Class B Push-pull Transformer Amplifier Circuit
The above circuit shows a standard class B amplifier circuit that divides the incoming waveform signal into two equal halves and uses a balanced input transformer with an out-of-phase 180o with each other. Another center-tapped transformer at the output is used to reassess two signals that provide more power to the load. The transistors used for this type of transformer push-pull amplifier circuit are NPN transistors, both of which are interconnected emitter terminals. Here, the load current is shared between two power transistor devices, since it decreases in one device and increases in the other during the signal loop, reducing the output voltage and current to zero. As a result, both halves of the output waveform are now released into a stream twice as quiet as zero, thereby reducing distribution. This has the effect of almost doubling the efficiency of the amplifier to about 70%. Assuming that the input signal does not exist, each transistor carries the normal stream of calm collectors, the value of which is determined by the base polarization at the breakpoint. If the transformer touches the center correctly, the two collector currents will flow in opposite directions (ideal condition) and there will be no magnetization of the transformer core, thereby minimizing the possibility of deterioration. And when an input signal that reduces the transistor current collector in equal amounts and vice versa is present on the secondary of the T1 drive transformer, the transistor base input is shown in the "anti-phase", so that if the transistor driving into the TR1 base heavy transmission goes positive, the collector current will increase but also the TR2 current base will go negatively off further into cutting. Therefore, negative halves are raised by a transistor, and positive halves by other transistors give this push-pull effect. Unlike the DC condition, these alternating currents cause two output half loops to be combined to reshape the sinus wave in the primary winding of the output Transformers, which then appears along the load. The operation of the class B Amplifier has zero DC polarity, since the transistors are polar when cut, so each transistor only works when the input signal is larger than the base emitter voltage. Therefore, zero input has zero output and no power is consumed. This then means that the actual Q point of the class B amplifier is in the Vce part of the load line, as shown below.
Characteristic Curve of Class B Amplifiers
The class B amplifier has a great advantage over its class A amplifier cousins, since when in a quiet state (that is, without an input signal), no current flows from the transistors, so no power is distributed in the output transistors or transformer. Therefore, the overall conversion efficiency of the amplifier (η) is greater than equivalent Class A, and efficiency reaches 70%, which leads to almost all modern types of push-pull amplifiers running in this Class B mode.
Class B Push-pull Transformer-Free Amplifier Circuit
One of the main drawbacks of the above class B amplifier circuit is that it uses balanced Center-tapped transformers in its design and makes it expensive to build. However, there is another type of B-class amplifier called the complementary Symmetry class B amplifier, which does not use transformers in its design, so it is transformer-free using complementary or matching power transistor pairs. Since transformers are not needed, this makes the amplifier circuit much smaller for the same amount of output, as well as there are no stray magnetic effects or transformer distortion to affect the quality of the output signal. Below is an example of the "transformer-free" Class B amplifier circuit.
Exit Stage of Class B Transformer-Free Amplifier
The above class B amplifier circuit uses free transistors for each half of the waveform, and while class B amplifiers have a much higher gain than class A types, one of the main drawbacks of class B push-pull amplifiers is that they suffer from an effect commonly known as cross-distortion. I hope we remember from our tutorials about transistors that it takes about 0.7 volts (measured from the base to the emitter) to obtain a bipolar transistor to start transmitting. In a pure class B amplifier, output transistors are not "pre-polarized" to an "open" operating state. This means that the part of the output waveform that falls below this 0.7 volt window will not be reproduced correctly as a transition between two transistors (when switching from one transistor to another). For each half of the waveform, each of the output transistors (positive and negative) will have an area of 0.7 volts in which they are not conductive. The result is that both transistors are "turned off" at the same time. A simple way to eliminate cross-distortion in a class B amplifier is to add two small voltage sources to the circuit to polarize both transistors at a point slightly above the cutting point. This will then give us what is called the EU-class amplifier circuit. However, it is impractical to add additional voltage sources to the amplifier circuit, so pn connections are used to have additional polarity in the form of silicone diodes.
EU Class Amplifier
We know that the base emitter voltage must be greater than 0.7 v for a silicon bipolar transistor to start transmitting, so if we replace the two voltage dividing polarity resistance connected to the base terminals of transistors with two silicon diodes. The polarity voltage applied to transistors will now equal the forward voltage drop of these diodes. These two diodes are often referred to as polarization diodes or compensation diodes and are selected to match the characteristics of matching transistors. The following circuit shows diode polarity. The EU-class amplifier circuit is a compromise between class A and class B configurations. This very small diode polarity voltage causes both transistors to move slightly, even when there is no input signal. An input signal waveform causes transistors to operate normally in their active regions, thereby eliminating any cross-distortion found in pure B-class amplifier designs. Without an input signal, a small collector stream will flow, but much less than the class A amplifier configuration. This means that the transistor will be "on" for more than half of the waveform, but much less than a full loop that gives a transmission angle of 180o to 360o, or 50% to 100% of the input signal, depending on the additional amount of polarization used. The amount of diode polarl voltage in the transistor's base terminal can be increased in multiples by adding additional diodes in series. It is highly preferred over class A designs for high-power applications such as class B amplifiers, sound power amplifiers and PA systems. Like the class A amplifier circuit, one way to greatly increase the current gain (A) of a class B propulsion amplifier is to use Darlington transistor pairs instead of single transistors in the output circuit. In the next lesson about amplifiers, we will take a closer look at the effects of cross Distortion in class B amplifier circuits and ways to reduce its effect.