0
\$\begingroup\$

Earlier i tried generating a +5V to -5V square wave using an op-amp as an astable multivibrator . The wave had a frequency of 1 kHz. The op-amp was given +Vcc of 5.5V and -VCC of -5.5V. I got the desired square wave at the output. Now i want to generate 0 to 5V square wave using the same circuit. However when I tried simulating the circuit taking -Vcc as 0V im getting a constant voltage of 500mV. Can you please help me to troubleshoot the circuit. The op-amp I have used is an LM324.

enter image description here

\$\endgroup\$
2
  • \$\begingroup\$ Your opamp is trying its hardest to swing low, but the lowest it can go is 500mV (a little above the negative rail). At that point the junction of R1/R2 is about 250mV, so the opamp neg input is still higher than its pos input so the opamp is forever stuck low. In your original design the opamp was free to swing all the way down to -4.5V, and R1/R2 would then set the pos input to about -2.2V, allowing the opamp to flip when C1 voltage reached -2.2V. \$\endgroup\$
    – td127
    Commented Aug 27, 2020 at 0:06
  • \$\begingroup\$ Can you suggest a circuit employing op-amp i can use to get a 0 to +5V pulsating DC waveform at 1KHz? if not, please suggest me the sources which i could use to figure out the connections for op-amp. \$\endgroup\$
    – Mihir
    Commented Aug 27, 2020 at 7:36

1 Answer 1

2
\$\begingroup\$

You need to add a resistor to the junction of R1/R2 to bias it away from GND.

For a square wave you'd want it (roughly) half of Vcc. Not exactly because the op-amp output does not swing as close to the positive rail as it does to GND.

Eg:

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

\$\endgroup\$
4
  • \$\begingroup\$ I Tried simulating the circuit you've given. However i am getting a constant 5V DC signal at the output instead of pulsating square wave of 0 to +5V at 1KHz. Can you guide me where i coudve made a mistake. I added a 10V DC source at the R1/R2 Junction through a 100k ohm resistor and changed the values of R1 and R2 as 100K ohm as well. \$\endgroup\$
    – Mihir
    Commented Aug 27, 2020 at 7:33
  • \$\begingroup\$ Either reduce the 10V source or increase the 100k resistor that is connected to it. \$\endgroup\$ Commented Aug 27, 2020 at 11:13
  • \$\begingroup\$ To get some intuition on this try calculating the threshold at which the output switches. \$\endgroup\$ Commented Aug 27, 2020 at 11:14
  • \$\begingroup\$ A real LM324 output can only swing from its negative rail to within 1.5V of its pos rail so should only be able to reach 3.5V max in your case. The fact that the simulation yields 5V under any circumstance is suspect. (Are you sure you didn't short the opamp output to 5V in your schematic?) Since the output swing is only 0 to 3.5V the R2/R3 ratio should be adjusted to half that, about 1.9V. Can do this by making R3 = 62K. If you make R4 = 62K too then the circuit should work and be pretty close to 1kHz. If you really need a full 5V swing you need to use a rail-to-rail opamp (e.g. LMV324) \$\endgroup\$
    – td127
    Commented Aug 27, 2020 at 17:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.