All three of these parts perform the same task - to increase the output signal power to such a level that you can drive a low-impedance load - a driver or headphones. How do they do it? It's very simple - the DC power supply of the PA is taken and converted to AC, but in such a way that the output signal shape repeats the input signal shape.

This is just shown in the picture. At the input we have a small (meow!) signal, at the output a large (MEOW!). At the same time, its form (meow! -MEOW!) has not changed at all. Thanks Cat.

But, unfortunately, everything is good only in theory. In practice, when designing radio equipment, we use non-ideal resistors, capacitors, and especially transistors. Therefore, the shape of the output signal can be very different from the input, and this trouble is called distortion. All cascades of the amplifier contribute their five kopecks to signal damage, but the lion's share of it - I would say, a whole ruble in small change, is brought in by the final cascade when it is incorrectly constructed or calculated.

Why is distortion bad? Well, in order not to engage in demagogy, just cut out, say, every fifth word from this article. What happened? No, the meaning, of course, is still clear, but it’s already somewhat different, right? The same is true with sound.

So, let's look at various ways to build the final stages of the PA, which are also called classes (or modes of operation) of amplifiers. Probably heard - class A amplifier, class AB amplifier - that's it.

Let's start by looking at the general schematic diagram of the PA output stage.

This is a push-pull output stage on complementary transistors. As you can see, the base circuits of the transistors include voltage sources that form the initial shift of the operating point of each of the transistors. So it depends on the value of this voltage in which mode (class) this or that output stage will work.

Well, let's start in order - mode А .

We will get this mode at a fairly large bias voltage , such that

where I0 is the cascade quiescent current. Thus, both transistors are in the active zone and as the collector current of one transistor decreases, the current of the other increases. As a result of all these dances, we get almost perfect linearity of the cascade and a complete absence of nonlinear distortions. BUT. There is always some BUT, have you noticed? First, the power consumed from the power supply is equal to twice the power of the output signal and is a constant value, independent of the input signal. That is, if the amplifier develops a maximum output power of 100 watts, then the power consumed from the power supply will be 200 watts, and it does not matter at what volume you listen to music. And if the amplifier is two-channel, that is, stereo? What if it's a home theater? Further. Output transistors, as you know, have a bad habit of heating up. That is, they dissipate some power. In the case of mode A, the power dissipation for one transistor is as follows:

where a is the output voltage swing.

What do we get? Another feature of class A is that the power dissipation of transistors is greater, the lower the input signal. That is, if you leave a working amplifier without an input signal, it will heat up like a stove, because in the absence of an input signal, the power dissipation of the transistor is equal to the maximum output power of the amplifier. By the way, I want to say that this has been tested in practice - my Technics A 900 Reference actually heats up more if no signal is applied to its input - I was very surprised at this circumstance at one time and even wanted to drag it in for repair . Another important parameter of the amplifier is efficiency. Well, you understand - with such heating of transistors, we will not get any human (Meow!) or cat efficiency.

The efficiency is calculated like this:

where a, as in the previous formula, is the range of the output voltage. Thus, the efficiency is not constant and increases as the input signal increases, and hence the output power, and reaches a maximum value of 50%. (Do you want to drink a bottle of beer? Meow, it won’t work - pour half the bottle into the toilet, drink the remaining half and run again for the whole one.) Yes, that’s about it, but it should be noted that this beer will be just excellent. True, the more offensive it will be to throw out half.

So, to summarize - what is good about class A? First of all, excellent linearity and lack of distortion - the output waveform remains the same as it was at the input. But for this we have to pay with deadly power consumption and extremely low efficiency of the amplifier. Not everyone can make such sacrifices, and this mode of operation of amplifiers is used only in very high-quality Hi-End class systems, the cost of which starts from 1000 trampled raccoons and they look like shaped coffins.

The next class of amplifiers is class B

Just like last time, consider a push-pull cascade on complementary transistors.

The circuit has been simplified a little due to the specifics of the amplifier's operation in this mode. As you can see, there is no bias at all, that is, the transistors open exclusively from the input signal. Thus, the peculiarity of this mode is that in the absence of an input signal, both transistors are closed, and the cascade consumes absolutely nothing from the power source - I0 = 0. If there is an input signal, the transistors work alternately - transistor T1 works for positive half-waves, and T2 for negative half-waves. Let's see how we are doing with power consumption, efficiency, and heating of transistors.

To begin with, we introduce a certain coefficient a - the so-called utilization coefficient.

that is, the ratio of the output voltage at a given moment to the maximum output voltage. Speaking in human terms, this figure shows the workload of the amplifier at the moment - either it drags electrons in buckets at a breakneck speed - a=1, or even sleeps - a=0.

So, the output power is calculated according to the following formula:

;

power dissipation of the working transistor:

power consumption:

Well, in general, in the case of mode B, everything is fair - the power consumption increases as the input signal grows and, accordingly, the output power. The maximum power consumption at a=1 reaches

The efficiency also increases with the signal level and reaches 78,5%. Well, it's a completely different matter. (Meow! Well, yes - pouring 20% ​​of the beer is not 50%.)

So we seem to be missing something. Well, for sure - they forgot about the distortions. And all the Cat with his beer. Distracts.

So, let's look at the distortions.

Uuuu ... that's where we got - look what's going on. In a pure class, a very large mmm awaits us ... (Meow! Ass!) Well, yes, something like that - non-linear or, as they are also called, transient distortions of the 1st kind. You see - on the graph - instead of the sine wave smoothly passing through zero, as it does in the input signal, we get a dip of some width in general - that is, the moment when the signal disappears altogether - there is none. Why is this happening? The thing is that in order for the transistor to open and start working, it needs some threshold voltage applied to the base - for silicon bipolar transistors it is 0,7 volts.

That is what we get. Suppose the magnitude of the positive half-wave begins to decrease. Transistor T1 starts to close. And there comes a moment when the value of the first half-wave drops below 0,7 volts and T1 closes, but T2 has not yet opened, and it will open only when the signal goes into a negative half-wave and its value reaches a voltage of -0,7 volts. Thus, we get a hole in the signal with a width of 1,4 volts. Ay ah ah, what do we do now, huh? (Drink beer, pouring 20% ​​down the toilet, meow!)

Well, in order not to end this part on a sad note, I’ll run ahead and say that the solution to this problem was found, it was found a long time ago and it’s called the mode AB . Some compromise between signal quality and power parameters. But we will consider this in the next part. (And we will also consider class D - a digital amplifier, meow!)

Publication: radiokot.ru

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