pwm amplifier question

Thread Starter


Joined Nov 8, 2008
today i was doing my experiment on pwm amplifier.but i have a few questions on that circuit.we feed in sine wave 10v pp at 700 kHz at the input 22k resistor and 12v at the 10u capacitor.

1)because lack of the ca3130,bd137 and bd 138. so we were told to replace them with lm324 ,bd139 and bd 140 respectively.i want to ask that is this possible to make the circuit working?because i was wondering the value of capacitor and resistor are calculated based on the original circuit,so it cannot simply change the component without altering the values.

2)what is the result supposed to be?the lecturer told us that there will be square wave at the node before the 39uH inductor,and then triangular wave at the +ve input of the lm324.but once again i wondering why what we feed in is sine wave and then will exist triangular wave at the positive input while the circuit is operating.

3)we also given two speaker,but we do not know when to use it.any suggestion?

4)also,she also call us to identify the pwm output and find the oscillating frequency but i do not know what is that oscillating frequency mean.

5)lastly,we also were instructed to perform FFT.basically what is the purpose of FFT iin this experiment?

6)when i try to simulate the circuit using multisim,it come out timestep too small.what does it mean and how to solve it?

sorry for so many question.but i hope somebody can help me.:p



Joined Apr 5, 2008

The CA3130 has a bandwidth of 15 Mhz.
The LM324 has a bandwidth of only 1 Mhz.
The LM324 will probably NOT work in the schematic you gave.
You could try a NE5532, that has a bandwidth of 10 Mhz.

The transistors wil give no trouble.

See also the attached datasheets.




Joined Apr 2, 2009
One major flaw.
Replacing the CA3130 with a LM 324.
Check the data sheet of these two chips.
U'll know what I mean.
PWM requires high speed components


Thread Starter


Joined Nov 8, 2008
thanks for fast reply.:)then can you help me on other question.i am very eager to know the answers.:p

Thread Starter


Joined Nov 8, 2008
sorry,i still cannot get the answer for my questions.:confused:anyone who can explain in detail would be appreciated much.