Audio Amplifier One Transistor for Crystal Detector Radio Receiver

Figure 1. One Transistor Audio Amplifier

Figure 1. shows the schematic diagram for a very simple one-transistor audio amplifier stage that is useable by itself or with a crystal detector radio. This amplifier has only five components, counting the small 300 Ohms headphones or a small speaker and the B1, 1.5-Volt power cell.

C1 is a good quality, low voltage electrolytic capacitor. It separates the input signal from its DC components. Q1 is a small signal, silicon, NPN, general-purpose transistor. The transistor is biased into its active region with a quiescent collector current (current without signal) of approximately to 0.5 to 1 mA. A three Ampere-hour AA size alkaline cell is capable to provide this current for longer then three thousand hours! A higher gain silicon transistor is preferable, but if that is not available a germanium is also useable. The 300 ohms speaker may be substituted with an eight ohms speaker removed from an old transistor radio along with its output transformer.

This circuit amplifies the input signal about ten times, and allows higher volume radio reception without using much power. A crystal detector radio - like the high performance crystal radio set - outfitted with this amplifier produces a louder, audio signal than it's usual output. It does that, however, at the expense of some extra work and components. Current consumption is so low, that a battery lasts almost as long as its shelf life. For operating convenience, add an optional power switch.

                       Parts list for this one transistor audio amplifier:

                          B1 - 1.5 Volt cell - an AA alkaline is fine
                          C1 - Electrolytic Capacitor 10 uF 6-10 VDC
                          Q1 - Transistor, NPN, silicon general purpose
                          R1 - Resistor, 100 K, 1/4 Watt Carbon
                          300 Ohms headphones or small speaker