Differential Amplifier, the Basics

This is a one transistor audio amplifier, and it's a very typical setup. We've got for the load a 1k resistor, and we've got an emitter resistor of 150 ohms. And then we've got two biasing resistors, the 10k and the 2.2k ohm resistors to bias the base of that NPN transistor. Now the input for the audio is going to come through that 10 microfarad capacitor and ground. To make a differential amplifier, we're going to need another one of these amplifiers, like this. And I'm going to link these together through the power supply, like this. And now this is our completed differential amplifier. On the left, we've got an input 1 and an output 1. And on the right, we've got an input 2 and an output 2. For my first demonstration, I'm going to a bi-directional red and green LED between the outputs of 1 and 2. And here is the setup. And you can see the two large 10 microfarad capacitors. That's the input. 1 is on the left and 2 is on the right. And I am going to be injecting an audio signal into each of those input capacitors. Okay, signal into 1, red. Signal into 2, green. Back to 1. And now I'm going to short between the two inputs. And look what happens. The LED goes out, even though I'm injecting a signal. Now I'm going to lift the short and the green will light again. Okay, this is what took place. When I injected a signal in 1, we saw that the red LED came on. And we did have current flow from 1 to 2 because 2 is not amplifying anything. Now when I injected the signal in 2, the green light lit. And that's because 1 is not amplifying anything and there was some current flow between 2 and 1. However, when I shorted the inputs and injected the signal at both ends, there appeared to be no output. There was no current flow between transistors 1 and 2. But there's a lot more going on than that. And this is going to be my next demo. I'm going to put an audio transformer between the two so we can hear what's going on. And I'm also going to take a look at the 1 and 2 outputs with an oscilloscope. Okay, here I'm going to hook up the scope to the output of transistor 1. Now I'm going to inject into 1. We hear the tone and we can see the trace on the scope. Now I'm going to move the scope over to transistor 2 and we'll see that there's almost nothing there. Now I'm going to move the inject into 2. We can hear the tone and see the trace. Move the scope over to 1 and there's almost no trace. Back to 1. Now I'm going to short the two inputs. The sound goes away but we still have a trace on the scope. And we also have a trace on 2. Now we don't have any sound because both signals are the same coming out of each transistor. Now when I lift the lead, we get our tone back. Okay, now what we just saw was when the outputs at 1 and 2 are identical, we don't hear anything because there is no difference between the signals. It's only when there is a difference either at 1 or at 2 is when we heard the tone. There are many uses for this type of amplifier and let me give you just one idea. Let's say that you had recorded a speech and there was some recorded music in the background. And later on you wanted to remove the music, that recorded music. Well, if you had another tape recorder that had just the music on it and you put the outputs of both of those into a differential amplifier and synced up the music both in time and in level, that differential amplifier will null out the music and just about the only thing that you'll hear coming out of the differential amplifier is the speech. Thanks for watching.