ac Load Line Analysis Maximum Symmetrical Swing BJT AMPLIFIER ac Load Line Analysis Maximum Symmetrical Swing

Common-emitter amplifier with emitter bypass capacitor

DC Load line: KVL on C-E loop

AC Load Line - KVL on C-E loop

Example Determine the dc and ac load line. VBE=0.7V, β=150, VA=∞

Solution To determine dc Q-point, KVL around B-E loop

dc & ac load line

Maximum Symmetrical Swing Steps to design a BJT amplifier Write DC load line equation (relates of ICQ & VCEQ) Write AC load line equation (relates ic, vce; vce = - icReq, Req = effective ac resistance in C-E circuit) Generally, ic = ICQ – IC(min), where IC(min) = 0 or some other specified min collector current Generally, vce = VCEQ – VCE(min), where VCE(min) is some specified min C-E voltage Combination of the above equations produce optimum ICQ & VCEQ values to obtain maximum symmetrical swing in output signal

Example Determine the maximum output symmetrical swing for the ac load line in previous figure. Solution From the dc & ac load line, the maximum negative swing in the Ic is from 0.894 mA to zero (ICQ). So, the maximum possible peak-to-peak ac collector current: The max. symmetrical peak-to-peak output voltage: Maximum instantaneous collector current:

Common-collector Amplifier Common-base Amplifier BJT AMPLIFIER Common-collector Amplifier Common-base Amplifier

C-C Small Signal Voltage Gain Common Collector Circuit Small Signal Equivalent Circuit

C-C Input & Output Impedance

Example Calculate the small signal voltage gain, the input and output resistance. Assume the transistor & circuit parameter are;β=100, VCC=5V, VBE=0.7V, VA=80V and ro=100kΩ.

C-C Small Signal Current Gain Can be determine by using the input resistance & the concept of current dividers.

Common Base Amplifier

C-B Small Signal Equivalent Circuit

C-B Small Signal Voltage Gain

C-B Small Signal Current Gain

Input Impedance

Output Impedance

Summary & Comparison Configuration Common Emitter Common Collector Common Base Voltage Gain Av > 1 Av  1 Current Gain Ai > 1 Ai  1 Input Resistance Moderate (kΩ) High (50-100kΩ) Low (Ω) Output Resistance Moderate to High