Proper transistor connection

Transistor double diode model

The bipolar transistor can be considered as a device made up of 2 diodes. In the case of NPN type transistors, the diode formed by the Base and the Emitter and the diode formed by the Base and the Collector. 

In this way, if we connect the Base of the NPN transistor (Positive crystal) to the positive pole of a battery and the Emitter (Negative crystal) to the negative pole, a configuration called "Direct Bias", it will be able to circulate electric current through the diode Base - Emitter and the red LED will emit light:

On the other hand, if we connect the Base of the NPN transistor (Positive crystal) to the positive pole of a battery and the Collector (Negative crystal) to the negative pole of the battery, a configuration that is also called "Direct Bias", it will also be possible to circulate electrical current through the Base - Collector diode and the red LED will emit light:

With any other configuration of connections, it will not be possible to circulate the current through the transistor in an adequate way. For example, if we connect the Collector of the NPN transistor (Negative crystal) to the positive pole of a battery and the Emitter (Negative crystal) to the negative pole of the battery, a configuration called "Reverse Bias", it will not be able to circulate electric current properly by the transistor, no matter how much we increase the value in volts of the battery:

Forward and reverse bias

However, if we associate the first configuration described, forward bias of the Base - Emitter diode, with the last, reverse bias of the Collector and Emitter terminals of the transistor, it will be able to circulate the current through both configurations:

It can be seen that this circuit has 2 "meshes". One the Base and other the Collector.

The Base mesh is made up of a 3v VBB battery, a switch, a 100 ohm Base resistor (RB), a green LED diode, and the Base-Emitter "diode" of the NPN type BC 337 transistor.

The VBB battery biases forwardly the Base-Emitter junction since the positive pole of the battery is connected to the transistor´s P crystal (the Base).

The Collector mesh is made up of a 9v VCC battery, a 5v lamp, a 20 ohm Collector resistor (RC), and the Collector and Emitter connections of the transistor.

The VCC battery biases reversely the transistor, since the positive pole of the battery is connected to one of the transistor's N crystals (the Collector).

With the Base mesh switch open, no current flows through this mesh and neither the green LED nor the Collector mesh lamp light up because no current is generated by any of the meshes.

When we activate the switch, current will circulate through the Base mesh, which we can objectify by seeing that the LED emits green light. This current through the Base mesh will "allow" there to be current through the Collector mesh, turning on the lamp:

This is due to the action of the VBB battery, a small positive voltage is applied to the thin P crystal of the transistor Base (which must be greater than 0.7 v in the case of Silicon) and in this way the electrons of the N crystal of the transistor Emitter are attracted towards the Base and towards the positive pole of VBB and above all, towards the crystal N of the Collector of the transistor, which is connected to the positive pole of the VCC battery, with a higher positive potential than of the VBB battery, attracting to the electrons with greater intensity.

Real way of electrons

Bear in mind that this direction of circulation of the electron current is the real one and that the conventional direction, which is in the opposite direction, is the one that will always be considered unless otherwise indicated.

So in our circuit, following in the real direction of the electron current, a small current is produced towards the Base (IB), which makes the green LED emit light and a current of greater intensity towards the Collector (IC), which makes the lamp shine.

In addition, the greater the forward bias voltage of the Base - Emitter diode (VBB), the greater the number of electrons that circulate towards the Base and, above all, towards the Collector; so it can be said that this voltage and the Base current (IB) control the value of the Collector current (IC). Let's say that this forward bias voltage of the Base and the small current of electrons towards the Base that is generated, open more or less the transistor so that a greater number of electrons can flow towards the Collector to a greater or lesser measure depending on the value of the first.

It is like a water faucet. If we open it more or less (Base Current), we will get more or less water to flow from the supply network pipe (Emitter Current) to the faucet outlet nozzle (Collector Current):

But we must insist that this is the real direction and that by convention it has been decided that the direction of the electron current is the opposite, as indicated below:

Conventional direction of current in a transistor circuit

Voltages and currents in the designed circuit

Returning to the circuit designed in the simulator, if we measure voltages and currents with virtual multimeters:

We get that

IB = 2,413 mA

V de RB = 241 mv (IBxRB = 2,413 mA x 100 𝛀)

V del LED verde = 1,965 v

VBE = 793 mv

IC = 196,66 mA

V de la Lámpara = 4,917 v

V de RC = 3,933 v (ICxRC = 196,66 mA x 20 𝛀)

VCE = 150 mv

We can observe that in the mesh of the Base it is fulfilled that:

VBB - IBRB - VLED - VBE = 0

3 - 0,241 - 1,965 - 0,793 = 0

The greatest voltage "consumption" occurs at the LED, since it requires a potential difference of at least 2 v to emit light.

The Base resistor (RB) is added to protect the LED. If it is too small, the LED will be exposed to an excessively high voltage with the consequent risk of burning out. But if its value is too high, the RB itself will "consume" an excessive amount of the VBB cell voltage and the LED will not emit light.

Similarly, in the Collector mesh:

VCC - VLamp - ICRC - VCE = 0

9 - 4,917 - 3,933 - 0,150 = 0

In this mesh there is no current circulation if there is no current circulating through the Base mesh. Only when there is current in this last mesh, there will be current through the Collector mesh and the lamp will light.

Note that while a small Base current (IB) circulates through the Base mesh, 2.4 mA, a much larger Collector current (IC) circulates through the Collector mesh, 196 mA.

Construction with real components of the designed circuit

We can build the circuit designed in the simulator with real components and thus check its functioning:

We have resistors whose measured values ​​are 22 and 100 ohms.

If we activate the switch, current will pass through the Base mesh in the conventional sense and we can check it by seeing the green LED light up. In the same way, when passing current through the Base of the transistor, the passage of current from the Collector to the Emitter of the transistor is "allowed", circulating current through the Collector mesh. We can verify this by seeing the lamp turn on.

Next we will measure voltages and currents with the multimeter in the circuit in operation and we obtain that:

VBB = 3,08 v

IB = 2,97 mA

V de RB = 280 mv

V del LED verde = 1,92 v

VBE = 810 mv

VCC = 7,24 v

IC = 210 mA

V de la Lámpara = 2,27 v

V de RC = 4,68 v 

VCE = 140 mv

We observe that also in the mesh of the Base it is fulfilled that:

VBB - IBRB - VLED - VBE = 0

3,08 - 0,280 - 1,92 - 0,810 = 0

and that likewise, in the Collector mesh:

VCC - VLamp - ICRC - VCE = 0

7,24 - 2,27 - 4,68 - 0,140 = 0 

Summarizing:

We have shown how to connect the terminals of the transistor so that it works properly. The Base - Emitter "diode" will be directly polarized, connecting the positive pole of a battery to the P crystal of the transistor, the Base and the negative pole of the battery to the N crystal of the Emitter. The Collector and Emitter terminals will be reverse biased, connecting the positive pole of another battery to the Collector (crystal N) and the negative pole of this battery to the Emitter (crystal N of the transistor).

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