Measuring Parameters of an Amplifier

Let's take a circuit of a class B amplifier i posted earlier and calculate some of its parameters one by one. 1. Class of operations - this topology is set in a class B mode of operation. The way to determine the class is to inspect the idle current (with input grounded) of the power transistors. Keep as a rule of thumb these values. - Class B - idle current is anywhere from a few mA to around 20-30mA - Class AB - anywhere from 30mA to...well it depends how far into the class A region you want to put it in. Let's say 200mA for instance. - Class A - depends on the output power but ideally it should be 1/2 Imax. In reality it's a little higher. 2. Output Power - 2 out of 3 parameters should be known. If the current through the load is known the formula is P=Irms^2.Z (where Z is speaker impedance). If the voltage is the known parameter the formula is P=Urms^2/Z. For instance the current through my load is 6.33A. The Irms is 6.33.0,707=4.47A. The Z is 4 ohm. So P=4.47^2.4 = 79Wrms 3. Consumption and efficiency - You take a look at one of the supplies. You take its average being 3.17A in my case and you multiply it by the voltage. P=Iavg.U. In my case 3.17.35=110W. The efficiency formula is the output power from section 2. divided by the consumption we got in section 3. So the formula will look like η=(79/110).100 [%]. η= 0.718.100. η= 71%. This means 71% of the used power is spent usefully. 4. U,I,P gain - Uout/Uin, Iout/Iin, Pout/Pin. The result is a dimensionless number that represent how much times is the output greater than the input. Alternatively it can be expressed in dB. 5. Frequency response. - in this particular circuit the lower frequency response is below 1Hz. The higher the decoupling capacitors values are the lower the limit is. Since i don't have an output decoupling capacitor due to the dual rail power supply i have nothing to restrict the lower boundary. The upper boundary is somewhat unpredictable when it's not defined by some component (that being the 100pF cap) and should be no less then 20Khz. The higher the value of the capacitor the lower the upper boundary is 6. Ripple rejection - this parameter shows how much the amplifier rejects/attenuates noise from the power supply. In this particular case it's very high at -96dB. Usually you won't need to use this parameter just know that the less the output hum is the larger the ripple rejection is. 7. THD+N - i think i made a tutorial on how to measure it but I'll give a try on exaining it. First you need to make a notch filter. These filters tend to reject only 1 frequency (the fundamental at which you'll be testing the amplifier) and leave all others plus the nonlinear distortions. This way you can measure everything on the output that has been superimposed on the fundamental during the operation. Let's say we measure a total level of noise and distortions (both harmonic and nonlinear) of 1mV (Ud) and the fundamental is 1V(Uf) The formula is THD+N = (Ud/Uf).100 [%] THD+N = (0.001.1).100 [%]. THD+N = 1% 8. Parameters like input impedance and output impedance are expressed by the voltage over current. For instance the output is 25,3V over 6.32A. That means 25.3/6.32 = 4 ohm. Same applies for the input.

published 9 years ago