Can someone please explain me this circuit

The input to the non-inverting terminal is voltage signal across a resistor. The resistor is connected between IGBT emitter and ground.

Thus the input is a voltage signal taken across that resistor. The frequency of the current is close to 25 kHZ.

The signal is applied to an opamp with gain of 100.

I wan to know how the designer has selected all the components used for designing the opamp circuit.

I am not concerned with the voltgae divider circuit at the output of the opamp, I just want to know the designing of opamp. Why the feed back capacitor, why the input RC filter. Why the pull up resistor and capacitor in parallel to pull up resistor.

Note opamp is designed using LM339 AN by fair child

Hi,

LM339 is a comparator and no OPAMP.
But the feedback is on inverting input, this is very unusual for a comparator.
Please check if pin4 and pin5 is connected like in your schematic. (the other way round makes more sense)


Klaus

nalawade said:

........................
The signal is applied to an opamp with gain of 100.
...........
Note opamp is designed using LM339 AN by fair child

Click to expand...

The LM339 is not an opamp - it is a comparator.

An opamp has an internal frequency compensation capacitor so that it has no voltage gain at a frequency that its phase shift would cause high frequency oscillation when negative feedback is used.

A comparator almost NEVER uses negative feedback because a comparator does not have an internal frequency compensation capacitor.

BUT your circuit has a very high value capacitor C10= 0.1uF to ground at its output and its collector output resistor R17= 2k so it cannot oscillate at a high frequency.

Simple arithmetic shows that the high frequencies are cut:
1) The output C10 and R17 cut frequencies above 800Hz.
2) The input C11 and R19 cut frequencies above 3.6Hz.
3) the negative feedback C and R18 cut frequencies above 1600Hz.
4) The output divider C13 and R21, R24 cut frequencies above 3.5Hz.

So the circuit is not amplifying the 25kHz, instead it amplifies DC.

A comparator has an open collector output so it needs the pullup resistor.

Hi check the low frequency opam builts using comparator LM139

**broken link removed**

nalawade said:

Hi check the low frequency opam builts using comparator LM139

**broken link removed**

Click to expand...

That could be handy if you have a spare comparator left over in your circuit and you need a simple, low-frequency op amp function. But you would not use that circuit in place of a normal op amp otherwise.

Hi,

Yes, i agree with crutschow.

If you need an OPAMP, then use an OPAMP.
If you need a comparator, then use a comparator.

An OPAMP datasheet needs a gain and phase chart. This information is urgent to calculate stability of the OPAMP circuit.
In the comparator datasheet such importand OPAMP related data are missing.

The (misused) comparator circuit may work, but use an OPAMP and you can calculate that it reliably must work.

Klaus

Thanks a lot friend. I have used a quad comparator. Out of four only 2 were used for comparison. So I used the left over comparator for amplification

Hi,

You designed the circuit on your own?
And asked us to explain your circuit?
Funny.

Klaus

Hi check the low frequency opam builts using comparator LM139

Click to expand...

No matter if it's a clever design idea or a an odd suggestion, there are two important detail differences between the ST application note and your circuit. As long as you don't have the analog design knowledge to see it, you should better stay with regular OPs.

In any case you have the problem that LM339 parameters like output impedance and open loop gain aren't specified, so the "low frequency OP" dimensioning can't be systematically chosen for stable operation, it's a purely emprical (trial and error) design.

Did you notice that the comparator used as a low frequency opamp in the ST datasheet has an output RC timing of 0.5uF x 15k ohms= 7.5ms and does not have a capacitor parallel to the feedback resistor?

But your circuit has an output RC timing of only 0.1uf x 2k ohms= 0.2ms and has another capacitor parallel to the negative feedback resistor that ADDS negative feedback at high frequencies.