Question

Draw a labelled circuit diagram of a common emitter amplifier using a $p-n-p$ transistor. define the term voltage gain and write an expression for it. Explain how the input and output voltages are out of phase by ${180}^o$ for a common-emitter transistor amplifiers.

Answer 1

Voltage gain = it is equal the ratio of small change in output voltage at collector to that of change in input voltage

$A_V=\frac{Outputvoltage}{Inputvoltage}=\frac{△V_{CE}}{△V_{EB}}=\frac{△I_C{R}_{out}}{△I_B{R}_{in}}$

$A_V={\beta }_{AC}\times \frac{R_{out}}{R_{in}}$

Where ${\beta }_{AC}$ is AC current gain

Relation between input and output signals of n-p-n transistor amplifier, when positive half cycle is fed into input circuit, forward bais of emitter base circuit decrease, i.e collector current decrease , output voltage.

$V_O=V_{CE}-I_C{R}_L$

Decrease in collector current cause increase of output voltage.