1. By Squadron Leader Zahid Mir CS&IT Department, Superior University PHY-BE -19 Transistor as an Amplifier

2. DC & AC Quantities DC Quantities I B = dc transistor base current I E = dc transistor emitter current I C = dc transistor collector current V BE = dc B/E junction voltage V CB = dc C/B junction voltage V CE = dc C/E junction voltage V B = dc voltage at base w.r.t. ground V E = dc voltage at emitter w.r.t. ground V C = dc voltage at collector w.r.t. ground

3. DC & AC Quantities AC Quantities I b = ac transistor base current I e = ac transistor emitter current I c = ac transistor collector current V be = ac B/E junction voltage V cb = ac C/B junction voltage V ce = ac C/E junction voltage V b = ac voltage at base w.r.t. ground V e = ac voltage at emitter w.r.t. ground V c = ac voltage at collector w.r.t. ground

4. DC & AC Quantities DC & AC Resistances R E = external dc emitter resistance R C = external dc collector resistance R B = external dc base resistance R e = external ac emitter resistance R c = external ac collector resistance R b = external ac base resistance r’ e = internal ac emitter resistance r’ c = internal ac collector resistance r’ b = internal ac base resistance

5. CE Transistor Amplifier

6. Transistor Amplification Transistor amplifies current because the collector current is equal to the base current multiplied by the current gain, β. An ac voltage, V in is superimposed on the dc bias voltage V BB by connecting in series with the base resistance, R B. The dc bias voltage V CC is connected to the collector through the collector resistor, R C. The ac input voltage produces an ac base current, which results in a much larger ac collector current. The ac collector current produces an ac voltage across R C, thus producing an amplified, but inverted, reproduction of the ac input voltage in the active region of operation.

7. Transistor Amplification The forward-biased B/E junction presents a very low resistance to the ac signal; this internal resistance is designated r’ e I e = I c = V b /r’ e The ac collector voltage, V c, equals the ac drop across R C V c = I c R C V c = I e R C since I c = I e

8. Voltage Gain V b can be considered as the transistor ac input voltage where V b = V in – I b R B V c can be considered the transistor ac output voltage. The ratio of V c to V b is the ac voltage gain, A v, of the transistor. A v = V c /V b A v = I e R C /I e r’ e A v = R C /r’ e This equation represents that the transistor provides amplification in the form of voltage gain, which is dependent on the values of R C and r’ e. Since R C is always considerably larger in value than r’ e, the output voltage is always greater than the input voltage,

9. Practice Problem Determine the voltage gain and the ac output voltage of the amplifier if r’ e = 50Ω