# "gm/id" method for 2N3903 bjt

#### yef smith

Joined Aug 2, 2020
235
Hello,I am trying to understand the effect of the biasing on bjt to the gain of the AC signal going into 2N3903 bjt which data sheet is shown bellow.
Bellow we have the collector current plot ,i know that Ic=beta*ibase so what do i do with this plot to find the beta and finding the gain value of common bjt?
Thanks.

https://www.onsemi.com/pub/Collateral/2N3903-D.PDF

#### ericgibbs

Joined Jan 29, 2010
12,884
hi yef,
E

#### Bordodynov

Joined May 20, 2015
2,865
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#### Bordodynov

Joined May 20, 2015
2,865
See

#### crutschow

Joined Mar 14, 2008
27,171
i know that Ic=beta*ibase so what do i do with this plot to find the beta and finding the gain value of common bjt?
Your title mentions "gm" which is the transconductance or ΔIc / ΔVbe (not Ic / Vbe).
Beta is the current gain or Ic / Ib

#### Bordodynov

Joined May 20, 2015
2,865
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#### yef smith

Joined Aug 2, 2020
235
Hello Bordodynov i got a plot of Ic/Ib as shown bellow.
I just want to know what biasing to do i order to a small signal analysis.

#### Bordodynov

Joined May 20, 2015
2,865
In the title of the topic you have a transistor 2N3903, and in the calculations 2N3904!

Last edited:

#### Audioguru again

Joined Oct 21, 2019
3,172
The datasheet for any transistor shows a wide range of base-emitter voltage an beta. Also, when the temperature changes then the base-emitter voltage changes that affects the transistor circuit.

To make a product work with all transistors having the same part number then you must add some AC and DC negative feedback.
For a 2N3904 its datasheet shows in a graph that is "typical" base-emitter voltage at a collector current of 0.5mA is 0.65V. It might vary 0.4V with different transistors even if they have the same part number. A graph shows how much temperature change affects the base-emitter voltage.

Usually a transistor has an emitter resistor so that base-emitter voltage differences are reduced. The emitter resistor reduces distortion and voltage gain.

Usually the base-emitter-resistor is biased from a two-resistors voltage divider. Here is a typical circuit:

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