AMPLIFIER on one transistor
Assemble an amplifier on a single transistor. It turns out it’s not so simple.
You need to know some subtleties.
And the main one is how to choose the operating point of the transistor? Let’s not get into the jungle of formulas and graphs, but let’s try everything in practice. I will still give a few graphs and a form. But I will not focus on them. It’s just for order.
Why do I need the Decoupling Capacitors in the Amplifier
We will understand all this with the help of experiments and according to the results of circuits and oscillograms. And after reading this article to the end with comments and explanations, it will also become clear to everyone how it all looks in reality.
But who is too lazy to read the article and study the pictures, you can scroll down there to see all this in the form or by the Link: https://youtu.be/TGHea-vxNN0
Let’s start with a very simple circuit that beginner radio amateurs often assemble: a transistor, a power supply, an amplifier load and an input signal. Well, just like in the diagram:
We start experiments
Experiment 1
We will have a sine wave generator of 500 Hz and an amplitude of 100 mV as a signal source.
Let’s get power and we don’t see any signal at the output, there is a constant component of 5 V at the output. That is, our transistor is completely closed.
Although, as can be seen from the waveform, there is a signal at the input.
Yellow signal at the input of the transistor.
Blue signal at the output (collector) of the transistor.
Experiment 2
All novice radio amateurs immediately begin to increase the amplitude of the input signal. Let’s do that too. And we increase the input signal with an amplitude of 500 mV.
And again we look at the oscillogram
The input signal is increased but the output is no result.
Experiment 3
We increase the input signal to one 700 mV.
And finally, the result begins to appear. At the exit, our line also came to life. And there were failures. At this point, if we connect some kind of sound emitter to the output , then we can already hear at least some sound.
Experiment 4
Let’s start talking from a beginner’s point of view.
Once the result has appeared, we continue to further increase the amplitude of the signal, set the signal to 1 V.
The output is amplified. But with them, the distortion also grows. Because we see from the waveform that the output signal is very far from the sinusoid.
We can also see the separation of the signal at the input. If you look at the circuit again, then we have two oscilloscope probes connected at the input. One directly to the generator, the second to the base of the transistor. And until the moment we crossed the point of about 650 mV, the signals were the same. And then distortions began on the positive half-wave.
And here you need at least a glimpse of some of the current-voltage characteristics of the transistor.
Refinement of the SCHEME
Since the base of the transistor is a pn junction , something similar to that of an ordinary diode . Then a voltage drop occurs on it from just about 0.6 to 0.7 volts.
Well, it’s a theory again. And I promised to show experimentally. We need to try to move the base point of the Transistor just above 0.6 volts. This can be done using a separate power supply. But we already have a power source and we can take the voltage from it.
To do this, you need a resistor, which we will connect to the plus of the power source and to the base of our transistor.
Therefore, our scheme has changed a bit and began to look like this:
The scheme has changed by only one detail. Well, for the experiment, I connected two more voltmeters
Experiment 5
And now we start experimenting again. As can be seen from the diagram, we now have a 100 kΩ resistor in the base circuit. Let’s reduce the input signal to 100 mV. Let’s look at the waveform.
It is already better that the output signal at the base has received a bias, but not yet enough for the normal operation of the transistor. The bias must be increased, that is, the resistance of the base circuit resistor must be reduced.
Experiment 6
Let’s put a 70 kΩ resistor and look at the waveform again:
And As can be seen from the waveform, the sinusoid is already taking shape. But it is not symmetrical. The positive half-wave is more compressed than the negative half-wave.
Experiment 7
Let’s set the resistance to 54 com.
And the signal on the oscillogram acquires an almost ideal shape at the output.
As can be seen from the waveform graph, the sinusoid does not start immediately, but with a delay of 1 ms. This is done to make it easier to understand what a Working Point is.
When we still don’t have a signal at the input, then there is a voltage of 2.5 V at the output of the transistor on its collector. And that’s half of our 5V supply voltage.
We experimentally achieved the ideal signal at the output when we applied such a bias to the base of the transistor that a constant component equal to half the supply voltage was present on the collector.
This can also be clearly seen on the graph How the output signal is distorted when the operating point is shifted:
I also suggest watching the video Maybe it will be more clear there: https://youtu.be/TGHea-vxNN0
