|Radio and Electronics|
|13. CLASS B AMPLIFIERS|
In power amplifiers of older types of receivers we find often circuits which are called PUSH-PULL-AMPLIFIERS. In this kind of an amplifier one transistor amplifies only one half of the input signal wave, the other half of the signal is amplified by another transistor.
One circuit which has been commonly used in such push-pull amplifiers is shown in fig. 162. It has two transistors and two centre-tapped transformers.
T1 has a centre-tapped secondary coil and it is used to split the input signal into two halfwaves, and to feed the bases of the two transistors alternatively.
T2 has a centre tapped primary winding and is used to combine again the two amplified halfwaves of the output-signal.
Transistor V1 is part of an PREAMPLIFIER. It controls the collector-current of V1 through coil a of T1. If this collector current increases, there will be a voltage induced at the secondary coil b of T1. This has here a downward direction, so we find now at the base-emitter-junction of V2 a voltage which is directed forward while at V3 it has a reverse-direction. ® V2 will be conducting and ® there will flow a collector current through V2 and the part 1 of coil b of T2. This induces a voltage in the secondary coil of T2 and a current through the loudspeaker.
If this collector current through V1 decreases, there is a voltage induced at the secondary coil b of T1. This has here a upward direction, so we find now at the base-emitter-junction of V3 a voltage which is directed forward while at V2 it has a reverse-direction ® V3 will be conducting and ® there will flow a collector current through V2 and the part 2 of coil b of T2. This induces an opposite voltage in the secondary coil of T2 and an opposite current through the loudspeaker.
The two resistors R1 and R2 are necessary to avoid a distortion which would be caused if there would be no biasing of the two bases of transistors 2 and transistor 3. As demonstrated in fig. 155 without biasing the start of the output halfwave is delayed till the input signal has passed the level of the so called THRESHOLD VOLTAGE of about 0.6. Volts.
This kind of push-pull-amplifier was used a rather long time, but it is almost no more found in modern transistor radios because of the bulkiness and the high price of transformers. These disadvantages have been the cause for a change in technology of power amplifiers nowadays.