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# Need help on the type of op-amp, please

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#### Redfrost

##### New Member
My mentor doesn't agree with me on this type of op-amp configuration. (Please see the attached file). This op-amp configuration suppose to be a comparator, that's what the website said - **broken link removed**

However, my mentor said that it was a differentiator, with a threshold point, caused by R4 and R5.

I tried searching the web for this particular configuration but couldn't find any. I really need help, desperately, as my coursework depends on my presentation on this particular circuit.

I need to know:
1) What type of op-amp is this? (eg comparator, differentiator...etc)
2) What is the function of the feedback resistor R2 and R3 ?
3) What will happen when the signal from MIC goes through C1?
4) What will happen to the signal before and after C2?
5) What would you expect the output waveform to be? (please draw the diagram if possible)

I would appreciate greatly if any of you could give some elaboration.

#### Attachments

• clap1.gif
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If the RC time constant is comes in the audio frequency range, then this circuit will act as differentiator, otherwise not. If the capacitor value is large enough, it will just act as a DC isolating capacitor.

1) This type of configuration is called Inverting Amplifier for AC signals (sincle coupling capacitors are used)
2) Resistors R2 and R3 determine the gain of the amplifier. The AC voltage gain in this case will be -(R2+R3)/R1.
3) You will get an amplified version of the signal at the output.
4) The signal before C2 will have a DC signal content of 1/2 Vcc, which will be removed by C2 and only AC signal will be obtained at the output. 1/2 Vcc reference is set by the resistors R4 and R5 and I am considering R4= R5.
5) if you apply sine wave of 1mV peak to peak to input and if the gain is set to 1000 then you'll get inverted sine wave (180 degree out of phase) of 1V peak to peak amplitude.

Depends on your point of view ...

I would say the op-amp is a bog standard inverting amp.
To calculate the gain you need to know the input impedance (R1 in parallel with the mic impedance, all in series with the impedance presented by C1), if this is glossed over then you could say that the sum of R2 & R3 set the gain (but strictly speaking you need that input impedance to find the ratio).
That cap, C1 is the differentiator, if there really is one, in combination with its series resistor, the impedance of the mic.
R4 & R5 set the op amp's bias, the point it sits at when there is no other input, in my mind a threshold is only used to determine a switching point so I wouldn't agree with this being called that.

C2 is an AC decoupling cap but cannot be considered without R6 & R7 (cropped off your diagram), it stops the DC bias of one circuit stage upsetting the next stage (in this case a 555 monostab trigger input). 1/2VCC on one side, 2/5VCC on the other.
You could also argue that C2 and its series resistance is a differentiator.

The thing with these "differentiators" is their intended purpose.
In my mind the caps are just for DC blocking as Kinjal said. If the mic gave a step-change in DC output then a differentiator would 'do its thing' but all we want here is to pass mid-to-high audio frequencies such that the amp can, given a loud enough input, trigger the monostab.
All that matters here is volume. To prove this the input stage could be redesigned to use DC coupling and thus eliminate C1, or C1 could be swapped for 100uF, if a differentiator is reqired that would stop the circuit working (which it won't !).

Thank you very much for the reply. For the past few days I was trying to study and digest the information posted here by Kinjalgp and Mechie. I have a little more question more to ask.

--------------------------------------
Referring to Kinjalgp's post.....

When you said, "RC time constant comes in the audio frequency range", do you mean that it includes the sound frequency audible to us? If the value C1 is 0.1uF, is it considered large in value?
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ok... i understand from two posts that this op amp is just a plain inverting amp with a bias point of 3V.

How would R3 affect the sensitivity of the amp, so that the normal room noise would not disrupt the amp operation?

How would you know that the capacitor C1 is a coupling capacitor? What is the function of coupling capacitors? From what I know, coupling capacitor couples the AC signal from the MIC but I don't know what it means. Does it mean that it converts AC into DC? or it totally disallowed dc from passing through it?

The output of the op amp is connected to the trigger lead of a monostable 555 Timer, could the amplified sine wave from the op amp drive the Timer? From what I know, square wave signal could only trigger the monostable Timer.

More the value of R3 more will be the sensitivity of the amplifier. Set R3 to a optimum level where you get noiseless output from the speaker (if used for audio purposes).

Audio frequency range is 20Hz - 20kHz which is the audible sound range.

0.1uF is not a large value. For coupling 1uF-10uF capacitors are generally used.

The function of a coupling capacitor is to block DC signal and pass only AC so that the DC bias cascaded or coupled stages is not altered due to output of previous stages. The reactance Xc (or resistance) of a capacitor is given by
Xc = 1/(2 * Pi * F *C)

Pi = 3.14 Constant
F = Signal Frequency
C = Capacitor value in Farads

Now, for DC signal (F = 0 Hz) Xc tends to infinity, so it is said that it blocks DC.
For AC, Xc is of low value, so it passes this signal.

For proper triggering of monostable, sine wave needs to be converted to square using a schmitt trigger.

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