Transistors

• Videos of general transistor operation:

  • Transistors - Veritasium .

  • Transistors - EngineeringMindset .

    • Clip with good 'Swing Gate' metaphor to explain the current in the Emitter .

  • Transistors - RealPars .

  • Transistors - BenEater .

Current in the Transistor

• The current I_E (Emitter) depends on I_B (Base) and I_C (Collector), being equal to {I_E = I_B + I_C}; this can be approximated to {I_E = I_C}, considering that I_B is basically insignificant when compared to I_C.

• The current I_C depends on V_CE (Voltage Drop between Collector and Emitter), where V_CE has its value changed based on I_B, via the Beta factor relationship. When the Base "saturates", we will have that V_CE will reach its minimum value, enabling the highest allowed value of I_C.

• Extreme cases:

  • If {I_B = 0}, this implies that {I_C = 0} and {V_CE = V_CC}, because the circuit will be open (Cut-off).

  • If {I_B = >>}, this implies that {I_C = >>} and {V_CE = 0}, that is, V_CE will be shorted, leaving I_C at its maximum value. This point is called I_C(sat), known as the Saturation Point.

• Illustration:

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• Videos:

  • Explanation of the curves that associate I_B, I_C and V_CE: ElectronXLab video .

    • At 13:19  of the video, an example of determining I_B for the Transistor to saturate is shown. 'DC Load Line' is also briefly explained.

  • Calculation of I_B, I_C and V_CE, in addition to drawing the 'DC Load Line' and the 'Q Point': by ElectronXLab  and by ElectronXLab .

    • The 'Q Point' (Quiescent (quiet) Point), is defined as the point of Current I_C and Voltage V_CE when the circuit has no signal; represented as I_CQ and V_CEQ. It can also be said to be the 'Operating Point'.

Voltage in the Transistor

• The Voltage between the Emitter and the Base will be given by the manufacturing material:

  • Silicon: 0.7 V.

  • Germanium: 0.3 V.

Beta Factor

• It is a physical characteristic of the Transistor, not of the circuit. It is defined as {Beta = I_E / I_B}.

Transistor Saturation

• I_B(sat) (Saturation Current) is defined as {I_B(sat) = Beta * I_C(max)}. Any current I_B > I_B(sat) also puts the Transistor in saturation; the only care that must be taken is not to overload the I_BE current, burning the Transistor.

Darlington Transistors

• Darlington Transistors - OrganicChemistryTutor .

• Different explanation and application as 'Switch': Darlington Transistors - ElectronXLab .

Voltage in the Transistor

• The Voltage between the Emitter and the Base will be given by the manufacturing material, always being the sum of the Voltage between the '2 Internal BJT Transistors':

  • Silicon: 0.7 * 2 = 1.4 V.

  • Germanium: 0.3 * 2 = 0.6 V.

Current in the Transistor

• It is obtained by considering the Beta factor formula.

• Current in Transistor Simulation .

Beta Factor

• Beta_D (Darlington's Beta) will depend on Beta_1 and Beta_2 (Beta of the internal BJTs), so that: {Beta_D = Beta_1 * Beta_2 + Beta_1 + Beta_2}; normally the term {Beta_1 + Beta_2} is disregarded, since the contribution of this term in Beta_D is basically insignificant.

• Example of application with Beta calculation - GVEnsino .

Circuits with Transistors

'Common Emitter' Amplification [?]

• It has a "low gain" in Current and Voltage, but a "high gain" in Power.

• When comparing the 'input signal' and the 'output signal', we have that the 'output signal' is inverted in relation to the 'input signal'; for example, the sine function will be inverted.

• Demonstration of 'Microphone -> Speaker' amplification with 'Common Emitter' .

• Demonstration of "amplification / switch" with Transistors .

'Class A' type Amplification

• [I found it confusing] Demonstration and explanations .

• Demonstration and brief explanations:

  • "Power Amplifier" .

  • Class A .

'Class B' and 'Class AB' type Amplification

• [I found it confusing] Demonstration and explanations: video .

'Voltage Divider Bias'

• Used to minimize the circuit's dependence on the Beta value, and therefore minimize variations in the circuit's behavior with temperature.

• The calculation of the 'Equivalent Resistance' of the 'Voltage Divider' is not obvious, but is done using the 'Thevenin's Theorem'.

• Videos:

  • Explanation and example with calculation - ElectronXLab  and ElectronXLab video .

  • Use of 'Voltage Divider' for audio amplification - Afrotechmods .

'Buffers and Inverters' with Transistors

• Video demonstrating the creation of an Inverter - BenEater .

• Explanation and comparison between Buffer and Inverter - BenEater , in addition to the explanation of what a 'Pull-up Resistor' is.

'Flip Flops' with Transistors

• Flip Flops - EngineeringMindset .

'Toggle Button' with Transistors

• [I found it confusing] Toggle Button Example - EEVBlog .