I have a question on two components of a simple amplifier circuit. Can you help please?

OBW0549

Joined Mar 2, 2015
2,987
The 22k resistor on the opamp's input provides a DC path to ground for the input in the event no driving circuit is connected; without it, the output will saturate against either the positive or negative supply rail.

The 1 ohm resistor and 0.22 uF capacitor on the amplifier's output are necessary for stability, to prevent oscillation.
 

Thread Starter

botey

Joined Oct 4, 2016
46
OBW0549 and Dodgydave, thank you for your answers! Now I have a better understanding of the circuit. According to a quick research just did, Zobel impedance matching is used when a passive crossover is used for the speaker driver. This is because the impedance of the speaker changes when frequency changes and that affects the passive crossover network functionality in a bad way. If i don't need a passive crossover network for my speaker driver, that means there is no need to use the 1 ohm resistor and the 0.22uf capacitor. I hope i'm right.
 

Alec_t

Joined Sep 17, 2013
10,384
If i don't need a passive crossover network for my speaker driver, that means there is no need to use the 1 ohm resistor and the 0.22uf capacitor. I hope i'm right.
As OBW said, it's needed for amplifier stability; with or without any crossover network.
 

OBW0549

Joined Mar 2, 2015
2,987
Oh! It is a must then. I wish I knew how they are calculated.
Yes, that series RC network on the output is mandatory. It's needed to suppress high frequency oscillations in the amplifier output stage and has absolutely nothing whatsoever to do with impedance matching, Zobel or otherwise. Such series RC networks are used in all kinds of opamp applications that require driving reactive loads. As for the component values, just use the recommended values and don't worry about calculating them.
 

AnalogKid

Joined Aug 1, 2013
8,109
The output impedance of the amplifier changes as a function of frequency. Combined with the inductance of the speaker, this introduces a phase shift into the feedback signal. At high frequencies the shift can be enough to cause the circuit to oscillate. The Zobel network introduces a neutralizing phase shift that prevents oscillation at those high frequencies.

ak
 

Thread Starter

botey

Joined Oct 4, 2016
46
Yes, that series RC network on the output is mandatory. It's needed to suppress high frequency oscillations in the amplifier output stage and has absolutely nothing whatsoever to do with impedance matching, Zobel or otherwise. Such series RC networks are used in all kinds of opamp applications that require driving reactive loads. As for the component values, just use the recommended values and don't worry about calculating them.
I understand. Thank you!
 

Thread Starter

botey

Joined Oct 4, 2016
46
The output impedance of the amplifier changes as a function of frequency. Combined with the inductance of the speaker, this introduces a phase shift into the feedback signal. At high frequencies the shift can be enough to cause the circuit to oscillate. The Zobel network introduces a neutralizing phase shift that prevents oscillation at those high frequencies.

ak
I think that adds more info to OBW's answer even though I don't know what OBW has to say about it. Thank you and I appreciate your more detailed answer!
 
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OBW0549

Joined Mar 2, 2015
2,987
I think that adds more info to OBW's answer even though I don't know what OBW has to say about it.
My only comment would be that the use of such an RC network goes beyond applications involving loudspeakers or power opamps or even opamps at all: here is one instance of a series RC snubber being used to stabilize the output of a precision voltage reference IC (see the "Output Bypassing" section on p. 8). Linear Technology has an excellent application note, AN-148 "Does Your Op Amp Oscillate?" that discusses op amp stability and gives a good explanation of how a series RC snubber works; see Fig. 9 on p. 7, and the related text.
 

Thread Starter

botey

Joined Oct 4, 2016
46
My only comment would be that the use of such an RC network goes beyond applications involving loudspeakers or power opamps or even opamps at all: here is one instance of a series RC snubber being used to stabilize the output of a precision voltage reference IC (see the "Output Bypassing" section on p. 8). Linear Technology has an excellent application note, AN-148 "Does Your Op Amp Oscillate?" that discusses op amp stability and gives a good explanation of how a series RC snubber works; see Fig. 9 on p. 7, and the related text.
That application note, looks like it has more info than I need. That is great! I will make sure that I read it carefully. Thank you so much!
 
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