University of Tripoli Faculty of Engineering Electrical and Electronic Engineering Department EE219 Basic Electronic Circuits Fall 2019 (Groups A & B) Eng. Taissir Y. Elganimi Design of Bipolar Junction Transistor (BJT) Amplifiers 1. BJT Amplifier Biasing Design: For the amplifier circuit shown in Figure (A) as an example, the DC equivalent circuit as shown in Figure (B) can be simplified by Thevenin theorem as shown in Figure (C). Figure (A) Figure (B) Figure (C) By applying Thevenin theorem, 1 EE219 - Basic Electronic Circuits Design of BJT Amplifiers π π΅ = π 1 //π 2 = π 1 π 2 π 1 + π 2 ∴ ππ΅π΅ = ππΆπΆ & ππ΅π΅ = ππΆπΆ π π΅ π 1 Prepared by Eng. Taissir Y. Elganimi π 2 π 1 π 2 = ππΆπΆ π 1 + π 2 π 1 π 1 + π 2 (π 1 π·ππ πππ πΈππ’ππ‘πππ) And it can be rewritten as follows: ππ΅π΅ = ππΆπΆ π 2 π 1 + π 2 ππΆπΆ π 1 =1+ ππ΅π΅ π 2 → ∴ π 2 = → π 2 = π π΅ 1 − (ππ΅π΅ ⁄ππΆπΆ ) π 1 π π΅ (ππΆπΆ ⁄ππ΅π΅ ) = (ππΆπΆ ⁄ππ΅π΅ ) − 1 (ππΆπΆ ⁄ππ΅π΅ ) − 1 (π 2 π·ππ πππ πΈππ’ππ‘πππ) And for stability, choose: π π΅ ≤ 0.1π½π πΈ (π π΅ π·ππ πππ πΆπππππ‘πππ) --------------------------------------------------------------------------------------------------2. DC and ac Load Lines: To find the DC operating point (ππΆπΈπ , πΌπΆπ ) of this circuit, KVL can be applied with considering each capacitor acts as an open circuit, and the ac voltage source is a short circuit: ππΆπΆ − πΌπΆ π πΆ − ππΆπΈ − πΌπΈ π πΈ = 0 ο· In order to draw the DC load line, find two points on this DC load line equation: The first point is when πΌπΆ = 0 → The second point is when ππΆπΈ = 0 ππΆπΈ = ππΆπΆ . → πΌπΆ = ππΆπΆ π π πΆ + πΈ πΌ ο· In order to draw the ac load line, find the ac load line equation with considering each capacitor acts as a short circuit, and the DC and ac voltage sources are short circuit. By writing the KVL expression as: 0 − ππ (π πΆ //π πΏ ) − π£ππ = 0 where ππ is the ac collector current, and π£ππ is the ac collector-to-emitter voltage. Since the total instantaneous collector current can be expressed as: ππΆ = πΌπΆπ + ππ And the total instantaneous collector-to-emitter voltage can be expressed as: π£πΆπΈ = ππΆπΈπ + π£ππ The last KVL expression can be rewritten as follows: 0 − (ππΆ − πΌπΆπ )(π πΆ //π πΏ ) − (π£πΆπΈ − ππΆπΈπ ) = 0 2 EE219 - Basic Electronic Circuits Design of BJT Amplifiers Or: ππΆ = πΌπΆπ + Prepared by Eng. Taissir Y. Elganimi ππΆπΈπ π£πΆπΈ − π πΆ //π πΏ π πΆ //π πΏ This is the ac load line equation. We need to find two points on the ac load line equation to draw it with the DC load line equation as follows: The first point is when ππΆ = 0 → π£πΆπΈ = ππΆπΈπ + πΌπΆπ π ππ . where π ππ = π πΆ //π πΏ . The second point is when π£πΆπΈ = 0 ππΆπΈπ → ππΆ = πΌπΆπ + π πΆ //π πΏ . Note that at the Q-point with π£πΆπΈ = ππΆπΈπ , the currents ππΆ and πΌπΆπ will be equal. Therefore, the ac load line passes through the Q-point of the DC load line as shown below. Then, the DC and ac load lines can be plotted as follows: ππΆ (ππ΄) ππΆπΈπ πΆ //π πΏ πΌπΆπ + π ac Load Line πΌπΆπ Q-point DC Load Line 0 ππΆπΈπ ππΆπΈπ ππΆπΆ π£πΆπΈ (π) πΌπΆπ π ππ As it is shown, the ac load line limits the voltage swing to the minimum ππΆπΈπ and πΌπΆπ π ππ . Thus, the maximum voltage variation from the Q-point is the minimum of ππΆπΈπ and πΌπΆπ π ππ , without clipping at the output voltage waveform. When a sinusoidal wave signal is applied to the base of a BJT amplifier, it will vary the base current ππ΅ which will produce ππ΅ (ππ) and therefore ππΆ (ππ) and π£πΆπΈ (ππ). Then, π£πΆπΈ (ππ) will become the output voltage at the collector of this circuit (Common Emitter Amplifier without π πΈ ). 3 EE219 - Basic Electronic Circuits Design of BJT Amplifiers Prepared by Eng. Taissir Y. Elganimi If the Q-point is at a high collector current πΌπΆπ , the lower half cycle of the output voltage wave will be clipped due to the saturate πΌπΆ as shown above. Similarly, if the Q-point is at a low collector current πΌπΆπ , the upper half cycle of the output voltage wave will be clipped due to the cutoff πΌπΆ as shown below: ππΆ (ππ΄) ππΆπΈπ πΆ //π πΏ πΌπΆπ + π ac Load Line πΌπΆπ Q-point DC Load Line 0 ππΆπΈπ ππΆπΈπ ππΆπΆ π£πΆπΈ (π) πΌπΆπ π ππ --------------------------------------------------------------------------------------------------3. BJT Optimum Q-point Design: For maximum output voltage swing, we need ππΆ = 2πΌπΆπ when π£πΆπΈ = 0. In other words, we need ππΆπΈπ to be equal to πΌπΆπ π ππ . ∴ ππΆπΈπ = ππΆπΆ − πΌπΆπ π π·πΆ = πΌπΆπ π ππ where π π·πΆ = π πΆ + π πΈ πΌ in this example. ∴ πΌπΆπ = ππΆπΆ π π·πΆ + π ππ (πππ‘πππ’π πΌπΆπ π·ππ πππ πΈππ’ππ‘πππ) If the input wave increases the base current wave ππ΅ , then both ππΆ and π£πΆπΈ will be increased where both half cycles of the output voltage waveform will be clipped simultaneously due to cutoff and saturation in the case of the optimum collector current, and the peak-to-peak output voltage swing (before clipping) will be equal to 2πΌπΆπ π ππ as shown below: 4 EE219 - Basic Electronic Circuits Design of BJT Amplifiers Prepared by Eng. Taissir Y. Elganimi ππΆ (ππ΄) ππΆπΈπ πΆ //π πΏ πΌπΆπ + π ac Load Line πΌπΆπ Q-point DC Load Line 0 ππΆπΈπ ππΆπΈπ ππΆπΆ π£πΆπΈ (π) πΌπΆπ π ππ Good luck… 5