# Common base voltage gain

#### ntetlow

Joined Jul 12, 2019
39
Hello. Can someone tell me the formula for the voltage gain of the attached common base amp. The theory would have it that it is Rc||Rl/Re but I cannot find it so? It seems to be 10, 1 volt / 100mv but how that is arrived at has me baffled.

#### Attachments

• 1.4 KB Views: 9

#### Delta prime

Joined Nov 15, 2019
610
Hello there
welcome to AAC.
The voltage gain for the common base amplifier is the ratio of V OUT /V IN, that is the collector voltage VC to the emitter voltage VE. In other words, VOUT = VC and VIN = VE. as the output voltage VOUT is developed across the collector resistance, RC, the output voltage must therefore be a function of IC as from Ohms Law, VRC = IC*RC

#### ronsimpson

Joined Oct 7, 2019
1,121
Rc||Rl/Re but I cannot find it so? It seems to be 10, 1 volt / 100mv but how that is arrived at has me baffled.
I agree with Rc||Rl. R2 and R6 are in parallel. 10k||10k=5k
Gain = Rc/Re 5k/470 = 10.6 That is what I see in SPICE.

R6 could go to +12V not ground and it works exactly the same. It might be easy to see R6||R2 when R6 is connected to +12V.

#### ntetlow

Joined Jul 12, 2019
39
I agree with Rc||Rl. R2 and R6 are in parallel. 10k||10k=5k
Gain = Rc/Re 5k/470 = 10.6 That is what I see in SPICE.
View attachment 223469
R6 could go to +12V not ground and it works exactly the same. It might be easy to see R6||R2 when R6 is connected to +12V.
Yes. Thank you. A gain of 10 looks as if it might be correct and tallies with the formula. However, when you halve the 470 resistance to 235 and expect the gain to double, in fact the voltage goes down from about 1 volt to about 100mv.
Can't explain this , can you?

#### ronsimpson

Joined Oct 7, 2019
1,121
Can't explain this , can you?
Look at the dc operating points. It is likely that there is distortion where the output voltage gets too close to the supply or too close to no voltage across C-E of the transistor.
--------edited
If you increase the collector current "470/235" you need to increase the base current 470k/100k. It maybe that you do not have enough base current to work.

Last edited:

#### GeBJT

Joined Nov 3, 2020
14
In simulation, I put a 1uF capacitor from Q1 base to ground. The collector voltage swing went up considerably, but with severe clipping. A 2N3904 transistor and 4k R2 (collector) gave a smooth sine at 1.9 - 6.6V (4.7V p-p).
The AAC common base tutorial (post #4) shows the base connected directly to ground ie zero impedance. In your circuit (post #1) the base is connected to a voltage divider, impedance 100k||470k to ground.

#### GeBJT

Joined Nov 3, 2020
14
Later - apologies, I forgot to answer the question! Small signal voltage gain (from memory) is something like transistor gm*(R2||R6).

#### sparky 1

Joined Nov 3, 2018
401
Another example having the descriptive details needed ( less the transmission theory) along with the mechanics of the solution presented matching instrument ( scope shot) CB problem page 11 University of Nevada. subheading : Solved Example Problem #2: Common-Base Amplifier. Also an acceptable frequency response for the particular example problem is shown in figure 13. Taking a picture of the scope would show a nice sine wave no roll off (no decrease in amplitude down 3dB at 5MHz) because your hand calulations to the lab exersize are correct and freshman EE lab #8 completed!
http://eelabs.faculty.unlv.edu/docs/labs/ee320L/ee320L_08_experiment_8.pdf

Last edited: