Just a touch of gain

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In yesterday's post we covered a tough subject: the differential pair that serves as our input to the op amp. From here it gets a lot easier so if you've made it this far, you're in for an easy ride. Remembering that the input to our op amp consists of two transistors that form our diff pair, all we need is one more transistor to form a fully functional op amp. This simplest of op amps uses only three transistors and here's a picture of how that would look. This schematic might look a little intimidating at first but it's really simple. It shows the insides of an op amp and then below that what the symbol of the op amp as a schematic would look like. Note in the inside view we have our two transistors that form the diff pair and connected to the first transistor's output (its collector) we have attached another transistor which is the gain stage. This gain stage is identical to the single transistor amplifier we're familiar with now - the only difference is it is being fed from another transistor. So the output of one transistor is feeding the input of another transistor. This is what's known as a two stage amplifier or a compound configuration. Real technical talk for simply adding another transistor to the output of the first. This gain stage transistor has two purposes: it provides additional gain and it flips over the phase of the signal one more time. Remember? If we feed a transistor through its base, the output of that transistor is inverted or flipped over. Do that twice and you get two flips so you're right back where you started. In an op amp this is how we manage to get a non-inverted output - two flips. Take a look at the drawing and note that our input is on the non-inverting input - I marked it here as number 1. When it comes out of number 7, the output, it is in phase because it flipped twice. Now, look what I did on the second input marked number 2 our inverting input. I took the output (number 7) and fed that back into this input through a resistor. OK, let's read that again - I took the output and put it back into the input. Guess what this is called. You guessed right, it's feedback. Just like the name suggests, we are feeding back the output to the input. Now, why would we want to do that? There are a couple of reasons: to control the gain and to lower the distortion. Remember our diff pair lesson from yesterday? If I take the same signal and put it into both inputs at the same time, what do I get for amplification? Nothing, no amplification. Since the signal at the output is essentially the same as the signal at the input, when I place that into the - input I get no amplification so 1 volt in equals 1 volt out. But now let's imagine that I don't put everything back from the output - let's put only 1/10th of the output back to the - input. What happens? I get amplification of 10 times (20dB) - or equal to whatever I don't put back into the input. How do I control the amount of feedback? I simply divide the amount with resistor 6 labeled here as "set gain divider". Remember in our first post about how to design an op amp we used a 10K resistor and a 1K resistor? Same thing. Here we could use the 10K resistor for the feedback and the 1K resistor to throw away or divide what goes into the - input by 10 times. It could not be any simpler. The second advantage of feedback is to reduce distortion. How does this work? Because we are taking the output and feeding it back to the input, we are also comparing the output to the input. Remembering that if there are any differences in the two inputs - those differences will be amplified - if something is different on the output than on the input, our little circuit will amplify those differences until they are equal. Therefore, since the goal is to have the output the same as the input, applying feedback makes sure that happens and we get lower distortion. The last bit of our op amp is the output stage which is even simpler. Remember yesterday we showed you how by using the emitter of a transistor to get power gain rather than voltage gain through its collector we could drive a set of headphones, a loudspeaker or anything needing a bit of power? Well that's all we need to do now. Take a look at this drawing. It's exactly the same thing but we took what we learned yesterday and simply added one more transistor on the output - but instead of getting voltage gain from the collector we now get power gain through the emitter. This allows us to drive cables, headphones, whatever we want up to the limits of the output stage current. Whew! OK, that's the op amp. Tomorrow let's see what's good and what's not so good about IC op amps.
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Paul McGowan

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