Thursday, July 23, 2015

working of amplifier

Single Stage RC Coupled CE Amplifier

The figure shows a single stage CE amplifier. C1 and C3 are coupling capacitors, they are used for blocking the dc component and passing only ac part they also ensure that the dc basing conditions of the BJT remains unchanged even after input is applied. C2 is the bypass capacitor which increases the voltage gain and bypasses the R4 resistor for ac signals.

bjt application amplifier The BJT is biased in the active region using the necessary biasing components. The Q point is made stable in the active region of the transistor. When input is applied as shown below the base current starts to vary up and down, hence collector current also varies as IC = β × IB. Therefore voltage across R3 varies as the collector current is passing through it. Voltage across R3 is the amplified one and is 180° apart from the input signal. Thus voltage across R3 is coupled to the load and amplification has taken place. If the Q point is maintained to be at the centre of the load very less or no waveform distortion will take place. The voltage as well as current gain of the CE amplifier is high (gain is the factor by which the voltage of current increases from input to output). It is commonly used in radios and as low frequency voltage amplifier.
bjt amplifier
To further increase the gain multistage amplifiers are used. They are connected via capacitor, electrical transformer, R-L or directly coupled depending on the application. The overall gain is the product of gains of individual stages. Figure below shows a two stage CE amplifier. various parameters of CE amplifier are
DefinitionExpression (with emitter degeneration)Expression (without emitter degeneration, i.e., RE = 0)
Current gainA_{\text{i}} \triangleq \frac{i_{\text{out}} }{ i_{\text{in}} } \,\beta \,\beta
Voltage gainA_{\text{v}} \triangleq \frac{v_{\text{out}} }{ v_{\text{in}} } \,\begin{matrix}-\frac{ \beta R_{\text{C}} }{ r_{\pi} + ( \beta +1 ) R_{\text{E}} }\end{matrix}\,\approx-g_m R_{\text{C}}
Input impedancer_{\text{in}} \triangleq \frac{v_{\text{in}}}{i_{\text{in}}}\,r_{\pi} +( \beta +1 ) R_{\text{E}}\,r_{\pi}
Output impedancer_{\text{out}} \triangleq \frac{v_{\text{out}}}{i_{\text{out}}}\,R_{\text{C}}\,R_{\text{C}}
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