First, the circuit must be supplied with DC voltage so that the transistor is ready to work, this is indicated by the presence of VBE.
Basic Pre-amp
- The transistor will function if given bias / current.
- The capacitor will function as a DC current retainer and pass AC current (dedicated for coupling).
- This circuit improves weak input audio signals. One of its applications is in the initial circuit of RF oscillators, where this circuit helps improve the quality of transmitters that are very sensitive to sound.
- Gain = Vout/Vin
VBE = 0.5 - 0.6 volts (if silicon material) or 0.1 - 0.2 volts (if germanium material). In addition, there are currents flowing, namely IB, IC and IE, these unidirectional quantities will always be in the circuit around the transistor which is limited by the capacitor, the quantities contained in the amplifier can be calculated using the formula:
Vo = VCC - IC (R3)The IC current strength is very dependent on IB, so:
IC = hfE . IBAfter everything is ready, the input signal is fed into the amplifier through a capacitor, where the capacitor functions to block the DC magnitude, so that it cannot enter the signal source, but on the other side the capacitor will amplify the input signal into the amplifier, likewise in the output part the signal is taken from the collector through a capacitor too, the magnitude of the input or output signal can be measured directly with a Volt meter or with an oscilloscope. The comparison between the output signal and the input is called Gain.
For a good Gain, the output signal must always be linear to its input (amplitude). For an ideal Audio amplifier, a bandwidth will be obtained that is limited by a frequency of 20Hz - 20Khz where the Gain remains constant. In a complete audio system, several levels of amplification (cascade) will be used so that the output amplitude will be greater, as well as the output power. The formula for amplification in current, voltage and power:
Ai = Io/Ii
Vi = Vo/Vi
Pi = Po/Pi