obtained from a certain AC input voltage is often of greater interest. When these devices are used as cathode followers (or source followers), they all have a voltage "gain" of just under 1, but with a large current gain. Applications Although S.G. Brown's Type G Telephone Relay (using a magnetic "earphone" mechanism driving a carbon microphone element) was able to give power amplification and had been in use as early as 1914, it was a purely mechanical device with limited frequency range and fidelity. It was suited only to a limited range of audio frequencies - essentially voice frequencies. The triode was the first non-mechanical device to provide power gain at audio and radio frequencies, and made radio practical. Triodes are used for amplifiers and oscillators. Many types are used only at low to moderate frequency and power levels. Large water-cooled triodes may be used as the final amplifier in radio transmitters, with ratings of thousands of watts. Specialized types of triode ("lighthouse" tubes, with low capacitance between elements) provide useful gain at microwave frequencies. Vacuum tubes are obsolete in mass-marketed consumer electronics, having been overtaken by less expensive transistor-based solid-state devices. However, more recently, vacuum tubes have been making somewhat of a comeback. Triodes continue to be used in certain high-power RF amplifiers and transmitters. While proponents of vacuum tubes claim their superiority in areas such as high-end and professional audio applications, the solid-state MOSFET has similar performance characteristics. Characteristics In triode datasheets, characteristics linking the anode current (Ia) to anode voltage (Va) and grid voltage (Vg) are usually given. From here, a circuit designer can choose the operating point of the particular triode. Then the output voltage and amplification of the triode can be evaluated graphically by drawing a load line on the graph. In the example characteristic shown on the image, suppose we wish to operate it at a quiescent anode voltage Va of 200V and a grid voltage bias of −1 volt. This implies a quiescent plate (anode) current of 2.2mA (using the yellow curve on the graph). In a class-A triode amplifier, one might place an anode resistor (connected between the anode and the positive power supply). If we choose Ra = 10000 ohms, the voltage drop on it would be V+ - Va = Ia × Ra = 22.V for the chosen anode current of Ia = 2.2mA. Thus we require a power supply