Function of Capacitors in this context? (basic audio amplification system)

Thread Starter

CallMeJunior

Joined Sep 29, 2019
3
Hi,

I'm building a basic audio amplification system with two transistors only I don't fully understand the function of the capacitors here. Is it some kind of safety measure? Because my circuit behaves perfectly fine without em.

I've added 2 text documents that you can load into falstad
Note: the resistor's values are a bit off because they are on sliders (have fun :) )

www.falstad.com/circuit/
 

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OBW0549

Joined Mar 2, 2015
2,987
There aren't many people here who use Falstad, mainly other newbies who won't have even the slightest idea how to answer your question anyway.

Post a proper, human-readable schematic diagram of your circuit so the rest of us will be able to see what you're doing and give you some guidance.
 

OBW0549

Joined Mar 2, 2015
2,987
The three capacitors in that circuit function as coupling (i.e., DC-blocking) capacitors: the leftmost cap makes the DC operating point of Q1 independent of the voltage drop across the mic; he cap in the middle makes Q2's operating point independent of Q1's collector voltage; and the 10 μF capacitor on the output prevents DC current from flowing in the speaker.
 

Papabravo

Joined Feb 24, 2006
12,407
A stable operating point for a transistor is useful to avoid having an input signal push the amplifier into it's non-linear region, and to insure the biasing doesn't push it too close to saturation or cutoff.
 
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dl324

Joined Mar 30, 2015
8,958
I'm building a basic audio amplification system with two transistors only I don't fully understand the function of the capacitors here.
That's a pretty crappy amplifier. Is there a reason why you don't just use an audio amplifier like LM386?
Because my circuit behaves perfectly fine without em.
Don't see how it can work without the capacitors unless you're direct coupling.
 

Wolframore

Joined Jan 21, 2019
1,280
Hey those transistors have some high gain. Yeah I don’t like their dependence on hfe but having emitters tied to grounds gives you more gain. It’s not a very reliable circuit. Suggest you redesign.
 

Veracohr

Joined Jan 3, 2011
688
Here you go!

I've tried LTspice but the mac version is pretty bad.
No it's not. The only difference from the Windows version is that the interface is slightly less graphic. I use it every day on Mac and Windows. Just keep a post-it note on your monitor with these commands until you memorize them:

F2 : Place Part
F3 : Wire/connection
F4 : Net label
F5 : Cut
F6 : Duplicate
F7 : Move
F8 : Drag

You also need to learn the format of the analysis commands since there's no graphical interface for setting those on the Mac version. It's a little bit of a learning curve but it's really not that bad, especially if you also have access to the Windows version and you can see how the analysis command is constructed as you enter values into the GUI.

Plus, there's these documents:

LTSpice Shortcuts for Mac OSX
LTSpice User's Guide - I've used LTSpice for about 10 years and I still refer to this frequently
 

AnalogKid

Joined Aug 1, 2013
8,109
my circuit behaves perfectly fine without em.
Really? I don't see how it can work "fine" *with* them, let alone without.

The second transistor (reference designators?) is dangle-biased. This means that the collector operating point and the circuit gain is totally dependent on the characteristics of that particular transistor's gain, which varies greatly with changes in ambient temperature and aging, and can be very different from part to part.

The 1.2K collector load and the 8 ohm circuit load form a 43 dB attenuator.

ak
 

Thread Starter

CallMeJunior

Joined Sep 29, 2019
3
Thanks for all the help and feedback guys. This will really help me orientate myself in the world of electronic design.

I suspect that CallMeJunior is doing this for the experience it gives him. Using an LM386 of similar denies him the experience of seeing how the guts of an amplifier works.
Thats exactly the reason!

I'll be busy studying the terminology used here. Electronics is by far the hobby with the steepest learning curve I've ever dove into.
 

Wolframore

Joined Jan 21, 2019
1,280
There’s a reason why there are a number of books written on the subject and transistors. You would benefit from learning to correctly bias the circuit.
 

dl324

Joined Mar 30, 2015
8,958
I suspect that CallMeJunior is doing this for the experience it gives him. Using an LM386 of similar denies him the experience of seeing how the guts of an amplifier works.
Then we should be telling him how poor the design is and help him to design a class A amplifier.
 

dl324

Joined Mar 30, 2015
8,958
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Audioguru

Joined Dec 20, 2007
11,251
If your class-A audio power amplifier was designed properly so that it actually produced some audio output, the microphone can hear the speaker then you will have acoustical feedback howling. Put the microphone and power amplifier in one room and connect the speaker in another room with the door closed between them.
 

dl324

Joined Mar 30, 2015
8,958
Electronics is by far the hobby with the steepest learning curve I've ever dove into.
Electronics is a difficult subject to bootstrap yourself in without some formal education. If you have no formal education, that would be like an architect trying to build a skyscraper on a weak foundation.

Most of us went to college for years and it still takes years of actual practice for things to gel.

If it was easy, everyone would be an electrical engineer and make ~$200K/year...
 
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