Signal Amplifier, need help with our design

[gruppe6]

Hi,

We're trying to make a design for a 60W signal amplifier as a school project, supposed to give an amplification of 20. We're making our design in Multisim 12. As you can see on the simulation, there is a problem with the output signal, and we are having trouble finding out where. Is there anyone here who can take a look and see if there are any obvious mistakes?

Design made by our teacher:
http://www.filedropper.com/forsterker-60w2013skjema

Our design in Multisim:
http://www.filedropper.com/forsterkerkretsskjema2

Print-screen of input (sine) and output (not so sine) signals:
http://www.filedropper.com/simulering

[kellys_eye]

Upload your pictures to a site that doesn't require registering before viewing and we'll have a look.

[deth502]

i had no trouble downloading the file, only had to do one of those annoying captcha things.

although, it was an odd file extension i had no idea what it was so i immediately deleted it. ".ms12"

[MacFromOK]

FWIW... it seems they are pdf files (the first one anyway), which I have no great love for.

[Sch3mat1c]

Temporary hosting:

Instructor
http://www.seventransistorlabs.com/Forsterker-60W.pdf

Screenshot
http://www.seventransistorlabs.com/Images/simulering.jpg

I don't have Multisim 12 so I can't open the project itself.

Offhand,

The provided schematic is a pretty basic push-pull amplifier, with differential input stage, dominant pole compensation, variable output bias (Vbe multiplier), and a Sziklai pair complementary output stage. Overall voltage gain of about 22. Two unusual features are 1., the "Q capacitor" bootstrap (R8, R9 and C14); today, conventionally, a simple one- or two-transistor current source fills this role. 2., Q6 and Q7 and associated components provide foldback current limiting, which is a handy feature, but not usually necessary to teach the fundamentals of amplifier design and operation; the circuit could be simplified, for example, by removing many diodes and resistors, which would alter the performance (and probably reduce the amplifier's tolerance of short circuits), but would significantly clarify the design for the student.

As for what your waveform is doing, it looks like crap. Check current flow through all your transistors, you probably have something shorted. This is one thing that simulators are more kind about versus reality: when you make a stupid mistake, the simulator will go on assuming you know what the hell you were doing. Reality, instead, delivers a rude kick to the face as the magic smoke is released. Never forget to test your circuits in reality, don't trust the simulator.

Non sequitur, I like the look of the MS12 oscilloscope in Windows 7.

Tim