op-amp circuit design help needed

[serid]

Hi,

Just started learning electronics and wanted to design very simple circuit consisting of 2 op-amps voltage buffers

and RC to smooth PWM signal (in between). I am having difficulties in getting desired output. What I wanted to achieve is:

1) 500 Hz 5Vpp 2.5V offset going into buffer

2) RC circuit to "smooth" it out

3) From RC to second buffer

4) Signal from second buffer to drive transistor

However - not working at all ..... Getting readings which puzzle me (eg to big voltage coming out from second buffer, no current at all flowing

to base of transistor even better - getting small negative readings).

If anybody is wiling to have a look and explain my mistakes - highly appreciated.

Thanks,

Adrian

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[johnsold]

Adrian,

I am an electrical engineer with 40 years of experience designing circuits, so I may be able to help.  I do not have Multisim so I cannot see what is in the simulation.

1. I do not understand the excitation. The function generator XFG1 is set for Vp = 2.5 V and Voffset = 2.5 V, so I would expect a 5 V peak to peak signal centered on 2.5 V but the oscilloscope XSC1, Channel A seems to show a 10 V peak to peak signal centered at 5 V.  Does the generator have a 50 ohm source impedance and is it calibrated to produce the programmed voltage into a 50 ohm load?

2. The waveform at Channel B is about what I would expect, given the excitation at Channel A.  The rise time is consistent with the R1*C1 time constant.

3. What is the burden of the ammeters? Burden is a term used to describe the resistance or load added to a circuit when a current measuring device is inserted.  Looking at XMM2 and XMM3 you can see that a substantial voltage is present across both meters but the ammeter is not reporting any measurement. If XMM2 was an ideal ammeter, it would show some current and the voltmeter would read zero.  XMM5 is showing negative current which also seems unlikely.

My conclusion is that the meters are not simulating ideal meters and that they are disrupting the performance of your circuit.  Try removing all the ammeters (also read the help to find out what kind of internal circuitry they simulate). Replace XMM2 and XMM4 with small resistors, perhaps 1 milliohm. Measure the voltages across the resistors and calculate the currents.  Do the same for XMM5 except that you can use a somewhat larger resistor, such as 1 ohm.

Then you will find that Q1 is in saturation or has been destroyed.  It might also destroy the amplifier.

Let me know what you find.

Lynn

[serid]

Hi Lynn,

Thanks so much for getting back to me. Yes indeed ! I was expecting 5Vpp on input and around 2.5V "steady"

DC on output due to 50% duty cycle. I have in the meantime run the simmulation elsewhere and looking ok - please have a look:

https://www.circuitlab.com/circuit/2p7kdq/op-amp-rc/

Also - the reason I wanted to simmulate this circuit is cause I have already "fried" some wires on my breadboard and thought - there is

something wrong with it so I better check the simmulation. Now, I am thinking it could be due to my messy way of placing the wires ....

Anyway - still feeling puzzled due to how the simmulation is performed on Multisim ..... I would have thought it is one of the best products

out there ? At least to my knowledge .....

One more thing - I am lucky enough to have all the basic equipment - Arb. Sig. Gen, Oscilloscope, multimeters etc. I will try to build the

real circuit today and take measurements and will get back to you with the real stuff. Would you recommend any specific basic

op-amp to play with ? I have salvaged some ICs packs with two of them in each - they are:

- lots of 4558D but different numbers underneeth them, eg: 7112G, 6367B, 7807G, K12230

- 5218A

- LM324N

Also - if you could perhaps explain me why most of these little ICs have quite small I output ? I read in the datasheets they are around ....

if I can remember properly 20 - 40 mA ?

Many many thanks,

Adrian

[Tom__Sedlack]

To get the proper output level from the function generator, you need to ground the common (the center connection). The negative (-) connection of the generator gives you 0 to -5V output (with respect to the common). That's why you saw 10V on the output.  

[serid]

Of course !

Checking it now - will get back with the details.

Thanks,

Adrian