Need help with circuit

Thread Starter

Arjun Swani

Joined Aug 17, 2017
3
http://www.circuitbasics.com/wp-content/uploads/2015/04/Build-a-Great-Sounding-Audio-Amplifier-with-Bass-Boost-from-the-LM386-Minimal-Amplifier-Schematic.png

Hello,
I've been trying to educate myself about amplifiers (using op amps), with the final goal to build an audio amplfier of my own.
I stumbled upon this design and tried to analyse it.

From my understanding this is a negative feedback circuit. C1 acts as a high pass filter. R1 controls the gain. I can't figure out what C2's purpose is.
I would like some help there and would also like to know if any of my observations are wrong.

Thanks in advanced
 

OBW0549

Joined Mar 2, 2015
3,107
C1 acts as a high pass filter.
No, it does not. C1's job is to provide AC coupling for the speaker, preventing DC at the LM386's output from appearing at the speaker.

R1 controls the gain.
No, it doesn't. The gain of the LM386 is fixed internally.

I can't figure out what C2's purpose is.
R1 and C2 form what is called a "snubber" network which presents a heavy load on the LM386's output at high frequencies. Without this load, the LM386 tends to oscillate. You see this kind of network on many audio amplifier designs.
 

Thread Starter

Arjun Swani

Joined Aug 17, 2017
3
No, it does not. C1's job is to provide AC coupling for the speaker, preventing DC at the LM386's output from appearing at the speaker.


No, it doesn't. The gain of the LM386 is fixed internally.


R1 and C2 form what is called a "snubber" network which presents a heavy load on the LM386's output at high frequencies. Without this load, the LM386 tends to oscillate. You see this kind of network on many audio amplifier designs.
No, it does not. C1's job is to provide AC coupling for the speaker, preventing DC at the LM386's output from appearing at the speaker.


No, it doesn't. The gain of the LM386 is fixed internally.


R1 and C2 form what is called a "snubber" network which presents a heavy load on the LM386's output at high frequencies. Without this load, the LM386 tends to oscillate. You see this kind of network on many audio amplifier designs.
Thank you for correcting me. I clearly need to learn more.
 

BobTPH

Joined Jun 5, 2013
2,140
It is not totally incorrect to say that C1 is a high pass filter. Replace it with a 10uF and you will see what I mean, or rather hear what I mean. It does act as a high pass RC filter with R being the impedance of the speaker. For an 8 Ohm speaker, the cutoff frequency is about 20 Hz.

Bob
 

OBW0549

Joined Mar 2, 2015
3,107
C2/R1 is a Zobel Network. http://www.wavecor.com/html/zobel_networks.html You can do other searches.
While the R1/C2 network may superficially resemble a Zobel network in form, its function in the circuit is entirely different: whereas a Zobel network is used to render a speaker's effective impedance essentially constant, R1 and C2 in this circuit are there to keep the amplifier from oscillating at RF frequencies-- something the LM386 is very prone to doing since it lacks the frequency compensation cap present in most op amps for exactly that purpose.

The values of C2-- 0.1 μF in the TS's circuit, and 0.05 μF in the LM386 data sheet-- are orders of magnitude smaller than the Zobel network capacitor values listed in the reference you provided, a sure tip-off that R1 and C2 here are NOT there to function as a Zobel network. While the pole frequency of a typical Zobel network might be a few hundred Hertz to a kHz or so, the pole frequency of the RC network in the LM386 circuit is over 150 kHz.
 

Tonyr1084

Joined Sep 24, 2015
3,955
From my understanding this is a negative feedback circuit.
I'm surprised nobody else caught this. No, it's not a negative feedback amp. There is NO feedback whatsoever that I see. In a feed back amp you would see the output fed back via a wire or a resistor (or resistors) back to either the positive (non-inverting) pole of the amp or the negative (inverting) pole. (see AAC Education, Semiconductors, Operational Amplifiers https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/negative-feedback/ for more information on negative feed back).
 

ian field

Joined Oct 27, 2012
6,539
No, it does not. C1's job is to provide AC coupling for the speaker, preventing DC at the LM386's output from appearing at the speaker.


No, it doesn't. The gain of the LM386 is fixed internally.


R1 and C2 form what is called a "snubber" network which presents a heavy load on the LM386's output at high frequencies. Without this load, the LM386 tends to oscillate. You see this kind of network on many audio amplifier designs.
Its actually a Zoebel network (it has at least one other name) - well documented on the web.

The TS should add supply decoupling - at least 47uF electrolytic and parallel 0.1 or 0.22uF.
 
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