Cross Distortion WaveForm
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.
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.
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
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
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.