Small signal gain for common emitter amplifier - Page 1
Forum section: electronics / projects

author: sy
author_rank: Regular Contributor
timestamp: January 01, 2025, 04:47:03 am
content: Hello, I would like some assistance with biasing my common emitter amplifier as I am not getting expected results.The approach I have taken for the design is as follows:1. I am using the 2n3904 NPN silicon transistor. I have chosen my supply voltage as Vcc=9V and collector current Ic=10mA.2. Next I have set the collector emitter voltage Vce=4V (halfway between Vcc-1V as suggested by Horowitz and Hill). So the collector resistor is about Rc=4V/10mA=400.3. I believe the small signal gain can be approximated by Av=-Rc/Re. If I wanted a gain of 10, then my emitter resistor would be Re=40.4. From the datasheet, at Ic=10mA, the expected beta is around 100<beta<300. I have chosen to approximate using beta of 200, so the required base current is Ib=10mA/200=50uA.5. Then I did KVL at the base, which yielded: (Vb-9)/R1 + Ib + Vb/R2 = 06. To find the base voltage I did another KVL from the base through the emitter, so Vb = 0.7 + IeRe = 0.7 + (Ib+Ic)Re = 0.7 + (50uA+10mA)40 = 1.10V.7. I chucked this base voltage into the equation in step 5, and arbitrarily set R1=10k, then solved for R2=1.5k (approx). (edit: the values might be different in the picture because i was messing around with the resistor values trying to fix the problem)When I put this into the LTSpice to simulate with an input signal of 1mV, I don't quite get an output of 10mV. It's slightly less with a gain of around 8-9ish. What can I do to make this more closer to 10? I am not very good with designing circuits which is why I want to practice with this, so I would appreciate your insights:) If possible, please provide me with formulas to explain how calculations are made. Also Happy New Year to anyone reading this

author: moffy
author_rank: Super Contributor
timestamp: January 01, 2025, 04:57:48 am
content: That's about right as the effective emitter resistance is about 2.6R at 10ma, to increase the gain just reduce R4 or increase the value of the collector resistor R3, this will shift the bias point slightly.

author: TimFox
author_rank: Super Contributor
timestamp: January 01, 2025, 06:16:41 am
content: Another small reduction in the gain is due to the effective collector resistancehoethat is in parallel with the external load resistor from collector to the DC power supplyVcc.

author: sy
author_rank: Regular Contributor
timestamp: January 01, 2025, 07:04:38 am
content: Ok, I have reduced the value of my resistor R4 and got the expected gain.However, I have noticed that this only works from some small signals on the input, and beyond some certain amplitude, the amplifier fails.Why is this the case? In the image below, I have sent 2Vpk-pk signal, but the output voltage only remains around 8.8Vpk-pk, so the gain is much less than 10.

author: moffy
author_rank: Super Contributor
timestamp: January 01, 2025, 07:32:46 am
content: Quote from: sy on January 01, 2025, 07:04:38 amYou only have a 9V supply and the amplifier cannot exceed that so it clips instead, saturation of the output signal.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 01, 2025, 07:54:20 am
content: Plenty distorted, too.