[SOLVED] How to convert a sawtooth signal to an impulse using a 741 opamp as a comparator

[mystic07]

In fact, it is suggested that the sawtooth signal sholud be compared with a Dc voltage which is adjustable using a potentiometer. but i'm not quite understanding the theory behind. could anyone please help me.?? thanks

 

[betwixt]

Using the op-amp with little or no feedback so it's gain is very high will make the output swing from minimum to maximum (or the other way around if inverting) when the input voltages are the same or one exceeds the other. The idea is to put the sawtooth signal on one input and a DC voltage on the other. When the sawtooth voltage is less than the DC, the output is in one state, when it exceeds it the output state changes. By adjusting the DC (the 'threshold') you can set at which part of the sawtooth the effect occurs. A low DC voltage will make the output switch at the bottom of the sawtooth and as it is increased, it will switch nearer the peaks.

It's a system used frequently to change analog signals to digital in some amplifiers and ADC units. The 741 is a poor choice of op-amp, there are many comparator ICs which will work better, they have a more defines switching point and faster output change.

Brian.

 

[is_razi]

hi,you can use a differentiator (an RC and an opamp ) to get signal which it's level is proportional to the slope of sawtooth signal.see this link (a representation) for the circuit and its explanations.
good luck

 

[mystic07]

ok thanks. i understand a little bit better now. but when i mounted the circuit on breadboard and checked the output with a CRO, i obtained impulses for both +ve n -ve voltages along the time axis. i dn't understand why the negative impulse:???:

---------- Post added at 05:38 PM ---------- Previous post was at 05:37 PM ----------

i didn't get the link...:S

 

[betwixt]

Can you post a schematic?
If you are using a dual supply, the output might swing positive and negative. You might also be seeing instability causing strange signals, particularly at the threshold voltage. Do you have the supply lines decoupled?

Brian.

 

[mystic07]

Thanks but i finally understood. in was in fact a dual supply(+5V, -5V). the negative impulses occured at the edge of the sawtooth signal.

 

[Ahmed Alaa]

You can compare the sawtooth with a constant DC voltage that is equal to the sawtooth's max peak.