--- title: "Common base amplifier circuit and gains" subject: "Electronics Devices and Circuit" chapter: 2 key: "semester_3/electronics_devices_and_circuit:cb-amplifier-analysis" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/cb-amplifier-analysis site: BCT Engineering --- # Common base amplifier circuit and gains Electronics Devices and Circuit · Chapter 2: The Bipolar Junction Transistor - Asked in: 2083 Baisakh, 2+2+2+2 marks - Video: [BJT- Common Base Amplifier Explained](https://www.youtube.com/watch?v=MckBbhfxdkk) — ALL ABOUT ELECTRONICS - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/cb-amplifier-analysis - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-2 ## Question Draw the circuit diagram of Common Base (CB) amplifier using BJT and derive the expressions for input impedance, output impedance, and voltage gain. ## Short answer CB amplifier analysis: input at emitter, base AC grounded. Zin = RE||re ≈ re = VT/IE (very low, ~10–50 Ω). Output impedance Zo ≈ RC. Voltage gain Av = α(RC||RL)/re ≈ (RC||RL)/re, non-inverting. Current gain Ai ≈ α < 1. ## Answer #### Circuit Common base amplifier with voltage divider bias: the input vs (through C1 and the source resistance Rs) is applied at the emitter; the output is taken from the collector through C2 to RL; the base is grounded for ac by the capacitor CB. ```text C1 C2 vs--Rs------||-----+----- E C ----+----||----+---- vo | \ / | | RE -------- RC RL | | | | GND B +VCC GND | R1 to +VCC, R2 to GND, CB to GND (ac ground) ``` #### AC equivalent (T-model) With the base grounded, re is connected from the emitter to ground (through the base), RE is in parallel with it at the input, and the current source α ie feeds the collector load RC || RL. Here ie is the T-model emitter current, flowing from the base through re and out of the emitter terminal. #### Input impedance The input current flows into the emitter, ii = -ie, and the emitter voltage is ```text vi = -ie re Zin(emitter) = vi/ii = re Zin = RE || re ≈ re = VT/IE ``` It is very low (about 10 to 50 Ω). #### Output impedance ```text Zout = RC || ro ≈ RC ``` (The collector looks into a current source, whose resistance is very high, so RC dominates.) #### Voltage gain The collector current α ie flows into the collector, so ```text vo = -α ie (RC || RL) , ie = -vi/re Av = vo/vi = α (RC || RL) / re ≈ (RC || RL)/re = gm (RC || RL) ``` The gain is positive (no phase shift) and can be large. #### Properties | Parameter | CB | |---|---| | Zin | low (≈ re) | | Zout | high (≈ RC) | | Av | high, in phase | | Ai | ≈ α < 1 | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the input impedance of a CB amplifier and why? A. High, about beta re, because the base is grounded B. Equal to R1 || R2 C. Equal to RC D. Low, about re = VT/IE, because the input is at the emitter looking into the forward-biased junction Answer: D. At the emitter the source sees re (in parallel with RE), which is only a few ohms to tens of ohms. (Key point: CB input impedance) 2. What is the voltage gain expression of the CB amplifier? A. Av = alpha (RC||RL)/re ≈ gm (RC||RL), positive B. Av = -beta RC/rpi C. Av = -(RC||RL)/re D. Av = RE/(RE + re) Answer: A. vo = alpha ie (RC||RL) and vi = ie re, so Av = alpha(RC||RL)/re with no phase inversion. (Key point: CB voltage gain) 3. What is the output impedance of the CB stage? A. Zout ≈ rpi B. Zout ≈ re C. Zout ≈ RE D. Zout = RC || ro ≈ RC Answer: D. Looking into the collector is a current source, so RC dominates and Zout ≈ RC. (Key point: CB output impedance) 4. Which statement about CB gain properties is correct? A. Voltage gain is below 1 and current gain is high B. Voltage gain is inverting and current gain is about beta C. Voltage gain is in phase and current gain is very high D. Voltage gain is in phase and current gain is about alpha (< 1) Answer: D. CB gives non-inverting high voltage gain and Ai ≈ alpha, slightly below 1. 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