This experimental (3) transistor class A audio power amplifier delivers 25mW into an 8Ω load, or 50mW into a 4Ω load using only a 1.5V power source. At such low voltages, there are many issues to consider and much to learn. To the best of my knowledge the following information is new to the world. Background Months ago, I indicated an intention to write a piece on low voltage transistor application for another of my Single Transistor Amplifier Revisited series. Then a few weeks ago, Daniela, who participates in our forum, asked the simple, but profound question: what is the minimum operating voltage of a transistor? While, Mr. Marian attempted to narrow the scope of the question, no answers were given. This is on the heels of a similar article concerning Low Voltage Operation of the 555. Theoretical minimum vs. practical minimum voltage For a silicon bipolar transistor, the initial voltage requirement is to exceed the Vbe junction voltage of 0.6V. Then, to be able to current regulate this voltage via a series resistor, the source voltage must be about double this or about 1.2V. (The rule of thumb for shunt voltage regulators is that the source voltage is recommended to be double the regulated voltage.) This is perhaps the theoretical minimum Vcc. The practical minimum takes into consideration low voltage power sources in this case, the ubiquitous 1.5V single cell battery is perhaps the standard low voltage power source this is the power source for my circuit. Experimental circuit I have been toying with this idea for months and finally got around to bread boarding it. While it functions well, I am not suggesting that it is all that useful due to battery life limitations. I merely suggest that it is a really great experiment that shows what happens at low voltages. While the power is relatively low (25mW), the sound volume is adequate and would be quite loud with earphones. Beyond this, the loudspeaker efficiency and enclosure acoustics plays an important role in the output volume. The frequency response is good due to the use of direct coupling it is limited on the low end by C1 to 8HZ, and on the high end to well beyond 50kHZ. The output stage quiescent current flows through the loudspeaker not recommended for higher power applications. The direct coupling of the transistor arrangement (NPN, PNP, NPN) provides superior base drive required to saturate the output transistor. Efficiency is about 20% that is close to the theoretical maximum of 25% for a class A amplifier. Note that unlike class B or AB, the class A amplifier DC supply current remains unchanged regardless of the output signal level this makes it a real battery-killer!