Cin in NIVGC...

[gabanyayaya]

hi guys

I need some wise opinion here regarding Cin in LM3886 gc circuits.

Referring to BrianGT LM3886, he said Cin is optional. I need to know is there any improvement over this capacitor when use in the circuit?

What types of cap should be suitable...? polarized such as BG or some non polarized like Muse ES or perhaps the expensive film like Mundorf.....

any comments on this by any gurus....???

[pacificblue]

The main function of Cin is to set the DC gain to 1. That way the DC offset at the input is not amplified with the same gain as audio signals. According to the datasheet the typical DC offset is 10 mV, maximum 150 mV. Imagine 150 mV amplified with the 33,35 times gain set by 22k/680R. You get 5 V DC at the output. That is more than 3 W for an 8 Ohm speaker or more than 6 W for a 4 Ohm speaker. Even big woofers cannot cope with that for a long time.

The size determines that it will be an electrolytic capacitor. Choose a low ESR type. Try a small film cap in parallel, if you want to experiment.

[AndrewT]

build an AC coupled amplifier first.
When you know why it works and how it works, then you can get adventurous and try the various mixed AC & DC coupled designs or even the full DC coupled designs.

[gabanyayaya]

Originally posted by pacificblue
The main function of Cin is to set the DC gain to 1. That way the DC offset at the input is not amplified with the same gain as audio signals. According to the datasheet the typical DC offset is 10 mV, maximum 150 mV. Imagine 150 mV amplified with the 33,35 times gain set by 22k/680R. You get 5 V DC at the output. That is more than 3 W for an 8 Ohm speaker or more than 6 W for a 4 Ohm speaker. Even big woofers cannot cope with that for a long time.

The size determines that it will be an electrolytic capacitor. Choose a low ESR type. Try a small film cap in parallel, if you want to experiment.

How does Cin works to set DC gain into 1?

and how does DC offset influence the gain of the amplification in gc case?

[pacificblue]

The gain is set through a voltage divider that is formed by Xf and Xi. In the above schematic Xf is only a resistor called Rf. Without Ci, Xi would be Ri. Then you have the gain equation a = (Xi + Xf) / Xi = (Ri + Rf) / Ri. The gain is the same at all frequencies.

Now lets put Ci back in. Xf remains Rf. Xi becomes ((Ri + Xci) + Rf) / (Ri + Xci), where Xci is a frequency dependent resistance according to Xci = 1 / (2 * PI * f * Ci). Now if you use different values for the frequency f, you see that for high frequencies Xci becomes ever smaller. The goal is to keep it so low in the audio band that it becomes negligible and the gain is approximately as high as without Ci.

For lower frequencies however Xci becomes bigger until it is infinite at 0 Hz = DC. At that point the gain equation would then be (infinite + Rf) / infinite, Rf becomes negligible and infinite divided by infinite is 1. So the gain is more or less 33,35 at audio frequencies and 1 at DC. The -3 dB point is at f = 1 / (2 * PI * Ri * Ci).