Dear Sir Rich,
Nice, fabulous, mind blowing and reasonable explanation for sure. Undoubtedly, you are knowledgeable.
Sign of expert !
Thank you very much for taking part in this circuit.
Even I have spend almost 10 years in studying Electronics circuits, seems like all my concepts are confusing.
But, I love to study circuits, even thousand time I fail to understand. People hired me as an Electronic Engineer that does not mean I understand circuits!
Let me draw points on your comments,
T
The first stage is a non-inverting AC amplifier.
Ac signals are being amplified, do you mean the source of AC signal is from MIC? Is it fluctuating dc type AC?
Dont you think, this idea is better to reduce the effects of finite "input offset voltage."
In that case it may be preferable to omit the capacitor and trim the offset voltage to zero, large capacitor has chosen.
The R2/R4 divider is there to provide 1/2 of the supply voltage (4.5V) to the non-inverting input. This ensures that the AC output signal is centered 1/2 way between ground and power, so there is room for the signal to move higher and lower by up to about +/-4 volts. Without the ability to go up and down in voltage, you would get clipping - or some of the audio signal cut off - which would sound lousy.
Yes, this non inverting amplifier is not an ideal one. The way the AC signals are allowing can understandable, the frequency at which the impedance of the capacitor is taking part. As we are dealing with audio output, case is more sensitive!
A general idea of an op amp is the output goes where ever it needs to to try to make the two inputs balanced. That's not a big technical theory, but it sort-of works out that way and I find it an easy way to understand op amps. In this case, the output will go up and down with the incoming audio in an effort to keep the two inputs balanced as the audio signal changes.
A crucial point raised here, that is input balancing. Very basic building block may be, "the op-amp voltage gain is so high that a fraction of a millivolt between the input terminals will swing the output over its full range", can we ignore the voltage difference? Input draw no current?
The first stage indeed does have negative feedback. That is entirely necessary and normal for an op amp that without some gain control might have gains of hundreds of thousands. In effect, even a slight voltage offset of the input (which is typical for op amps), could be amplified to the point that an op amp without negative feedback would simply drive itself to either the positive output limit or the negative output limit, what is called "going to the rails".
Like some other stupid, some time I feel that the negative feedback is effect of reducing the amplifier's gain. But people says "True, it does lower the gain, but in exchange it also improves other characteristics, most notably freedom from distortion and nonlinearity, flatness of response (or conformity to some desired frequency response), and predictability." You are right in this case as you are talking input and output limiting.
The second stage is as you said, connected with an RC filter. It is somewhat a by-product of the technology. The capacitor is there to ensure the signal from the first stage gets through but the DC bias of 4.5 volts does not get thru to screw-up the second stage's bias voltages. Because it is going to the non-inverting input, the input has a nearly infinitely small load on the signal.
Yes, meaning full, that means this RC filter is making a bridge in this 2 stage, eaither for small load on signal or DC bias for second stage.
This second stage (which you may notice also has the required negative feedback), also needs to be biased but in this case only one side of the audio signal is needed because the output LED will only conduct one-way anyways, and putting the high resistance to the + input does that. So the two parts - the resistor and the capacitor - are needed, and almost by coincidence form a low-pass filter. Not a problem though, because the input draws such an extremely small amount of power, very little current is absorbed, so even an affordable 100nF capacitor in combination with a 47,000 resistor will have a very low cutoff frequency (the point where the bass drops off 3dB, a just audible amount). The cuttoff frequency of the low-pass filter is greatly affected by the input's need for current. A 2nd stage with a larger current draw would need a much larger capacitor value to retain the same cutoff frequency.
This explanation is for sure important, as second stage is performing for small amount of power. Need negative feedback. To set the cutt off frequency of the audio signal RC values are fixed for 3db gain. Only one side of audio signal might play a vital role to glow the LEDs, but does R7 is limiting base current or biasing the transistor Q1? Otherwise, transistor need to be turned on for making a paths from battery to ground for driving the LEDs. At this point, the purpose of transistor is very measurable, because amplified audio signal need to be well controlled.
Not sure what current source you are looking for ... there should be current everywhere if there is a good battery! But maybe you are asking about the current to drive the LEDs to light. That current comes from the battery, thru the LEDs, and then thru the transistor to ground thru a current-limiting resistor. The transistor is driven from the 2nd op amp stage, so the current in the LED path is controlled by the transistor and it varies according to the level of amplified audio. That is where the negative feedback is taken for the 2nd stage. It also serves the purpose of including the transistor in the feedback path, so any odd non-linearities of the transistor is reduced. That means almost any general-purpose NPN transistor could do a good job here. Of course there is hardly a chance that anybody would notice a non-linearity between the volume in the microphone and the LED brightness - they will only see flashing lights when they talk, sneeze, screem, burp, clap, or generate a "trouser cough".
Excellent! I wont thought about non linearities of transistor. can imagine the disadvantage that the control input floating, 100ohm has taken to ground underneath the negative feedback, that you said "ground thru a current-limiting resistor" I was thinking "Current source with grounded load"
