Yükselteçlerde Çapraz Bozulma(Distorsiyon) / Crossover Distortion in Amplifiers

Cross-Distortion is a common feature of Class B Amplifiers, where the fact that the two switching transistors are not linear, does not change linearly with the input signal. However, we also know that by replacing the output stage of the amplifier with a Class B push-pull type configuration, we can improve the amplifier and almost double its efficiency. However, in terms of efficiency, this arrangement is great, but most modern Class B Amplifiers are transformerless or complementary types with two transistors at the output stage. This results in a fundamental problem with push-pull amplifiers, as the two transistors do not fully combine in both half of the waveform at output due to unique zero-cut pre-polarization arrangements. This problem creates some "distortion" in the form of an output wave when the signal changes or "switches" from one transistor to another at zero voltage point. This results in a condition that is often called Cross-Distortion. Cross Distortion produces a zero voltage "straight point" in the form of an output wave as it moves from one half of the waveform to another. This is because the transition period does not stop or start at exactly zero crossing points as transistors move from one to the other, thereby causing a small delay between the first transistor being "OFF" and the rotation of the second transistor. This delay, when transistors are "ON", causes both transistors to be rendered "OFF" by simultaneously producing an output wave shape as shown below.

Cross Distortion WaveForm

cross-distortion In order to avoid any distortion in the output waveform, we must assume that each transistor begins to transmit the voltage from the base to the emitter when it rises just above zero, but we know that this is not true because the base-emiter voltage for silicon bipolar transistors must reach at least 0.7v before starting the transistor transmission due to the advanced diode voltage drop of the base-emiter pn-connection, thus producing this flat point. This cruciate distortion effect also reduces the overall top-to-top value of the output waveform, which causes maximum power output to decrease, as shown below.

Nonlinear Transfer Characteristics

This effect is less pronounced for large input signals, since the input voltage is usually quite large, but it can be more severe, causing sound distortion in amplifiers for smaller input signals.

Pre-Polarising output

The problem of Cross-Pass Distortion can be significantly reduced by applying a low forward basic front polarization voltage to the bases of the two transistors through the input transformer, so that transistors are now polarized at zero breakpoints.

Push-Pull Amplifier with Front Polar

cross-distortion This type of resistance pre-polarization causes one transistor to be "ON" at exactly the same time when it makes the other transistor "OFF", since both transistors are polarized slightly above the original cutting points. However, to achieve this, the pre-voltage must be at least twice the normal base emitter voltage to make transistors "ON". This polarism can also be applied on transformerless amplifiers using complementary transistors, replacing two potential divider resistances with prone diodes, as shown below.

Pre-Polarising with Diodes

cross-distortion For the amplifier circuit with or without transformer, this polarizer voltage has the effect of moving the amplifiers' Q-point beyond the original cutting point, allowing each transistor to operate within its active zone, for just over half or just over 180o. The amount of diode polarl voltage in the base terminal of the transistor can be increased in multiples by adding additional diodes in series. This later evolved into an amplifier circuit, often called the EU Class Amplifier.

EU Class Exit Properties



To summarize, cross-distortion occurs in Class B amplifiers because the amplifier is polar at the breakpoint. This then causes two transistors to be made "OFF" at the same time as the waveform passes the zero axis. By using a resistant potential dividing circuit or diode front polarization, this crossover distortion can be greatly reduced or even completely eliminated by positioning transistors in an "ON" position. The application of a pre-polar voltage produces another type or class amplifier circuit, often called the EU Class Amplifier. Then the difference between a pure Class B Amplifier and an improved EU Class Amplifier is at the pre-polarity voltage level applied to output transistors. One of the biggest advantages of using diodes according to resistances is that PN connections compensate for changes in the temperature of transistors. Therefore, we can say that the EU Class Amplifier is a Class B Amplifier with effectively added "Bias", and we can summarize this as follows: Class A Amplifiers – There is no cross-distortion since they are polarized in the center of the load line. Class B Amplifiers – Large amounts of cross-distortion due to polarity at the breakpoint. EU Class Amplifiers – Sometimes cross-distortion occurs if the pre-polarity level is set too low. In addition to the three amplifier classes above, we've also seen switching amplifier designs that use different switching techniques to reduce power loss and increase efficiency. Some of these amplifier designs use RLC resonators or a multichannel power supply to help reduce power loss and distortion.