supply voltage, transistor C-E junction voltage drop, collector current, and amplification factor beta. Transistor as an amplifier The common-emitter amplifier is designed so that a small change in voltage (Vin) changes the small current through the base of the transistor whose current amplification combined with the properties of the circuit means that small swings in Vin produce large changes in Vout. Various configurations of single transistor amplifiers are possible, with some providing current gain, some voltage gain, and some both. From mobile phones to televisions, vast numbers of products include amplifiers for sound reproduction, radio transmission, and signal processing. The first discrete-transistor audio amplifiers barely supplied a few hundred milliwatts, but power and audio fidelity gradually increased as better transistors became available and amplifier architecture evolved. Modern transistor audio amplifiers of up to a few hundred watts are common and relatively inexpensive. Comparison with vacuum tubes Before transistors were developed, vacuum (electron) tubes (or in the UK "thermionic valves" or just "valves") were the main active components in electronic equipment. Advantages The key advantages that have allowed transistors to replace vacuum tubes in most applications are No cathode heater (which produces the characteristic orange glow of tubes), reducing power consumption, eliminating delay as tube heaters warm-up, and immune from cathode poisoning and depletion. Very small size and weight, reducing equipment size. Large numbers of extremely small transistors can be manufactured as a single integrated circuit. Low operating voltages compatible with batteries of only a few cells. Circuits with greater energy efficiency are usually possible. For low-power applications (for example, voltage amplification) in particular, energy consumption can be very much less than for tubes. Complementary devices available, providing design flexibility including complementary-symmetry circuits, not possible with vacuum tubes. Very low sensitivity to mechanical shock and vibration, providing physical ruggedness and virtually eliminating shock-induced spurious signals (for example, microphonics in audio applications). Not susceptible to breakage of a glass envelope, leakage, outgassing, and other physical damage. Limitations Transistors may have the following limitations: They lack the higher electron mobility afforded by the vacuum of vacuum tubes, which is desirable for high-power, high-frequency operation – such as that used in some over-the-air television transmitters and in travelling wave tubes used as amplifiers in some satellites Transistors and other solid-state devices are susceptible to damage from very brief electrical and thermal events, including electrostatic discharge in handling. Vacuum tubes are electrically much more rugged.