25 June 2012

Class B Push-pull Transformer Amplifier Circuit


* The audio power amplifier used in transistor receivers, tape recorders, record players, PA systems are make use of the push pull circuit.

* A combination of two class B amplifiers working together is called push-pull amplifier.

* To improve the full power efficiency of Class A amplifier by reducing the wasted power in the form of heat, it is possible to design the power amplifier circuit with two transistors in its output stage producing what is commonly termed as a "push-pull" type amplifier configuration. 

* The circuit above shows a standard Class B Amplifier circuit that uses a balanced centre-tapped input transformer, which splits the incoming waveform signal into two equal halves and which are 180o out of phase with each other.

* Another centre-tapped transformer on the output is used to recombined the two signals providing the increased power to the load.

* The transistors used for this type of transformer push-pull amplifier circuit are both NPN transistors with their emitter terminals connected together.

* If the transformer is accurately centre tapped, then the two collector currents will flow in opposite directions (ideal condition) and there will be no magnetization of the transformer core, thus minimizing the possibility of distortion.

* The collector terminals of the two transistors are connected to the supply voltage VCC through part of the primary of the transformer (T2).

* The resistors R1, R2 and the emitter resistance RE are used to bias the amplifier under class B condition so that practically no output current flows in the absence of an applied signal.

* When a signal is present across the secondary of the driver transformer T1, the transistor base inputs are in "anti-phase" to each other, thus if TR1 base goes positive driving the transistor into heavy conduction, its collector current will increase but at the same time the base current of TR2 will go negative further into cut-off and the collector current of this transistor decreases by an equal amount and vice versa. Hence negative halves are amplified by one transistor and positive halves by the other transistor giving this push-pull effect.

* These two output half-cycles are combined to reform the sine-wave in the output transformers primary winding which then appears across the load.


* We see that during one half cycle of the input signal, the circuit pushes the signal high and during the next half cycle, the circuit pulls the signal low at the output. For this reason, the circuit is called Push-Pull amplifier.

Advantages:
* The output is true replica of the input signal.

* The d.c. components of the collector current in the output transformer are in opposite directions. This prevents the transformer core from getting into saturation in the transformer.

* Higher efficiency with less distortion can be secured with the class B push-pull amplifier. The maximum efficiency = 79%.

* There will be no hum in the amplifier output, since the ripple currents due to power supply flow in opposite directions in the output transformer.

* There is no current drain, when the signal is zero.