How to amplify audio linearly with a few transistors
This is for beginners and interested people. Here you see how to create a simple amplification stage with one or two NPN transistors, usable for audio and HF, most suitable for audio. The transistors are in a grounded emitter figuration. You can give every individual stage its own amplification factor by turning the variable resistor on the emitter. The right values of the resistors connected to the basis are the key to success. When you home-brew on the basis of this schematic, always use a variable resistor on the basis of each individual transistor (25 K to 50 K, try 25 K first) to set every transistor to the own correct working range. I cannot stress this enough, some schematics cannot be successfully reproduced because the author had a certain "luck" in designing these resistors. When the amplifier works correct (check it with the scope) take out the variable resistors, measure them and replace them by fixed value resistors. This schematic is usable for simple high frequency stages up to maximum approx. 10-15 MHz. The BC547 b (hfe = 300) is usable up to 8-9-10 MHz. The 2N2219 is usable as typical HF transistor, up to 15 MHz in this schematic. For higher frequencies other types of schematics and specialized HF transistors work better. The schematic is very good usable for audio applications (20 Hz-20 KHz) for instance as a microphone amplifier. In that case often 1 stage is enough. You can set the amplification between 2 and approx. the maximum that the transistor can give. Example: stage 1 = 10, stage 2 = 100 (supposed both transistors amplify (hfe) more than 100). Total amplification = 1000, one microvolt AC in will be 1000 microvolt (1V) AC out.
It is possible to put a third stage in.
In most cases not necessary, given the fact that a modern NPN transistor amplifies approximately 300 and 300 x 300 = 90000. If you use electrolytic capacitors the + should be to the "most positive" side in the schematic (=the point with the highest potential