At the beginning of this quarter, we decided to buy a circuit we could build on our own to practice soldering. We bought the flashing LED sweetheart and while building the circuit we realized that it was an astable multivibrator. An Astable multivibrator switches between two states without stopping by using their output signal to recharge the input signal. This works because of a transistor or amplifier that amplifies the output signal and forwards the charge to the input. In most astable multivibrators there are two capacitors in between the input and output terminals, one is fully charged and the other is fully discharged, which allows voltage levels to be stepped up or down. As the charged capacitor discharges and transfers its energy to the second capacitor, the second capacitor recharges and prepares to discharge the energy back to the input. This allows the astable multivibrator to switch between a high state and a low state on a continuous cycle. Astable multivibrators are used in radio equipment to receive and transmit radio signals. Astable multivibrators are also used in Morse code generators, timers, and systems that require a square wave, including television broadcasts and analog circuits. Some advantages of astable multivibrators is that they continuously switch between one state and another. This allows them to power themselves and perform work at a fast rate without influence from any outside forces or events. Astable multivibrators are also inexpensive to produce and are relatively simple in design, and can remain functional for extraordinary amounts of time. Some disadvantages of an astable multivibrator is that they do not transfer the entire output signal to the input. This is because of resistance within the circuit, lack of a completely closed loop at the output terminals, and the tendency for one capacitor or transistor to absorb energy at a slightly different rate than the other. Although the amplifier restores the lost energy when it amplifies the signal, the signal will be too small to be of any use. The basic transistor circuit for an astable multivibrator produces a square wave output from a pair of grounded emitter cross-coupled transistors. Both transistors either NPN or PNP, in the multivibrator are biased for linear operation and are operated as Common Emitter Amplifiers with 100% positive feedback. Amplifiers, which are devices that have varying input signals and produces an output signal that varies just like the input but on a larger amplitude. The large output signal contains the essential waveform features of the input signal. The input signal might be a current, voltage, mechanical motion, or any other signal; the output is usually of the same nature as the input. There are many ways to use amplifiers in an electronic equipment such as, radio and television receivers, high-fidelity audio equipment, and computers. The most common type of amplifiers have transistors or electron tubes as their principle components. A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. A transistor is composed of semiconductor material with at least three terminals for connection to an external circuit. Amplifiers in the simplest form are built around a single transistor. In a single transistor amplifier, or common-emitter circuit, the input voltage is fed to the base of the transistor, and the output appears at the transistors collector; the ratio of the output voltage to the input voltage is called the voltage gain. A bipolar junction transistor (BJT) is a type of transistor that counts on the contact of the two types of semiconductor for it to work. BJT’s can be used as amplifiers, switches, or an oscillator. It is called bipolar because their operation involves both electrons and holes. The regions of a BJT are called the emitter, base, and collector. BJT’s come in two types, known as PNP and NPN based on the doping of the main terminal regions. A common-emitter amplifier is a basic single-stage bipolar junction transistor amplifier topologies, usually used as a voltage amplifier. The CE amplifier configuration in which the emitter is the common terminal to an ac signal or ground. In the CE circuit the base terminal of the transistor serves as the input, the collector is the output, and the emitter is common to both. Common emitter amplifiers give an inverted output; they can have a very high gain that may differ from one transistor to the next. The gain is a function of both temperature and bias current, and so the actual gain is somewhat unpredictable. A common-base amplifier has am input between the emitter and the base. A CB amplifier has an output that is between the collector and the base. It is called the common-base because the signal source and the load share the base of the transistor as a common connection point. In the CB amplifier the input signal source must carry the whole emitter current of the transistor. A common-collector amplifier has the collector in common in both the input and output. It is called common-collector because both the signal source and load share the collector lead as a common point. CC is input applied to the base and collector. The output is from the emitter collector circuit. Common-collector has a low output impedance which allows a source with large output impedance to drive a small load impedance; the circuit has current gain instead of voltage gain. A small chance to the input current results in a much larger change in the output current supplied to the output load.