Transistor Radio Output Stages

Posted on Feb 5, 2014

The driver stage works in the same manner as the class A output stage described previously. However it is only working at a couple of milliamps, so efficiency is not an issue here. The biasing arrangement should be familiar by now. Some sets, including the circuit above, have an extra stage of amplification between the volume control and the drive

Transistor Radio Output Stages
Click here to download the full size of the above Circuit.

r stage. Such sets will be able to receive weaker stations better and probably produce higher volume or better quality on stronger signals. This would take the total to seven transistors, whereas sets without this stage have six transistors. These are the standard line-ups, and most early MW/LW sets will have either six or seven transistors depending on the model and original selling price. Because the class AB output stage current consumption varies with the signal being amplified, the current drawn from the supply (battery) varies in sympathy. This will cause the supply voltage to fluctuate. We need to prevent these rapid fluctuations from finding their way back to the driver and earlier stages, otherwise they could cause distortion and/or instability. In the circuit above we have a 100uF capacitor (C7) directly across the power supply. This would be mounted close to the output stage and would smooth the fluctuations. The supply to the driver stage biasing and earlier stages in the set is further filtered or decoupled by a 1k-ohm resistor (R13) and another 100uF capacitor (C5). If this capacitor fails the set will probably make a horrible noise known as "motor-boating" because it resembles the sound of an outboard motor. If the 100uF capacitor directly across the power supply were to fail the same effect would probably occur but only as the battery starts to run down. The main problem with the two-transformer circuit...

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