# How to control transistor mode using resistors???

#### babaliaris

Joined Nov 19, 2019
160
So I'm trying to figure out the behaviour of transistors and which resistors to choose in order to control its modes. Let's assume that I'm using a resistor for all the connections of the transistor.

For Active mode, I did some math and I conclude that if I choose a BIG resistance in the base and SMALL for the collector and emitter then chances are that transistor will operate as an amplifier in active mode. Check the following to correct me if I'm wrong.

I noticed that some of my limits are wrong, but still, the idea stays the same. BIG R1 makes collector's voltage BIG and emitter's SMALL which is what I'm looking for to get closer to the inequation. Also, small R2 keeps collector's voltage BIG and SMALL R3 keeps emitter's voltage small.

Even if my conclusion is true, still its hard to choose resistor values to achieve that. For example, I tried R1=1K, R2=100, R3=100
but the transistor was in saturation. Then I changed R1=20K still in saturation. Then I changed R1=50K and finally, the transistor was
in Active mode with Ib=120μA and Ic=30mA .

Any other ways to achieve what I'm looking for??? I want to have control over the transistor and not making random stuff...

Thank you.

#### ci139

Joined Jul 11, 2016
1,898
i'm not shure if i get your modes correct . . . but basically you can operate your BJT over wide range of . . .
currents . . .
if
the signal is smooth (sine) the operation up to 100kHz and even up to 2MHz . . .
. . . can be achieved with relatively low current consumption

your primary concern is the overall circuit complexity -- e.g. -- if your transistor amplifies at nano-Amp range then it takes a state of art circuitry to interface/"read" such stage so that there's little distortion to signal and to the operation of such stage

basically your next stage defines your previous , if the signal can't be directly fed to next stg. the matching stages need to be introduced . . .

Back to Basics: Impedance Matching (Part 1)

#### MrChips

Joined Oct 2, 2009
30,456
I would add, if you want the BJT to operate in the active region, choose the base bias voltage so that VBE is about 0.6-0.7V.

#### DickCappels

Joined Aug 21, 2008
10,121
There are very few cases in which you will obtain good, predictable analog performance by running a transistor in its active (not cut-off nor saturated) range of operation without feedback or one sort of another. That is because a transistor's gain and base-emitter voltage wander all over the place by individual device and as a function of temperature.

Notice that in the video posted by danadak in post #2 that there is an emitter resistor in the circuit. The emitter resistor provides negative feedback to keep the transistor from either saturating or cutting off.

I think that one notable exception is the case of driving the transistor with a small voltage as opposed to current -the gain will vary widely by temperature but in some applications it is ok.

#### ci139

Joined Jul 11, 2016
1,898
here's another video (the working point is usually "drawn" from the collector load lines diagram but this has a slightly different approach)