Monday, August 17, 2020

Mind experiment in amp design part 2

 The last post explained how to set the gain of an opamp with the feedback resistors. Now that we know how to set the gain we will look at the effect of loading on the output. We need to look at the generator Z and the load Z The old aying is for max power transfer we need Zgen = Zload but it is not always that simple. consider a power plant generator producing 100 megawatts with the Zgen - Zload. It would send 50 megawatts to the city and drop 50 megawatts internally. Obviously we do not want that! The other old saying is to make Zgen 1/10th Zload. In my previous example we would still have 10 megawatts dissipated in the generator. In some cases a 1000 to 1 ratio is still potentially a problem. It is fairly simple make a single stage amp. The beginner will often find it more difficult to build a 2 stage amp because of the Z matching. 

Examine the circuit to determine the gain. With 10k and 1000k resistors the gain should be 1000k/10k = 100. The gain of 14 is far from it! Remember the generator has internal drops. In this case it is dropping 86% of the signal. Not good at all.

By increasing the load Z we now have 100mv output which is our calculated value. 

I "Z matched" the amp with a 220 ohm load and get maximum power transfer and 50 mv. If I had a high Z speaker with a Z of 220 ohm this would be good. For a voltage amp not so good. So the amp can produce power or voltage gain. The Z matching would be adjusted accordingly.

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