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September 08, 2011

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Account Deleted

When I apply an external supply from frequency generator to Push Pull Amplifier, Push Pull Amplifire's input gets loaded even if a small amount of supply is given to it..
If the input is sine wave then output is no doubt amplified but sometimes gives triangular like or distorted output..
To avoid this loading effect I had kept the amplitude to its minimum and the supply voltage is hardly measurable on CRO but it gives the correct amplified sinewaves.

Pedro Perez de Ayala

Well, I think that a very important lesson to learn from this experience is that you have to design based on the specs as much as possible. I only have 7 years of experience as an electronic engineer, but I have seen many times that a design stops working because a part has degraded some parameter but still within specs. The datasheet is the closer you will get to having a contract on the part's performance, use it as such.

This leads to another conclusion, which is that testing is highly overrated and can be dangerous if not done properly. Sometimes, people are tempted to validate everything through testing, which is better than nothing, but many times, the yield of the test is not clear or simply not enough. On the other hand, a design supported by good theoretical analisys, and proper part dimensioning has many chances to become a rock-solid machine. Of course testing is also necessary, but it doesn't cover everything. Don't feel confident just because there is a test, feel confident because the design is well thought, meticulously dimensioned, carefully implemented, AND properly tested with a good set of tests that give a good enough yield.

I hope to give some light on the subject,
Best regards,
Pedro

Jonathan Point

I have seen this issue with CMOS ICs as well. These were hex schmitt inverters - 40106/74C14/MC14584. The circuit was proofed (by others) using MC14584s. At some stage, they had switched to 40106s, but about 6 months into that, the product started showing flaky behaviour and the inverters started 'missing' pulses. Some 40106s worked, most didn't. Back to the MC14584s and no problems at all. It seemed temperature-related, as this was summer at the time.

More proof that it doesn't matter how many prototypes 'work', you MUST stay within the 'minimum' specs for any part, or there'll be trouble somewhere down the line. In this case, the devices were operating close to their maximum frequency from AC-coupled inputs =]

Regards,
Jon Point

Mark Peacock

Did you simulate in SPICE using the extreme (W/C) tolerances?

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