BySquadron Leader Zahid Mir

CS&IT Department , Superior University

PHY-BE -19 Transistor as an Amplifier

DC & AC QuantitiesDC Quantities

IB = dc transistor base current

IE = dc transistor emitter current

IC = dc transistor collector current

VBE = dc B/E junction voltage

VCB = dc C/B junction voltage

VCE = dc C/E junction voltage

VB = dc voltage at base w.r.t. ground

VE = dc voltage at emitter w.r.t. ground

VC = dc voltage at collector w.r.t. ground

DC & AC QuantitiesAC Quantities

Ib = ac transistor base current

Ie = ac transistor emitter current

Ic = ac transistor collector current

Vbe = ac B/E junction voltage

Vcb = ac C/B junction voltage

Vce = ac C/E junction voltage

Vb = ac voltage at base w.r.t. ground

Ve = ac voltage at emitter w.r.t. ground

Vc = ac voltage at collector w.r.t. ground

DC & AC QuantitiesDC & AC Resistances

RE = external dc emitter resistance

RC = external dc collector resistance

RB = external dc base resistance

Re = external ac emitter resistance

Rc = external ac collector resistance

Rb = external ac base resistance

r’e = internal ac emitter resistance

r’c = internal ac collector resistance

r’b = internal ac base resistance

CE Transistor Amplifier

Transistor Amplification• Transistor amplifies current because the collector current is equal to the

base current multiplied by the current gain, β.

• An ac voltage, Vin is superimposed on the dc bias voltage VBB by connecting in series with the base resistance, RB.

• The dc bias voltage VCC is connected to the collector through the collector resistor, RC.

• 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 RC, thus producing an amplified, but inverted, reproduction of the ac input voltage in the active region of operation.

Transistor Amplification• The forward-biased B/E junction presents a very

low resistance to the ac signal; this internal resistance is designated r’e

Ie = Ic = Vb/r’e

• The ac collector voltage, Vc, equals the ac drop across RC

Vc = IcRC

Vc = IeRC since Ic = Ie

Voltage Gain• Vb can be considered as the transistor ac input voltage where

Vb = Vin – IbRB

• Vc can be considered the transistor ac output voltage.

• The ratio of Vc to Vb is the ac voltage gain, Av , of the transistor.

Av = Vc/Vb

Av = IeRC/Ier’e

Av = RC/r’e

• This equation represents that the transistor provides amplification in the form of voltage gain, which is dependent on the values of RC and r’e.

• Since RC is always considerably larger in value than r’e, the output voltage is always greater than the input voltage,

Practice Problem

• Determine the voltage gain and the ac output voltage of the amplifier if r’e = 50Ω