# How do I determine base resistor value to achieve .7 V

#### OrlandoTech

Joined May 5, 2019
3
I have spent over an hour trying to find this answer. For example, Vcc is 7V, how do I calculate the ohm value of my base resistor to achieve a .7 base voltage? Or any other desired voltage.
Sorry for the typo but in the title, you can’t edit the title when editing a message.

#### crutschow

Joined Mar 14, 2008
24,302
The base resistor is to limit current, not generate a voltage.
The base-emitter junction looks like a forward-biased diode, so the base resistor limits the current to that needed for the desired collector current.
The 0.7V comes naturally from the forward-bias current through the base-emitter junction.
It changes only slightly with current at typical base currents.

If you tells more about the application/circuit, we can give a more complete answer.

#### MrAl

Joined Jun 17, 2014
7,101
I have spent over an hour trying to find this answer. For example, Vcc is 7V, how do I calculate the ohm value of my base resistor to achieve a .7 base voltage? Or any other desired voltage.
Sorry for the typo but in the title, you can’t edit the title when editing a message.
Hi,

Do you mean using a diode model? The calculation comes from the model, the resistor,and the source voltage.

#### Zeeus

Joined Apr 17, 2019
508
I have spent over an hour trying to find this answer. For example, Vcc is 7V, how do I calculate the ohm value of my base resistor to achieve a .7 base voltage? Or any other desired voltage.
Sorry for the typo but in the title, you can’t edit the title when editing a message.

Not too sure but : According to trial

Supply = 7.42V

NPN 2n3904

To get 0.7V...Just connect emitter of transistor to ground, collector to supply then use base resistor about 220k (yellow is last band of 3 band resistors)...Using 10M puts Vb at about 0.43V...Using smaller Resistor increases Vb

To get an approximate voltage at base...Find approx. value of Beta (here : 195)...

Not too correct formula(but it works to an extent) : NPN... emitter resistor and base resistor..Collector to supply

ignore base-emitter resistance Re

Rb : base resistor
B = beta
Used 8.2k as emitter resistor

R1 = Rb/(B+1)
V1 = 7.42 - 0.65

so Vb/V1 = 8.2k/(8.2k + R1) //should be Ve and not Vb?
So to get a particular Vb(base voltage) : solve for Rb

To get 4v at base...I used 1.47M as Rb and got 3.86V
To get 2v at base ...Used 3.82M and got 2.35V

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#### Tonyr1084

Joined Sep 24, 2015
4,128
Just connect emitter of transistor to ground, collector to supply then use base resistor about 220k
Are you sure you want to hook up a transistor in this manor? Collector to power and emitter to ground - that's a dead short through the transistor.

#### Zeeus

Joined Apr 17, 2019
508
Are you sure you want to hook up a transistor in this manor? Collector to power and emitter to ground - that's a dead short through the transistor.
yeah thanks...collector through resistor

#### OrlandoTech

Joined May 5, 2019
3
OK That makes sense now. (Current not voltage) Appreciate the friendly comments

#### LvW

Joined Jun 13, 2013
900
I have spent over an hour trying to find this answer. For example, Vcc is 7V, how do I calculate the ohm value of my base resistor to achieve a .7 base voltage? Or any other desired voltage.
Sorry for the typo but in the title, you can’t edit the title when editing a message.
Yes - you are right. Even one single resistor RB between Vcc and the base node is able to open the BJT by producing the necessary voltage Vbe=0.6...0.7 volts.
It is common practice to speak in this case from "current injection" - however, in reality we have nothing else than a voltage divider between RB and the B-E path (resp. its resistance, which is strongly non-linear).
The working principle of such a DC biasing can best be understood by using a graph.
For this purpose, I have attached a graph showing how the two most important biasing methods work.
As you can see, it is (a) not possible and (b) not necessary to strive for an exact value of VBE=0.7volts . The accuracy and DC stability for the second RB-method will be further improved when RB (with a recalculated value) is connected to the collector node instead of Vcc (negative feedback).

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#### MrAl

Joined Jun 17, 2014
7,101
I have spent over an hour trying to find this answer. For example, Vcc is 7V, how do I calculate the ohm value of my base resistor to achieve a .7 base voltage? Or any other desired voltage.
Sorry for the typo but in the title, you can’t edit the title when editing a message.
Hi,

What is the reason you want to set the base voltage at exactly 0.7 volts anyway?
That would help explain exactly what you want to do here.

[LATER]
Here is a quick example of doing it using a diode model suitable for your transistor.
First wrote the nodal equations then solve for Vbe symbolically:
Vbe=-((Is*e^(Vbe/NVT)-Is)*R1-(B+1)*E1)/(B+1)
where
Is is the saturation current of the diode,
NVT is N*VT where N is the ideality factor of the transistor,
B is the Beta,
E1 is the input voltage to ground,
R1 is the input base resistor,
Vbe is the voltage base to emitter.
It is assumed that the transistor is in the active region so you must verify that knowing the power supply voltage and the collector resistor and the collector current which you can calculate later.

This equation:
Vbe=-((Is*e^(Vbe/NVT)-Is)*R1-(B+1)*E1)/(B+1)

after substituting all the known quantities is solved numerically and the result is the base emitter voltage Vbe.

Note i left out the diode series resistance Rs for simplicity. To include that we have to include a base emitter resistor R3 and add the value of Rs to that resistance, then solve for the voltage Vbe above and add the voltage across Rs to get the actual real life base emitter voltage.

All this is actually not hard to do but you have to know the quantities for the diode for the transistor you are using. You might get this info from a Spice model. You can also change the form of the Vbe equation a little too for a little more simplicity.

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#### Dodgydave

Joined Jun 22, 2012
8,797
OK That makes sense now. (Current not voltage) Appreciate the friendly comments

#### ArakelTheDragon

Joined Nov 18, 2016
1,327
Bipolar transistors are current controlled. If you need 0.7 V on the base, you will have 0V on the emitter, which is a bad idea. You have to start by calculating the emitter voltage and current, collector voltage and current and from them see the needed current for 0.7V on the base resistor. You must not forget the voltage drop of the base emitter junction. Lookup in google transistor calculating tutorial.

#### Ford Prefect

Joined Jun 14, 2010
173
Have a read of this thread which was posted a couple of weeks or so ago.
Also my post #50 within the thread in which I said I basically ignore the hFE in transistor datasheets and use and start with a 'forced beta' of 10 to calculate the base resistor value which will force a transistor into full saturation and thus approximately 0.7v (Vbe) at the base of the transistor.
Have a look here:
Transistor Base Resistor Calculator
and here:
Transistor Hard Saturation - Rule of Thumb
You may find this helps.

#### WBahn

Joined Mar 31, 2012
25,222
I have spent over an hour trying to find this answer. For example, Vcc is 7V, how do I calculate the ohm value of my base resistor to achieve a .7 base voltage? Or any other desired voltage.
Sorry for the typo but in the title, you can’t edit the title when editing a message.
Trying to achieve a base voltage of 0.7 V doesn't give enough information. What is the emitter voltage? 0 V? Or some other voltage?

It sounds like you are trying to bias a transistor so that it is "on" (either in the active region or in saturation) and using the notion that the base-emitter voltage is 0.7 V when that is the case. But if so, then it is hard to tell what you mean by "or any other desired voltage".

The nominal 0.7 V for a forward-biased silicon PN junction is only an approximation. The actual voltage depends on the specifics of the device, the actual current being drawn, the temperature, and other less important parameters. In practice, we seldom try to establish a specific Vbe voltage. Instead, we design for a desired current and verify that the transistor will be in the desired operating mode.

For completeness, if you really wanted to design the circuit to get a specific Vbe (assuming that you are trying to get a reasonable value), then what you need to do is determine what the base current will be at that voltage. This information will come from the datasheet and will probably require some interpolation on the graphs usually provided. Then determine the voltage drop across the base resistor when the Vbe is what you want and use those two parameters to calculate the necessary resistance.

#### MisterBill2

Joined Jan 23, 2018
5,162
First, you need to state the whole situation and if there is a circuit involved you need to provide at least some description of it. And at least one of the responses you got so far is not close to relevant.
Lots of folks can give great answers if the question relates to a complete thought, and so the questions need to explain a lot more about the situation and what you are trying to do.