stg3 signal, I can get the transistor to work . See the test circuit I added, its not the best waveform, but it is much better than what I can get out of the problem circuit.
What is X5 in that drawing?
Its just a constant load at 1watt with a 6ohm minimum resistance. I added it just make it power something beefy (for me :) )
What is the purpose of the emitter follower (Q1) in that circuit?
The opamp already has a very low output impedance.
Why not just connect the load (the 100Ω resistor) to the opamp output?
Without it the signal collapses when I put it behind a transistor??? This is essentially the main problem I added it to help solve it.
Finally @DavidAlfa I will give that a try.
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What is the purpose of the emitter follower (Q1) in that circuit?
The opamp already has a very low output impedance.
Why not just connect the load (the 100Ω resistor) to the opamp output?
Without it the signal collapses when I put it behind a transistor??? This is essentially the main problem I added it to help solve it.
No, I meant in DavidAlfa's circuit, not yours. Your Q1 is not an emitter follower.
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Op-amps can't provide much current, that transistor amplifies it and makes possible to drive heavier loads.
In fact 100ohms will be too much for most op-amps.
You can remove the transistor if you're connecting a light load, like another amplifier input.
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Try this, very basic op-amp amplifier circuit:
Thanks for this. I watched the video and was able to get something working by adding that feedback resistor R11 (see new schem). I am happy but I want more power >:D . Why am I stuck at 2 volts, is it because it is because the transistor in that linear mode, or do I need something like a power mosfet?
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Not to mention mucho distortion on the output.
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I have watched a bunch of UT videos on transistors and op-amps but I got the most from this one. Its a bit long winded and repetitive, but it did explain a few important details that were light bulb moments for me.
https://www.youtube.com/watch?v=WgOKPF8LkA8 (https://www.youtube.com/watch?v=WgOKPF8LkA8)
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Your current circuit has a number of problems, but I am not dure it can be explained in a simple to get way. However, I will try.
1. Stop with the net labels and not connecting by wires, stg1 is the most obvious where teo labels are right next to each other but not connected.
2. Now to the more practical stuff. What you are building Is essentially a single ended audio amplifier with collector drive. You want to acheive 1V output and from a 0.1V input signal, 10x gain.
3. The circuit shown uses opamps as buffers only, with the transistor acting as the main amplifying device but with very limited control of gain and biasing.
My advice is the following (and I suspect you will need to use keywords for looking up, a full solution goes counter to learning): AC couple your signal to bias the first opamp around Vcc/2 (or adjustable for a DC load), implement your 10x gain in this stage (preferably closing the loop at the output of your buffer transistor), change your output transistor to emitter follower and choose PNP or NPN based on if your load is supply or 0V referenced.
Like the circuit previously posted this means you will need one opamp and one transistor.
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but it did explain a few important details that were light bulb moments for me.
Came back to thank you. This video was wonderful and gave me the light bulb moments as well. Took me a few days to get to it due to IRL stuff, but I am very happy I took the time to watch it. Also put it into play, and came out with a way better circuit. Thank you very much.
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0.1V isn't a tiny signal. It's high enough not to worry too much about noise.
Is the voltage negative as well as positive?
What's the frequency? Does it go down to DC?
If it has a DC component then it can't pass through any capacitors.
It it can go negative, as well as positive, then you either need a dual supply or a bridged output, for DC, or AC couple, if it doesn't have any DC.
The best way to do this is to put the power transistors inside the feedback loop of an op-amp.
Here's an example of an op-amp with a couple of booster transistors. The diodes reduce the distortion by keeping the transistors slightly on all the time, otherwise there will be crossover distortion when one transistor turns on and the other off. Incidentally, this is a problem with the LM358's output stage.
(https://www.eevblog.com/forum/beginners/inputoutput-caps-on-simple-headphone-amp/?action=dlattach;attach=178179;image)