Measurement of the output power of audio frequency amplifiers. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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Let's take a conventional low-frequency amplifier with a supply voltage of +12 Volts, a load resistance of 4 ohms, connect an oscilloscope to the load, and a sinusoidal signal generator to the input, (Fig. 1)

(click to enlarge)

turn everything on and observe "funny pictures" on the oscilloscope screen - a sinusoid until it reaches visible distortion (rice. 2a). (Scientist cat's note: less than 3% distortion is invisible to the naked eye. We'll talk about what distortion is in another article.)

The area occupied by a sinusoid can be calculated (or measured) and replaced by an equivalent DC voltage of the same area (rice. 2b).

This tension is called RMS voltage - SLE (English abbreviation - RMS), colloquially - "effective". Thus, you can find the equivalent voltage for any form of current (rice. 2c, d, e).

For triangular, rectangular, sinusoidal, exponential current, there are mathematical expressions for the equivalent conversion. For ease of understanding, the figures show half the periods of symmetrical signals. The advent of computer registration makes it possible to perform numerical integration of any function without searching for its mathematical expression. What is all this for? The found equivalent direct current will produce the same thermal work as our investigated current.

Any alternating current can be characterized by the following types of voltage:

Amplitude - blue arrows (it is clear from the name and pictures);

Secondary - arithmetic mean of all instantaneous signal values ​​for the measured period (not shown in the figures);

RMS - red arrows (discussed above).

To facilitate the understanding of these types of voltage, you can draw them on a graph paper and independently sum up the numerical values ​​\uXNUMXb\uXNUMXbof the voltage (for sinusoidal, rectangular and triangular voltage). Most AC voltmeters have an AC rectification circuit corresponding to the average voltage - as the simplest, and the graduation of the indicating scale - in RMS. When measuring sinusoidal currents and voltages, this does not cause any difficulties, and if the current or voltage differs from a sinusoid, you will have to enter correction factors.

Now let's remember the beginning of the beginning - Ohm's Law: I = U / R, as well as formulas for calculating DC power - P=U*I=I2R=U2/R.

For a sinusoidal current (and voltage), the formula for calculating power from the amplitude voltage measured by the oscilloscope will look like this:

P = (0,707U)2/Rн = U2/4Rн

where 0,707 is the conversion factor of the amplitude voltage U of the sinusoidal current into the equivalent DC voltage.

We have come up with a practical way to measure the output power of an amplifier by measuring the amplitude of the signal on the oscilloscope screen (rice. 2b). Mechanical power is work done in 1 second. Electric power does not contain the explicit time parameter; it is implied (but not observed, and it is precisely when measuring the power of low-frequency amplifiers) that this is also 1 second. For example, for a meander with a frequency of 100 Hz for a time of 10 ms at any moment of the SLE, the voltage is equal to its amplitude value (rice. 2v)

And who prevents to extend such approach and to a sinusoidal signal? For a part of the sine 100Hz for a time of 1ms (rice. 2nd) we get almost a rectangle, for which the coefficient of conversion of the amplitude voltage to the RMS is equal to 1, and, accordingly, the instantaneous power is twice as much as for the whole half-cycle of 10 ms.

But that is not all! You can measure the voltage swing when going from the minimum to the maximum value (rice. 2zh) in a very short period of time and get even more power! Here they are - tens of watts from a boombox and hundreds of watts from a household amplifier!

Let's summarize the results in a table.

We've looked at measuring power across a resistive load (such as a heavy wirewound resistor) commonly used in amplifier testing. An attentive radio amateur, measuring the resistance of the speaker with a digital ohmmeter, will find that it turns out to be less than 4 ohms, for example, 3,8 ohms. "Yeah, so I'll get more than what's on the chart!" - he will exclaim - and he will be right, but not quite. The fact is that the speaker has two resistance components - active, which can be measured with any ohmmeter, and inductive - depending on the number of turns of the speaker coil and its magnetic properties (measured by the RCL meter). Take for example a 3GD-32-75 speaker with a nominal DC coil resistance R = 4 Ohm; inductance L=150 microHenry. The impedance Z of the speaker consists of two components - active Rx and inductive XL. Let's calculate them for two frequencies:

Frequency		1000 Hz	10 kHz
Inductive reactance is calculated by the formula		0,94 ohm	9,4 ohm
Impedance - according to the formula		4,11 ohm	10,21 ohm

We see that at 10 kHz the resistance of the real load increased by 2,5 times, and the power delivered to this load, respectively, decreased by the same 2,5 times (rice. 3 b). Now remember that there is a capacitor at the input of the amplifier (and at the output).

Suppose Rin = 100 kOhm, capacitor capacitance Cin = 0,1 μF. At a frequency of 1 kHz, its resistance will be 1,6 kOhm; at a frequency of 100 Hz - 16 kOhm; at a frequency of 10 Hz - 160 kOhm, i.e. the voltage supplied to the input of the first stage of the amplifier will decrease by 0,38 times, and in proportion to this, the output power (rice. 3v).

A similar calculation for the effect of the output capacitance Cout = 1000 uF gives: 1 kHz - 0,16 Ohm; 100 Hz - 1,6 ohm; 10 Hz - 16 Ohm. In the latter case, only 4 output voltage will be supplied to the 0,2 ohm load, and the output power will decrease to 1/25 of the maximum possible (rice. 3g). Therefore, do not be lazy to calculate the minimum required capacitances of the input and output capacitors to obtain a given frequency response in the low-frequency region.

But again, that's not all! If our loudspeaker is a two- or three-way loudspeaker, it is rather difficult to predict the behavior of the loudspeaker impedance due to the influence of inductances, capacitors and crossover resistors, it is easier to measure (rice. 3nd). (Note from the wise cat. Yes, in general, this is not too necessary.)

To summarize

1.Output power measurement is best done by observing a sinusoidal unlimited signal on the oscilloscope screen, and convert the measured value of the amplitude voltage in RMS (for sinusoidal power), or leave it as it is (for peak power). Measuring voltage with an AC voltmeter is undesirable, since we will not see signal distortion at power close to maximum, and we usually do not know how the voltmeter is assembled and calibrated. The measurement of the amplitude peak power is doubtful - it can also be obtained purely by calculation. The formula for an approximate calculation of the power of a sinusoidal signal is as follows: P \u3d (Up: 2) XNUMX / Rn, where Up - supply voltage, Rn - load resistance at a given frequency. Precision zealots can subtract the voltage drop across the output transistors from Up and take into account the drawdown of Up with an unstabilized power supply.

2.Now we know how to relate to the power declared on the nameplate of a "cool" home theater: "the total power of all channels is 400 watts" with a power consumption of -100 watts from the network. 3.The most correct way would be to say: measured amplifier power - X watts at a harmonic coefficient of Y% and a frequency of Z hertz at a load of R ohms. (For the inquisitive - the old GOSTs implied a harmonic coefficient of 1% at rated power and 10% at maximum). About the harmonic coefficient (we will talk later, now I need food in the form of fish, not electric current! - note of a hungry cat).

4."But again, that's not all!" (Master, can you speak without using advertising slogans? note of a literate cat). The power dissipated on the terminal transistors of the amplifier is not constant (for the most common class AB amplifiers), and reaches a maximum in the range of 0,25..0,5 output power. Based on this, it is necessary to calculate the required area of ​​\uXNUMXb\uXNUMXbradiators.

Publication: radiokot.ru

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