Negative Resistance Op Amp Improvement

127

01:33:29

Let's start first with why one would want to use this. Well op amps usually behave quite well in terms of distortion when the output is loadless. As soon as you load the output (especially with a low impedance load) the op amp starts straying from the ideal characteristics mentioned in the datasheet, at some point even completely failing at delivering the given specs to the load. Well a simple solution is to cancel the load. How can we do that? Well a simple resistor presents a positive AC impedance, so it stands to reason that applying the same amount of negative resistance would cancel out the impedance that the first op amp sees. Rl+(-Rl)=∞Ohm theoretically. So this is how it works. The first stage acts as a voltage amplifier as any other amplifier does. But instead of handling the current amplification too it only carries the voltage one. The current amplification is shifted to the second op amp, but not in a regular way like you'd do, but through a negative resistance feedback. Basically the second op amp has a gain of two to compensate the attenuation through the negative resistance feedback (the other 200ohm resistor). Note that the parameters of the second op amp don't influence the performance of the first one. So basically you can get a power Op Amp and a low noise op amp, and use this topology to make a power amp with the distortion specs of an unloaded low distortion small signal op amp. 

published 9 years ago

hurz

9 years ago

Ohms law is also valid for negative resistance R+(-R)=0Ohm if you do R||-R its ∞Ohm

thebugger

9 years ago

Yeah it depends though. Negative resistance devices tend to be nonlinear in nature so ohms law is sometimes hard to apply, but yes ideally speaking Ohms law is just as valid for negative resistance as for positive impedance devices. Rl+(-Rl)=0ohm. But for an op amp that doesn't mean infinite conductivity, just the opposite (infinite impedance). Although that's easily thrown off balance. Replace the load with a reactive one and the whole thing goes off balance.

hurz

9 years ago

So many words, a typical bugger comment. However, A-A=0 and NOT ∞ !

thebugger

9 years ago

Yes but that means no load, not 0ohm load. Therefore the perceived impedance by the op amp is theoretically ∞

hurz

9 years ago

So, you mean zero siemens and not zero ohm. But writing ohm in your description!

thebugger

9 years ago

Yes zero siemens equals infinite ohm. What's the problem?

hurz

9 years ago

Its your problem posting description with wrong mathematical formulas.

thebugger

9 years ago

Well it's not wrong. It is to you, cuz you always have to pick on someone. 0ohm=∞S and vice versa. It's all the same, no difference in directly substituting it you petty little thing

hurz

9 years ago

1 Ohm - 1 Ohm = 0 Ohm and NOT ∞Ohm. I stopped at that point reading your description

thebugger

9 years ago

Yeah but you failed to notice what I'm referring to. I'm referring to how the op amp sees this cancellation, not in general. Come on you really think i wouldn't know that 1+(-1) is 0. That's fifth grade mathematics.

hurz

9 years ago

And its important to understand basics before inventing perpetuum-mobile!

thebugger

9 years ago

Haha i saw what you did there :D true true. Nothing is over unity!

hurz

9 years ago

http://everycircuit.com/circuit/5325827299868672

add comment in editor